Did the ancient Egyptians build their impressive monuments with the help of the world's first protractors? A paper published last week on the physics preprint server suggests that the mysterious object in this picture, found in the tomb of an ancient Egyptian architect, could have been used as one.

The architect was called Kha, and he lived during Egypt's 18th Dynasty, around 1400 BC (shortly before the reign of Tutankhamun). His intact tomb was discovered by the Italian archaeologist Ernesto Schiaparelli in 1906, located in Deir el-Medina, which was a village for the workmen who constructed the royal tombs of that time.

Kha was buried with his surveying instruments, including cubit measuring rods (one of them foldable) and an instrument that looks like a modern set square. It would have been stood up like an A frame, with a plumb line hanging down from the point of the A, to show whether a surface was level.

Then there's the strange wooden item pictured, with a circular section bulging out of a straight bar. It is hollow inside and has a hinged lid, so it seems to be a case for something, although it was found empty. Schiaparelli thought it held a levelling instrument. The Egyptian museum in Turin, where the objects are now on display, identifies it as the case for a balance scale.

But Amelia Sparavigna, a physicist at the Politecnico di Torino in Turin, Italy, now suggests that whatever the case originally held, it could also have functioned as the world's earliest known protractor. Sparavigna has applied her physics expertise to archaeology before. For example, she has used an imaging processing method developed for studying liquid crystals to identify archaeological features in satellite images, and has investigated the symmetry in ancient seals.

I've written a short story for New Scientist on Sparavigna's latest paper, but here's the story in a bit more detail. She has analysed the decoration on the circular section of Kha's mysterious case. It includes a rose pattern with 16 evenly spaced leaves, and a circular zigzag with 36 corners. Both numbers, 16 and 36, were significant to the ancient Egyptians. The fraction one-sixteenth features in a calculus system the Egyptians used, says Sparavigna, and they identified 36 star groups called the decans, which later formed the basis of a star clock. She suggests the object was "a protractor instrument with two scales, one based on Egyptian fractions, the other based on decans".

She thinks Kha could have used it for measuring the angle of a slope or wall, or even to orient temples of tombs to specific astronomical directions. If you placed the straight portion of the instrument on the slope and then you could read off the appropriate angle from vertical using a hanging plumb line - she was kind enough to send me this graphic of how she sees it working.

I asked Kate Spence, an archaeologist at the University of Cambridge who specialises in ancient Egyptian architecture, what she makes of the idea. She describes the idea was "intriguing" but is very sceptical. She says that ancient Egyptian measuring instruments tend to be carefully made and precise with neat labels. "They look like measuring instruments," she says. In contrast, the design on Kha's case isn't particularly even and looks decorative. What's more, to use the instrument as Sparavigna suggests would involve resting it on its hinged lid, which is slightly rounded. "If it was to be a surveying insturment, the solider and squarer the faces of the flat bit the better," she says.

There's no doubt that Egyptian architects and engineers were capable of very precise work, however. Spence says that if they wanted to, architects could orient a building to within a few sixtieths of a degree. But they wouldn't necessarily have needed a protractor. Instead, the Egyptians expressed slopes and angles in terms of proportional ratios - for example you might create a 45-degree angle by measuring along two cubits and up two cubits, rather than specifying the angle itself.

The Egyptians knew how to divide up a circle too. On the ceiling of the tomb of another 18th Dynasty architect called Senmut, there is a series of circles (pictured). These have been divided into 24 equal sections by drawing squares around the circles, and seem to be associated with the hours in the day. "They look mathematical, the case doesn't," says Spence.

Sparavigna stands by her suggestion, and I think it's a fascinating idea, although I do find Spence's arguments quite convincing. So, Kha's case aside, when did protractors first appear? I asked Tracey Rihll, an expert in Greek and Roman technology at Swansea University and she said that there's a Roman example of a surveying instrument similar to a protractor in the Aquincum museum in Budapest.

But it may have been the Greeks who invented them. The first-century astronomer Ptolemy described how to make a 360-degree ring, and a 90-degree quadrant, so it was apparently routine by then to make instruments to measure angles. And of course the Antikythera mechanism, which dates from the 2nd century BC, has a 360-degree dial on it too.

Thanks to Sparavigna for sending me the picture of Kha's case (copyright Jane Maria Hamilton).

Yesterday I posted my talk from the literary storytelling session at the recent World Conference of Science Journalists in Doha, Qatar. Here's a follow-up post, with some brief comments on the other talks, as well as some tips and recommendations from all the panellists that we didn't have time for in the session itself.

I was lucky enough to be on a panel with David Dobbs and George Zarkadakis. David writes features and essays on brain and behaviour for publications including the Atlantic, New York Times and National Geographic (including this great piece on the genetics of temperament) and is now working on his fourth book. George has experience in writing both fiction and non-fiction. He has a PhD in artificial intelligence and is a novelist, playwright and science journalist, as well as having worked as a magazine editor and book publisher.

The focus of the session was the challenges of writing longer articles, over four or five thousand words. David talked about the importance of getting the right structure. For long pieces, he pointed out, the conventional feature template of lede, back story, narrative and close doesn't work - each individual section is too long, and the reader will get impatient. He suggested cycling through those elements, having mini-features within a feature, to keep the reader interested. When you change the scene, he said, you can vary things like the voice you use and how zoomed in you are, like changing the focal length of a lens. He also talked about learning from the structures used in music or drama, for example the idea of introducing a new character towards the end of the second act.

I loved George's talk too, he spoke about the importance in fiction or narrative non-fiction of making connections - your chapters can't be separate blocks of information. He explained the use of mind maps for getting all of the elements of your story down on one page, so you can see them all at once and chart the links between them. These could be between different people, ideas, fields or time periods, for example. These are your added value - links that perhaps the researchers themselves haven't even made. Both George and David said that they keep diaries when they're writing a book to record what they've done each day and keep track of these connections.

Another thing we talked about was the importance of knowing what to cut. When I started writing my first book I got excited about finally having space to put in all of the fascinating (to me) details and anecdotes that I unearthed in my research. I soon realised that you have to be just as disciplined - perhaps even more so - than in a shorter piece in knowing what to cut.

That's because you're expecting people to invest a lot of time in your piece, it's quite an endurance test and the reader can get distracted easily. Sometimes I think it's like clinging to a t-bar ski lift as it pulls you up a mountain. If there are too many lumps and bumps the reader's concentration will waver - they'll fall off and they won't get on again. As a writer, everything you leave in has to play a role in supporting or moving on your narrative (something that I think applies to blogs too - having unlimited space doesn't necessarily mean you should use it...)

The famous "kill your darlings" quote came up of course. I'm not sure who said it first, some people say Hemingway, some say William Faulkner, but this blog post says that it comes originally from the British author Arthur Quiller-Couch (pictured), describing "style" in his 1916 publication On the Art of Writing:

"Whenever you feel an impulse to perpetrate a piece of exceptionally fine writing, obey it -- whole-heartedly -- and delete it before sending your manuscript to press. Murder your darlings."

George quoted a great line from the French writer and aviator Antoine de Saint-Exupéry: "A designer knows he has achieved perfection not when there is nothing more to add, but when there is nothing left to take away." And David mentioned a lovely passage written by Janet Malcolm, in which she describes the biographer's struggle of knowing what to throw out, and likens it to clearing the clutter from a crowded house (see his blog post).

Alok Jha, who chaired the session expertly, had asked us all beforehand to come up with some tips and recommendations for the audience. We got so carried away with the discussion that we didn't have time for them in the session itself, but George and David have kindly emailed their notes to me, so here they are below:

AN ARTICLE OR BOOK THAT BUDDING WRITERS SHOULD READ TO GIVE A SENSE OF GOOD WRITING

David recommends Beware of Greeks Bearing Bonds, a Vanity Fair piece by Michael Lewis. He says:

"It's about finance, not science, and that's part of the point. Lewis, one of our very best longform writers, faces a task similar to that of writing about science: Explain the workings and consequences of a seemingly arcane, jargonophilic discipline (finance) in a way that engages the reader. He doesn't just engage you: He provides wild, raucous, riveting entertainment - theater, really - in a way that gets all the essentials right and sets up a magnificent close. A huge percentage of what you need to know to write well and beautifully is in this piece."
(For another perspective on the financial situation in Greece, see George's recent blog post)

David's second recommendation is Song of the Dodo, by David Quammen:

"One of the best books, period, written in the last two decades, and easily one of the best science books written in the 20th century. If I could have only one grocery bag full of books to read for the rest of my life, this would be in it. As with Lewis, this is not an explanation but a story, a tour of a new and important idea about evolution told through visits with smart, driven, startlingly original and articulate scientists. Quammen is one of our very best; I don't know a science writer I'd rank above him. He's also grievously underrecognized, and I'm shocked far too often to discover that many science writers have never heard of him. He's magnificent, and this book is a tour de force. It also has one of my favorite sentences of all time (ranking close behind 'Shut up,' he explained.'): 'Cor! You don't see that every day, do you.' When you hit it you'll know why."

My choice was Gilgamesh, in particular an English version published in 2004 by Stephen Mitchell. The Epic of Gilgamesh is the oldest known work of world literature - written down on clay tablets in ancient Mesopotamia in the second millennium BC. I love the writing because it is simple, clean and direct, there are no wasted words. Yet the prose has an energy and a beauty, it feels alive. It switches effortlessly from the broad and sweeping to the personal and intimate. Much of this is Mitchell's work, but I wonder if it also owes something to the oral tradition which would have been like a continual editing process every time the story was told out loud - it would have been instantly clear to the narrator from the audience's reaction which bits worked and which bits didn't.

The prologue describes how the hero, the king of Uruk, has been on an epic adventure - experienced all emotions from exultation to despair, journeyed to the edge of the world and back - and carved his trials onto stone tablets. The narrator invites the reader to admire his gleaming city, the mighty walls, gardens, orchards, palaces and temples. Then it tells you to climb the ancient stone staircase:

"Find the cornerstone and under it the copper box
that is marked with his name. Unlock it. Open the lid.
Take out the tablet of lapis lazuli. Read
how Gilgamesh suffered all and accomplished all."

I love this because it sucks you right in, straight away you imagine opening the box, taking out those tablets and starting the story...

TIPS FOR SOMEONE LOOKING TO IMPROVE THEIR WRITING TOOLKIT

Here are David's:

1. Read lots of good writing --- and read hard. Read the great things things two or three times. And by all means, read a mix of science and of things NOT science. And learn to read to take it apart and see how it works and how they solve problems. Mark the good points and go back later to see how they work. Read to steal. Which is to say, not to take a tactic or strategy and imitate it, but absorb it and fully understand it and make it an integral part of how you write. (Knowing how to throw a curve ball isn't enough; you have to know when and where to throw it.)

But you can't steal unless you learn how to read first. How does John McPhee move so smoothly between places in time? (Answer: He finds the right structure first.) How does Janet Malcolm move herself and her own observations in and out of scenes? Why does Michael Lewis put himself in some stories as "I" and leave himself completely out of others? How does David Quammen get away with opening a book with 80 pages of backstory? What makes Vladimir Nabokov's sentence (in Lolita), These burst. so incredibly effectlve? How does Deborah Blum manage to generate sympathy for Harry Harlow's work even while conveying the horror of some of his experiments? What decisions does Rebecca Skloot seem to have made about handling her first-person presence in The Immortal of Henrietta Lacks?

Answering some of these questions will take you only minutes or hours. Others may take days or weeks. But every time you answer one, you'll add an invaluable arrow to your quiver.

2. When you get to your later drafts. READ THEM OUT LOUD. The sour junk will rise to the top, and you can throw it out.

George seconds those tips and adds two more:

1. Respect for the economy of language. In every language there are many ways of expressing the same thing but only one that is the simplest. Chose this one and make sure is from the heart.

2. Connect with the reader. A writer is a reader who mutated. Often this mutation causes selective amnesia: the writer forgets that she was once a reader. Reconnect with your previous self and talk to her about what moved you in the subject you chose to write a book about. Do not presume that being smart is enough. A book full of smart ideas but no emotion will vanish swiftly in the all-devouring quicksand of indifference.

I couldn't agree more with the above. My tips are just a couple of things that I do sometimes to get over that feeling of staring at a blank page.

1. Brainstorm then edit
If I want to convey an elusive feeling or thought and I'm not sure where to start, I write down individual words and phrases that seem to capture any part of it. I try to switch off any inhibition, I don't worry about sentences, or whether what I'm writing makes sense logically. It's just a cloud of words that pop into my head and feel as though they might be at least partly right. Then, once I have those raw ingredients on the page, I become more analytical/critical and try to make a piece of sensible writing out of them.

2. Stream of consciousness then edit:
If I want to get a story to flow, then I read through my notes and sources to get the events straight in my head. Then I put all those notes to one side and just write the story down. I don't stop to check dates, facts etc -- if necessary I'll just put in xxs where I can't remember exact details, I just try to write the story all in one go, as I'd tell it to a friend. I do it longhand because it stops the temptation of editing as I go along. I just write it all down in one go, with no stopping or thinking about it too much. Then, only once I've got the story down, I'll go back and edit - clean up sentences, check quotes, facts, figures, cut unnecessary bits. The aim is to end up with a narrative that hopefully has some pace and energy to it.

If anyone has any more tips or reading recommendations, or writing experiences to share, please do comment below.

When the Guardian's Alok Jha asked me to speak on a panel at this year's World Conference of Science Journalists on the importance of narrative in science journalism, I agreed straight away. After a decade writing and editing for Nature and New Scientist I felt well qualified to discuss the topic.

Subsequently, the title of the panel changed to "literary storytelling" - a small tweak but one that filled me with fear. Writing clear, engaging science news is one thing but when it comes to "literary" writing, well, that's a different world in which I feel a complete beginner.

The other speakers on the panel - David Dobbs and George Zarkadakis - are experienced authors, with many beautifully written long-form articles and books to their names. I have written just one book.

So I decided to talk about what it was like making that first step in the transition from "science reporter" to "author". What was the biggest mistake I made, and how did I have to change the way that I write?

A lot of the points I'm about to make may seem obvious to anyone trained as a "proper" writer, but for me, coming from a science background and used to writing for outlets like New Scientist and Nature, it required quite a paradigm shift. In fact, I think there are certain things about some kinds of science journalism that actively inhibit the parts of the brain and ways of thinking that you need for creative writing.

I was used to writing in a very logical, left-brain way - news journalism is pretty much about finding out facts, and putting them in the right order. Space is at a premium, so you have to convey information in a very economical way, with a fact in every sentence. What was done, who by, how, what did they find?

You have to explain complex technical concepts clearly and unambiguously. You talk about what things mean in a very analytical way. You don't generally put in anything of yourself - your thoughts or emotional reactions (or if you do, your editor will take them out). Constructing a story is a bit like constructing an argument.

Then I came to write my book, which is about a mysterious astronomical computer found on an ancient shipwreck. The first chapter should have been a dream of a tale. In 1900, sponge divers crossing the Mediterranean were blown off course by a storm, and took shelter by a rocky islet. The next day they dived into the water and discovered a wreck, filled with treasures from ancient Greece. They spent the next 10 months salvaging the artefacts in a treacherous mission during which one of them died of the bends, and two were paralysed.

I did painstaking research, raiding historical archives and translating old Greek documents to find out as many details as I could about the episode. When I wrote the chapter, it was clear, well-structured and packed with facts. But I discovered that what works for a New Scientist news story doesn't necessarily work for an 8,000-word book chapter. I gave my draft to a journalist friend to read and he came back to say that it was dense, flat and unmoving - a barrage of facts that was exhausting to read.

I realised that I needed to relax, and slow the pace right down. I couldn't just relay information, I had to really tell a story. It was a harsh lesson that to sustain a reader's attention over a long span, you have to say what things mean as well as just what happens. You have to transport the reader into the story, to carry them along with you.

I'm sure there are much better names for this but as I went through the book I started thinking of it as "inside out writing". Instead of processing and conveying facts in quite a superficial way, you need to internalise all of the information that you're gathering, to think very honestly and deeply about what it means, have an emotional reaction to it, and then try to bring that out in your writing.

I would do all my research for a particular scene, then try and turn off my left brain. I'd shut my eyes and imagine being there; think about the sounds, sights, smells; what the characters would have been feeling; the greater significance of the events I was describing.

I think there are a couple of overlapping aspects to this. The first is to describe a scene in a lifelike way, put the reader right in the middle of things. Think about involving all five senses to capture the feeling of a particular moment. Maybe it's looking up to see shafts of sunlight shining down through the water, or hearing the sound of helicopters whirring overhead like ravenous giant mosquitoes (the mosquitoes are from Andrew Smith's book Moondust: In search of the men who fell to Earth, which I think does this really nicely).

 To do this you don't have to be writing a narrative-led or biographical book, it can be important even in the more traditional type of science book, that discusses a particular subject. For example, Being Wrong by Kathryn Schulz (which I absolutely love) is about how holding false beliefs is part of being human, and how good we are at convincing ourselves that we're right even in the face of overwhelming evidence that we're wrong. There's a chapter about how often witnesses make mistakes when identifying criminals, and how the strongest, most emotionally charged memories are the most likely to be false.

In this chapter, Schulz tells the story of a woman who is raped while running alone on a beach. Her description is powerful and immersive, she makes you feel that woman's fear, her pain, her strength of will, her determination to remember, how she made sure that every detail of her attacker's face was etched onto her memory. After reading it, you feel some of the same shock and disbelief when you later find out that she had emphatically identified the wrong man.

The other aspect to inside out writing is that you need to get beyond simply saying what happened, to say what things mean, and what they mean to you.

Saying what something means is of course the role of a writer. You're trying to capture the essence of something, to give it a meaning that's personal yet universal. Often when I was writing I'd have this elusive feeling about why something is important - about how events or ideas were connected or why I wanted to include a particular anecdote. But I'd struggle to put it into words. That's the important bit, that's what you have to try to drag out from inside, and to express on the page.

In her book Longitude, Dava Sobel opens one chapter with her musings on time: "Time is to clock as mind is to brain. The clock or watch somehow contains the time. And yet time refuses to be bottled up like a genie stuffed in a lamp. Whether it flows as sand or turns on wheels within wheels, time escapes irretrievably, while we watch."

This passage doesn't contain any facts, and it doesn't move on the story at all. But I think it was important for Sobel to stand back at that point and give a very human account of what the essence of her story is about.

It is possible to do this even in shorter news pieces too, but it's rare. One example is an 800-word news article written by James Meek, which ran in the Guardian in 2003 (thanks to Ian Sample for passing it on to me). The article was about Iraqi families and soldiers who tried desperately to get inside government buildings after the fall of Saddam Hussein, clinging to the belief that their loved ones who had gone missing during the regime might still be alive, kept in secret underground cells.

The first two paragraphs describe fairly conventionally what is happening, then the first sentence of the third paragraph hits you like a bullet: "Those who have lost what they loved the most are always the richest in hope." For me, this one line crystallises the human meaning of the story.

Of course this is what proper writers do all the time. But as a news journalist, and I think especially a science journalist, it can feel like a huge change. In trying to take this approach I felt that I was breaking a lot of ingrained rules. To tell a real story you have to be subjective, creative, emotional even, but to write about science you have to be accurate, clear, unambiguous, and not mess with the facts themselves. There is always going to be a tension between those conflicting principles.

Despite all my experience as a science journalist, the biggest thing that writing a book taught me was how much I have to learn about what it actually means to write well about science (or anything else, for that matter). If there are any other scientists or science journalists out there dealing with similar issues I'd love to hear about your experiences.

*** The picture is a portrait of the Russian short story writer Vsevolod Garshin, just because he looks like he found writing tough too...

Here's a short video about the Antikythera mechanism posted last week by the Greek ministry of culture and tourism, the Swiss watchmakers Hublot, and the 3D video magazine NVP3D. It includes animations of the mechanism, shots of a wristwatch based on the device, and some lovely footage of the island of Antikythera. Definitely worth 7 minutes of your life. This video is in 2D, but there's a 3D version too (you need a stereoscope), or you can watch it in French.

I just got back from a really fun week in Doha, Qatar, at the World Conference of Science Journalists. I gave two talks there so I thought I'd put what I said into a series of blog posts, in case they're of interest to anyone who wasn't able to attend.

In the next couple of posts, I'll pass on what I had to say about "literary storytelling" in science journalism, and give some tips and recommendations from all the panelists that we didn't have time to talk about in the session itself.

But first, let's talk about the other panel session, on the role of history in science journalism. This panel was chaired by Tom Levenson of MIT, and the aim was to talk about how historians and reporters can use the past to write better science stories. The other panelists were the authors Deborah Blum, Reto Schneider and Holly Tucker.

Deborah spoke beautifully about how she explores the different paradigms or frameworks of thinking that enclose and define science at different points in history. For example, the assumption in the first half of the 20th century that "love doesn't exist" provided the context for the psychologist Harry Harlow's controversial experiments on mother-child bonding in monkeys, which she describes in her book Love at Goon Park.

Reto argued that science news is over-rated, and suggested that perhaps we should redefine it as interesting stories in science that people don't know. And he showed us how he is using Google Maps to plot the locations of different weird science experiments through history.

Holly described her book Blood Work, about the first blood transfusions, performed in the 1660s, and talked among other things about how looking at early science gives us context to discuss our own responses to technological innovation. It reminded me of a feature I wrote recently for New Scientist, about George Church's work on genome engineering. He predicts that his techniques could soon allow us to engineer virus-resistant humans. That might seem like ethically a really bad idea, but he argues that so did IVF and organ transplants when they were first introduced.

I also really liked a point Tom made about historical biographies. Although their subject is in the past, he said, biographies are actually about the present, reflecting the concerns and agenda of the writer's own time, rather than that of the subject.

My job was to talk from a more practical point of view as a science reporter. How can you use history to add depth and value to a piece of science journalism, whether it's a short news article or a whole book? For me, four approaches immediately came to mind:

1. Telling a story

A fairly obvious use of history is that including a chronological narrative - describing a series of events over time, for example following the trials and tribulations of the characters involved in a particular area of science - allows you to convey your subject as a compelling human story.

This can be a personal history - events as they happen to an individual or group of individuals, for example. The reader gets a feel for personal motivations, invests in the characters. He or she can root for particular people, or see them through challenges and setbacks. You can have feuds and battles, with winners and losers. Or it can be an intellectual history - in other words the history of the development of ideas. So the reader gets carried along by that intellectual journey as well. The ideal story would probably have both kinds of history.

A story like this can be set thousands of years ago, or last century, last year or last week. It can focus on a particular episode in time, or it can involve a grand sweeping narrative. I think this kind of approach is especially important for science writing, where you're often dealing with complex, technical subject matter. You can explain concepts bit by bit as the plot unfolds.

One successful example is Longitude by Dava Sobel. It's about a race to develop a clock that could keep time at sea - to enable sailors to calculate their position. Simply writing about the technology involved, the exact design of the springs and escapements, is unlikely to have excited people on its own. It would still have been history, but it's the way Sobel used that history, how she turned it into the story of a race between competing rivals, that made it such a fascinating book.

Another is Fermat's Last Theorem by Simon Singh, about the reclusive scholar Andrew Wiles' solution of Fermat's 350-year-old maths problem. Singh could have just described the maths involved in Fermat's theorem and Wiles' solution, but telling the history of both of these things is what really brought the subject to life. In both cases, the historical story gives you a human dimension and a plot.

In shorter articles, even news articles, using a historical sequence of events can still give you a plot line, an internal logic that drives the story forward. This can be just a small part of the article, it doesn't have to be the whole thing.

For example, I wrote a feature for Nature recently about an argument over Egyptian mummy DNA - some groups are publishing papers on this DNA, while other groups don't believe the results. In this article, I didn't want to just say "A thinks this, B thinks that". To show where the argument had come from, I went back to the origins of the ancient DNA field in the 1980s, explaining the excitement as researchers realised they could amplify DNA from ancient samples, and the harsh lessons they learned as it became clear how many studies had been ruined by contamination. The "sceptics" in the argument had lived through that experience, while many of the "believers" had come in later from other fields. This history doesn't tell you who is right, but it gives you a deeper perspective on the debate. And it gives you a story instead of a static argument.

Even when writing short articles I would always ask researchers how they got into a particular field and why they did a particular study, to get a sense of what is driving them and what has gone before. Even just a sentence or two on that can be enough to sweep the reader into a story.

2. Judging what's new, and what things mean

News is about the present, but an understanding of history is still vital because otherwise how do you know what's new? I think this is particularly important for science journalism because science is all about how our understanding progresses over time. You can't judge the significance of a step forward unless you know where you've come from. In science, it makes all the difference in the world whether something is being done for the first time or for the hundredth time.

Imagine you're writing a story about a cloned chimp embryo. Have scientists cloned a chimp embryo before? A primate embryo before? Has a cloned chimp been born before? You can't necessarily rely on press releases because they often have a vested interest in presenting a story as new. So if you're not already familiar with a field then you need to ask other experts how a new finding compares to what has gone before. This is important when choosing whether to cover a finding, but it is also important information to include in an article for the reader. It doesn't have to be written as a chronological sequence of events, but it does need to be in the story.

Having this perspective is what turns a vacuous account of events into a useful piece of analysis. I once spent some time doing work experience at The Economist, and I asked what was the secret of their success. One of the reporters said to me, it's the 3Cs: compare, contrast, context. To make meaning out of an event you have to set it in its place with what has gone before (as well as what else is going on at the time).

3. Transporting the reader

Writing about different historical periods in a long article or book is like travelling to different exotic places. It immerses you in another world where the landscape is different but so are beliefs, norms, the understanding of how the world works.

You can use the details of different historical periods to engage, surprise and transport the reader. You can also move between time periods to give variety and contrast. For example, my book Decoding the Heavens tells the story of a mysterious ancient Greek machine (called the Antikythera mechanism) that was found in a shipwreck, and the efforts of modern-day scientists to work out what it was. I wanted to combine stories from the astronomers and philosophers of ancient Greece, the archaeologists of the 1900s, and the high-tech scientists of today. That was all mixed in with other aspects of life in those time periods, from the Roman armies sweeping through the Mediterranean region in the first century BC to the sponge divers of the early 20th century with their clunky diving suits (pictured).

4. Gaining big picture perspective

Science is all about understanding, at deepest possible level, why things are the way they are. So I love looking at the big picture, being able to zoom out and see where a story fits in the grand scheme of things. Looking at things on a historical time line enables you to do that.

For example one aspect of understanding where the Antikythera mechanism comes from is to look right back in geological time to the forces that formed the copper and tin that went into the bronze that made it. Once you understand where the metal came from, you can understand why it was scarce, the trade routes that supplied the bronze, and the reasons why bronze objects were valuable and were almost always melted down and reused. This all feeds into why there are so few artefacts like this that survive from the ancient world.

Or, to understand how the survival of particular kinds of object colours our view of ancient societies, you can look far forward to imagine how our own society might look to the archaeologists of the future.

A nice example of this big picture historical perspective is Fermat's Last Theorem. Singh doesn't just go back to Fermat's work in the 17th century, but to the origins of mathematics in ancient Greece. Another lovely book, Life: an unauthorised biography by Richard Fortey, tells a 4-billion-year story. Time lines don't get much bigger than that.

The wreck of Antikythera, discovered by sponge divers in 1900, is most famous for the clockwork astronomical computer found on board. But it also contained one of the richest hauls of objects ever found from the ancient world - from bronze swords, statues and thrones to golden jewellery and luxury glassware.

I was reminded of this on Saturday, when I was interviewed for an episode of a programme called Museum Secrets, to be shown on National Geographic in the autumn. It will focus on the National Archaeological Museum in Athens, telling the stories of various objects held there, including the Antikythera mechanism. The rest of the sponge divers' haul is at the Athens museum too, so I thought I'd write a post on it: What else did they find? And is there anything still down at the bottom of the sea?

The Greek government hired the sponge divers to salvage what they could from the wreck, with the help of the navy, during a gruelling ten-month expedition in 1900-1901. According to the official report of the expedition, published (in Greek) by the Archaeological Society of Athens in 1902, their finds included bronze swords, pieces of a bronze throne, and bronze bedsteads engraved with busts of women and lions. There was also jewellery, for example a golden earring in the form of a baby holding a lyre, as well as a full-sized lyre, plus jugs, flagons, kettles, lamps, bottles and a silver wine jar. 

Gorgeous and perfectly preserved glassware (described here) included an elegant blue-green bowl carved with a floral design, and a set of mosaic dishes in which stripes of rose, green, purple, yellow and turquoise glass had been coiled into tiny spirals and melted together. There were piles of amphoras (storage jars with pointed bottoms used for transporting food and other goods), one of them with olive pits still inside. But most notable were hundreds of bronze and marble statues, mostly of men and horses, but also some women, and a colossal marble bull. 

After two thousand years in the sea the marble statues were horribly disfigured; their smooth sculpted surfaces had literally been eaten away by hungry sea creatures. As they arrived at the museum, they were piled up in a courtyard, to eerie effect (see pic, above). 

One of the few marbles that isn't completely ruined is this crouching boy - one of my favourite exhibits when I visited the Athens museum on a sunny November day in 2006 (see pic, left). This statue was half buried by sand, which protected it from the corrosive effects of the sea. The boy's right side is almost perfectly preserved, but on the left, his limbs have been munched away to stumps.

The bronze statues were mostly found in pieces - disembodied heads, hands, feet and smaller fragments. But they were actually in better shape than the marbles because their surfaces had reacted with seawater to form an inert layer that protected the material from further decay.

For example, there's a lovely bronze head of an old man, probably from the third century BC (see pic, below). It's in the style of a Greek philosopher, and because of realistic features such as the wrinkles, it's thought to be a portrait of a particular person, although no one knows who. It comes originally from a full statue - the arms and sandalled feet were also found.

But the most impressive bronze find was a graceful naked male, nicknamed the Antikythera Youth (see top photo). Originally found in more than twenty pieces, it was put back together in the early 1900s by a French sculptor called Alfred André. His efforts were severely criticised, however. In the 1950s the statue was taken apart again and reassembled in a slightly different position - to everyone's satisfaction it seems - by a Greek renovator called Christos Karouzos.

The Antikythera Youth is thought to be from the fourth century BC, perhaps not one of the very finest Greek statues ever made, but certainly of high quality. It isn't known where it is from, who the sculptor was, or who the statue was meant to be of.

There are some clues though. At nearly two metres tall the statue is slightly larger than life, so experts think it represents a god or hero rather than a mortal. And traces of bronze still attached to the statue's fingers show that it originally held something spherical in its raised right hand. Suggestions from Greek mythology have included Perseus holding up the severed head of Medusa (her hair in a tight spherical bun); Paris displaying, or preparing to throw, the apple of discord; or a young, beardless Herakles (the Greek version of the Roman hero Hercules), perhaps picking a golden apple in the Garden of the Hesperides. There's much more on the statue, including photos of the reconstructions and diagrams showing how it is put together, in this 2006 PhD thesis (pdf file). 

Surprisingly the ship itself turned out to be not Greek but Roman. Judging from the origin of storage jars, plates, coins etc on board, she sailed from the Asia Minor coast in 70-60 BC. When she sank she was probably on her way west to Rome, perhaps carrying looted cargo (Roman armies, led by the general Pompey, were sweeping through this region at the time).

Could there be more treasures still lying at the wreck site? The sponge divers believed that there was plenty left when a combination of bad weather and exhaustion finally forced them to end their salvage expedition in the summer of 1901. The diving pioneer Jacques Cousteau led another salvage project at the site in the 1970s, which he filmed for a documentary called Diving for Roman Plunder. His divers brought up plenty of small objects, including ship nails, coins, a lamp, two elegant bronze statuettes on rotating bases, and a human skull.

So it's unlikely that there is still anything lying exposed at the wreck site. But Cousteau had limited time for his expedition too, and without the gas mixtures that are available today for deep diving the divers could only spend short bursts of time working at the 60-metre-deep site. So it's very likely that there are still buried objects there, say cargo stored in the lower parts of the ship, now covered by the sediment that settled on the wreck over millennia. This would be hard for any subsequent expedition to get at (if indeed it was felt appropriate to carry out such an invasive excavation). But the items might be better preserved than those found so far.

The hull of the ship itself may also survive under the sand. Cousteau's colleague Frederic Dumas described poking around in the sand with a pipe during a 1953 dive at the site in his wonderful book 30 Centuries under the Sea:

"...the pipe ran right into the hull of the sunken ship, which was perfectly preserved under a foot and a half of sand. If only I had more time!"

A third possibility is that more items from the wreck are lying undiscovered in deeper water. The ship settled on a sloping shelf of rock, near to a cliff that drops down to greater depth. So some objects could presumably have fallen down there as the wreck sank. The Archaeological Society's 1902 report also says that several large boulders, lying on top of the wreck, were heaved out of the way and over the cliff before it was realised that these were actually enormous (but very corroded and overgrown) statues. And it describes how a large statue of a horse that tore loose from its chains as it was being winched onto a boat, tumbling down into the depths.

Here's an update on the fast-moving situation regarding Egyptian antiquities. I wrote on Monday about the resignation of Zahi Hawass, Egypt's top antiquities official. I reported that the new prime minister Essam Sharaf had dissolved the newly-created Department of Antiquities and restored the Supreme Council of Antiquities (SCA), under the ministry of culture. The head of the SCA's Lower Egypt division, Mohammed Abdel Maksoud, seemed to have been chosen as its new head.

Well, as I was writing that, hundreds of archaeologists were marching on government to protest that decision. In response, Sharaf agreed to make the SCA independent, in other words it will be under the direct supervision of the cabinet, rather than part of the culture ministry.

The last I heard is that the SCA will be choosing its new head/minister in a referendum on 18 March, from the following list of candidates:

 

1-د.علاء شاهين Dr. Ala Shahine

2-د.عبد الحليم نور الدين Dr Abd el Halim Nur al Din
3-د.صبري عبد العزيز Dr Sabry al Aziz (was Hawass's no. 2)

4-د.محمودعمر Dr. Mamdouh Amr
5-د.حسن سليم Dr. Hassan Selim, professor at Ain Shams University
6-د.ممدوح الدماطي Dr Mamdouh AlDamaty, former director of the Cairo Museum
7-د.علي رضوان Dr Ali Radwan, former professor at Cairo University

But perhaps it has all changed again by now!

Blogs like Margaret Maitland's Eloquent Peasant have continued to follow the latest on the looting situation. She links to this document, compiled by the Penn Cultural Heritage Centre, which details the antiquities currently known to be missing fom Egypt (including the lovely statue of Tutankhamun spear fishing, pictured). On 7 March, Tarek El Awadi, director of the Egyptian Museum, and other archaeologists in Egypt released an open letter to Sharaf urging him to make it a top priority to return police to archaeological sites to protect them against looters.

Meanwhile a group of Egyptologists based in the UK and Egypt has posted on the New Statesman's culture blog, complaining about the "racism and intolerance" that they say characterises many of the reactions abroad to the safeguarding of Egypt's antiquities following the revolution. The authors point out that many Egyptians have fought hard to protect these sites. They write: "Egypt can secure its heritage in its own way within its own borders, and has all the experts it needs in conservation and historical knowledge. Outside pressure needs to end."

They argue that the international community should examine its own role in supporting the international black market in stolen antiquities. The authors propose that Western governments should end practices that promote looting, for example making it illegal to sell or buy undocumented antiquities. They also say that requests from Egypt for the return of antiquities should be honoured, and that all countries should support a shift towards Egyptian archaeology that is done by Egyptians in Egypt.

For an outside observer at least, it is hard to imagine Egyptology without the country's top antiquities official, Zahi Hawass. Since he became head of the Supreme Council of Antiquities (SCA) in 2002, he has been a familiar figure in TV documentaries -- tracking down long-lost mummies, supervising state-of-the-art scientific projects and roaming the country in search of "new" finds.

He has featured regularly in newspaper headlines too. Hawass's energy and enthusiasm have raised the profile of Egypt's antiquities, and the numerous documentaries, as well as touring museum exhibitions of pharaohs' treasures, have brought in much-needed cash. But his critics have accused him of everything from using his position to boost his own media profile, to corruption (an allegation that he denies).

Shortly before President Mubarak was ousted by the recent uprising, he converted the SCA into a new government department of antiquities with Hawass as its minister. But last Thursday Hawass told the New York Times that if asked to continue in that role he would refuse, complaining that the police are not doing enough to protect Egypt's archaeological sites from looters. He had previously downplayed the risk of looting on his blog, saying that sites were safe, but on Thursday he reversed that position, posting a long list of affected sites across the country.

As a long-time ally of Mubarak, it is far from certain that Hawass would have kept his job under the new regime anyway. But his comments about the extent of the looting are still pretty worrying. On Saturday I asked Hawass about what he hopes to achieve by resigning, his fears for Egypt's archaeological sites, and whether he would consider staying on in any SCA or government role. I've included the full interview below.

I've written two stories about the situation, one for New Scientist on Friday about the possible extent of the looting, and one for Nature today about the implications of Hawass's departure. Last night the caretaker prime minister Essam Sharaf named a new cabinet, and got rid of the newly-created antiquities department, reinstating the SCA. Its new head has not yet been officially announced but I'm told it will be Mohammed Abdel Maksoud, the head of the SCA's Lower Egypt division and an archaeologist who has worked for many years in the Nile delta region.

My interview with Zahi Hawass (5 Mar):

ZH: I resigned because of three main things:

· During the earlier protests, Egyptian youths and the police protected the museums and monuments. Only the Egyptian Museum in Cairo was broken into and, thank God, all the important objects inside it were safe and only a few things were lost or broken. A report of exactly what is missing is still being compiled, however. Magazines were looted, but after initially appearing to get back to normal, the situation has recently become worse and there are many reports of thefts and illegal excavation. This is my most recent announcement: www.drhawass.com/blog/status-egyptian-antiquities-today-3-march-2011.
· Since the revolution, many people have continued to protest over other things, such as against me over jobs and salaries. Unfortunately, it is not possible to provide everything that everyone is asking for. In the Ministry of State for Antiquities, we need money to protect sites and to restore buildings and objects too. We need the money brought in by tourists who visit our sites and museums to fund these things and, at the moment, there are no tourists.
· Crooks in the Ministry and at the University of Cairo have started to attack me personally. I cannot stand this!

Most importantly, however, is that there are not enough police to protect the sites. I hope that my resignation will encourage the government to do something about this and also encourage the international community to put pressure on it do so as well.

JM: What does this mean for your position as head of the SCA - will that position still exist, and if so will you continue to hold it?

ZH: Some weeks ago, the SCA became the Ministry of State for Antiquities. Having been a minister, I cannot go back to being the head of the SCA. It is a lower position and in a sense no longer exists. I have to step down from everything.

JM: In the meantime, what efforts have you been making to protect Egypt's archaeological sites?

ZH: I cannot do anything to protect the sites now. I hope that my successor will continue the projects that I started though and achieve things like I did.

JM: What is your greatest fear for Egypt's antiquities?

ZH: I am most fearful for the Giza plateau, the love of my life, at which I have lived, excavated and looked after for most of my life and made some of my most important discoveries. I built a big wall around the pyramids to protect them from building work and was working on a site management plan that will help make it beautiful again and restore its magic. It will stop cars, camels and horses accessing the site and tourists will be able visit the monuments in electric trams: www.drhawass.com/blog/site-management-giza. However, the camel owners started to put up signs during the revolution saying that I was stealing antiquities, when they are the ones doing the damage there!

JM: What would you like to see the army doing at these sites?

ZH: The army cannot do anything. They cannot run after people; this is the duty of the police and that is why we are all waiting for them to come back and do their job as before.

JM: How easy is it to get clear information about any looting or illegal excavations that are going on?

ZH: I keep my website as up to date as I can with the information that I have. This is my most recent post: www.drhawass.com/blog/status-egyptian-antiquities-today-3-march-2011.

JM: Have you had offers of help from international organisations? If so, do you think these should be accepted, and whose decision would this be?

ZH: Other than offers to keep an eye out for stolen objects, I have not received any offers of help from international organizations and none at all since my resignation.

JM: If the situation improves, will you reconsider staying on in the cabinet?

ZH: I do not know.

JM: What plans do you have for your own future, if you do not stay in your current post?

ZH: I am still thinking about what I should do in the future.

Since making these comments, Hawass has posted further comments on his own blog, which you can read here and here.

Throughout the history of humanity, people have looked to the heavens and wondered what's out there. But what about the creatures looking back at us? What kind of weird and wonderful forms might they take?

Most efforts to search for life elsewhere in the universe focus on organisms with a very similar chemistry to our own - carbon-based, using water as a solvent and something similar to DNA or RNA to carry genetic instructions (chemists can't think of anything else that would do it as well). That approach makes sense - it's easier to look for what you know. We know that this kind of life is possible, we know roughly where to look (planets like Earth), and what kind of chemical signatures to watch out for.

But what else might be possible? Could you get gaseous life? Superconducting life? Quantum life? Life on neutron stars, brown dwarfs or in interstellar space? My interest in this was triggered back in 2003. I was a news editor at New Scientist, commissioning stories for the physical sciences beat, and I saw a bizarre-looking paper in a journal called Chaos, Solitons and Fractals. It was by two physicists in Romania, and barely written in English.

Mircea Sanduloviciu and Erzilia Lozneanu at Cuza University claimed to have created self-organising blobs of gaseous plasma that could grow, replicate and even communicate. Fascinated, I asked reporter David Cohen to write a news story about the work.

The researchers were simulating lightning strikes by sending sparks between two electrodes in a low-temperature plasma of argon (a plasma is a gas in which some of the atoms have been split into electrons and charged ions). These electric sparks caused the ions and electrons in the plasma to form spheres.

Each sphere had a boundary made up of two layers - an outer layer of negatively charged electrons and an inner layer of positively charged ions. Trapped inside the boundary was an inner nucleus of gas atoms. The spheres ranged in size from a few micrometres up to three centimetres across, depending on amount of energy in initial spark.

The spheres could replicate by dividing into two, and grew by taking up neutral argon atoms and splitting these into ions and electrons to replenish their boundary layers. They could even apparently communicate information by emitting electromagnetic energy, which made the atoms within other spheres vibrate at a particular frequency.

Sanduloviciu thinks such plasma blobs could have kick-started the origin of life by forming the first cells. They would have formed at high temperatures in electric storms in the early Earth's atmosphere, but he says they can persist at lower temperatures, the sort of environment in which normal biochemical interactions occur.

Perhaps life really did begin with a spark of electricity. But I don't think there's any need to invoke such a far-fetched idea - after all, fatty acid molecules spontaneously form spherical vesicles in water, with membranes very similar to those in biological cells. Harvard's Jack Szostak has shown that these vesicles can grow, divide, compete with each other, and even support replication of an added-in DNA template.

However the work does suggest that these plasma spheres form very easily. So could they be common in the universe, and given the right conditions, could they ever form the basis of a new kind of "plasma life"?

 I was reminded of all this on Monday, when I attended a Royal Society discussion meeting on the chemical origins of life. There were some fascinating talks on how the very first steps towards life might have happened - for example Donna Blackmond on solving the mystery of life's "handedness", John Sutherland on how the first RNA molecules could have formed, and Szostak on his protocells.

But one speaker, Martin Hanczyc of the University of Southern Denmark in Odense, was taking a very different approach to life. He is looking for life-like behaviours in drops of oil. I've written a news story about Hanczyc's work for Nature's website, and there are some nice videos of the oil drops here (moving autonomously) and here (responding to a pH gradient).

Hanczyc puts his oil droplets into a watery solution, and feeds them with a chemical "fuel" such as hydrogen cyanide (which would have been around on the early Earth). When the fuel reacts with water at the boundary of the drops it alters their surface tension, which causes them to move. As well as trundling about under their own steam, the droplets can sense and respond to chemical gradients. The droplets can also sense each other, a rudimentary form of chemical communication, and their past actions influence future ones - which you could interpret as a kind of memory. Watery compartments within the oil drops could start to create more complex structures. Hanczyc and his colleagues are now working on getting the droplets to divide and replicate.

While other researchers at the meeting weren't convinced that this has anything to do with how life actually started on Earth, Hanczyc says he wants his work to serve as a reminder about the wide variety of forms that life might be able to take. The behaviours seen in his oil droplets happen extremely easily, he says, just by "throwing things into a pot". And, he points out, many biological reactions happen more easily in oil than they do in water.

Oil-based life forms might exist now on Earth, he says, in parallel with our own water-based life. Or it could have arisen elsewhere in the universe, for example on Titan, where hydrogen cyanide is abundant.

Of course, there's a big downside to both the plasma blobs and the oil drops. The range of behaviours they display is impressive, but critics argue that they both lack any kind of genetic material, and it's hard to see how that could get added.

I can't help thinking, though, that we'd probably say that about water-based biological cells too if we didn't know otherwise. And Hanczyc argues that in any case, demanding that information be encoded within dedicated genetic molecules is a narrow-minded way of looking at life.

Characteristics and structures within the oil drops can be passed on from generation to generation, he says (I think he means things like chemical composition or presence of sub-compartments). In other words, heritable information is there, but it is embedded in the chemistry of the drops themselves, rather than encoded within a genetic molecule. Hanczyc admits that such characteristics would be dependent on the environment and easily lost if conditions changed, but says we should be open to the possibility of a form of life that is intimately entwined with its environment in this way.

I'm not sure... I wonder how complex could life really get if it wasn't able to store instructions somewhere safe like DNA. But what I love about this kind of work is that it forces you to think about the question.

[See related post: Searching for shadow life]

Did the Vikings have a magical sunstone that glowed to show the location of the Sun behind the clouds? Viking mythology includes the following intriguing tale:

"The weather was very cloudy, it was snowing. Holy Olaf, the king sent out somebody to look around, but there was no clear point in the sky. Then he asked Sigurd, to tell him, where the Sun was. After Sigurd complied, he grabbed a sunstone, looked at the sky and saw from where the light came, from which he guessed the position of the invisible Sun. It turned out, that Sigurd was right."

The story mystified historians until Danish archaeologist Thorkild Ramskou suggested in 1967 that no magic was required - the sunstone could have been a polarising crystal. With such stones, Viking navigators could have used polarised light to aid their long sailing trips across the north Atlantic, almost a millennium before polarisation was first officially described in the 17th century.

It's great theory, and despite the lack of direct evidence, it has become popular since Ramskou suggested it. In recent years, however, scientists have been investigating the plausibility of the idea - whether such sunstones would actually have worked. They review their efforts in a paper published today in Philosophical Transactions of the Royal Society B, which I've written about for Nature News.

Viking seafarers routinely sailed thousands of kilometres across the North Atlantic, for example from Norway to Greenland, between around 700 and 1050 AD. How they managed it has been a bit of a mystery - during the summer sailing season there would have been almost perpetual daylight, so stars would have been of limited use.

Ramskou figured that the Vikings must have watched the Sun instead. Part of a wooden disc with curved lines on it has been found dating from Viking times. Ramskou and others have interpreted this as a type of sundial called a Bearing dial - by lining up the Sun's shadow with the curved marks on the dial, the navigators could have ensured they were heading in the right direction.

This method wouldn't have worked in cloudy or foggy weather, which could easily have lasted for days. But if the Vikings had sunstones to tell them the location of the Sun, they could then use a flame or torch to light the sundial from that direction and check their course.

So how would the stones work? Polarising crystals such as cordierite, or a type of calcite known as Iceland spar, transmit light differently depending on the direction in which it is polarised (ie the plane in which the peaks and troughs of the light wave are oriented). Some appear bright or dark depending on their orientation with respect to the polarisation of the light hitting them, others change colour, for example from blue to yellow.

Sunlight becomes polarised as it bounces off air molecules in the atmosphere, with the line of polarisation tangential to circles centred on the Sun. In theory, then, by holding a polarising crystal up to the sky and rotating it, you can work out the direction of the Sun, even without being able to see it directly.

But is the pattern of polarisation strong enough to allow use of a sunstone, and in what weather conditions? Gábor Horváth of the environmental optics lab at Eötvös University in Budapest, Hungary, and colleagues from Spain, Germany, Finland and Switzerland have carried out a range of studies looking at this, published over the last few years.

In particular, Horvath and his colleague Susanne Åkesson, a migration ecologist from Lund University, Sweden, travelled to the North Pole on the Swedish icebreaker Oden, and measured the polarisation pattern of the sky's light in a range of weather conditions. The equipment to do this has only recently become available, and the maps they produced are also of interest for studying how some birds and animals navigate using polarised light.

The researchers found that contrary to what many had assumed, the pattern of polarisation of the sky's light is very similar in foggy or cloudy conditions to the pattern you get with clear skies. So the information regarding the Sun's position is still there. The polarisation isn't as strong, however, leaving it an open question as to whether sunstones would be sensitive enough to detect it. In their recent paper, Horvath and his colleagues conclude that in a totally overcast sky the oscillation in the intensity of the light through a polarising crystal would be hardly visible.

But when I spoke to Akesson she was much more positive, saying that she has tried using such crystals herself in these conditions, and that the variation in brightness as she turned the stone was easy to see. The researchers' next step is to carry out quantitative studies with the crystals to check how accurate they are, and determine whether it would have been possible to locate the Sun well enough to be useful for navigation.

Of course all this can only say whether or not the method would have worked, not that the Vikings definitely used it. That would require archaeological evidence - for example finding such a crystal on a shipwreck or in some other maritime context. Archaeologists such as Christian Keller of the University of Oslo also point out that even the identity of the wooden sundial is in question. It was found in a nunnery so there's no reason to think it had anything to do with sailors or navigation, he says, unless you want to tell stories about what sailors were doing in a nunnery...

What we do know from written records is that Viking sailors judged their route across open sea by combining lots of different pieces of information, including the position of the Sun (on clear days), wind direction, appearance of clouds over distant land, bird flight paths and whale sightings. Which I reckon is just as impressive as using sunstones.