Scholars have been trying to pin down the dates that ancient Egypt's rulers reigned for more than a century. Lists of kings dating from the time of the Pharaohs, and a history of Egypt written in the third century BC by Egyptian historian and priest Manetho have given them a pretty good idea of the order of events in Egypt's Old, Middle and New Kingdoms, and how long each ruler was in power for. But fixing this relative history to any definite dates has been fraught with controversy.

Not any more though. A small team of scientists using radiocarbon dating on 211 plant-based samples have "independently corroborated a century of scholarship in just three years", according to lead researcher Christopher Bronk Ramsey (also known for studying the Shroud of Turin). 

Previously the main source of evidence for matching the "floating chronology" of ancient Egypt's rulers with absolute dates was a few ancient astronomical observations from the Middle and New Kingdoms. But this left plenty of room for argument. Many of the celestial and lunar phenomena in question repeat at regular intervals, so it's often hard to be sure exactly which event an ancient writer is recording. To make matters worse, the exact timing of astronomical observations also depends on the location of the observer.

Bronk Ramsey and his colleagues dated samples from seeds, baskets, textiles, plant stems and fruits, held in various museums. (They avoided any mummified remains as these could be contaminated with material used in the mummification process, such as bitumen, which would mess up the dating.) In general their results have confirmed historians' consensus about what happened when, but in a couple of cases the analysis showed that events happened earlier than thought - for example Djoser, of the Old Kingdom, reached power between 2691 and 2625 BC, and the New Kingdom began between 1570 and 1544 BC. The results suggest that New Kingdom pharoah Rameses II, considered the greatest of the Egyptian kings, ruled between 1297 and 1273 BC, and King Tutankhamun held the throne between 1353 and 1331 BC. 

Radiocarbon dating has long been possible, but generally only gives age measurements with an accuracy of 100-200 years, not too helpful for pinning down the exact timing of someone's reign. To narrow the range of error, Bronk Ramsey and his colleagues combined the results from their samples with information about radiocarbon activity in the Nile Valley region and from the historical chronology.

The complex statistical analysis that the researchers had to use may still leave historians something to argue about. But the paper, published in Science this week, is an important step forward for Egyptology as it will put the whole field on a more scientific footing. The analysis will also help historians to correlate what was going on in Egypt with events in surrounding areas where we already have radiocarbon dating information, for example Libya and Sudan.

From the fearsome Tyrannosaurus rex to delicate ants trapped in amber, fossils provide us with valuable information about what life on Earth was like millions of years ago. But what did ancient people make of them? Often, fossilised creatures such as dinosaurs were seen as proof of the existence of mythical monsters - the griffin may derive from fossilised remains of Protoceratops, for example.

A blog post on the Heritage-Key website this week reveals that the Mayans in central America used fossils to derive accurate information about Earth's prehistoric past - in particular, the realisation that the land where they lived was once underwater. Archaeologist Martha Cuevas and geologist Jesus Alvarado carried out a 3-year study of 31 fossils found at the ancient Mayan site of Palenque, in southern Mexico. These included shark teeth, stingray spines and a variety of marine animals, dating as far back as 63 million years. The Mayans saw these fossils as highly significant, for example using them as funerary gifts, tombstones or offerings to the gods. They also painted vessels with representations of the fossils. 

The researchers explain that these marine fossils were a key source for the Mayans' creation myths. The Mayans believed that their land was once covered by sea. Humanity was created when the gods ordered the water to retire, and their city emerged. When a person died, they believed that the person's spirit would return to an aquatic underworld - hence the importance of the fossils at funeral.

The ancient Greeks invented the steam engine (a small demo version anyway), and when they came up with the Antikythera mechanism, they weren't that far from building a mechanical clock. So one of the questions I'm often asked is: why didn't the Greeks make better use of their inventions? Why did we have to wait so many centuries for the Industrial Revolution?

The late science fiction author Arthur C. Clarke blamed the Romans for destroying Greek culture - if the Greeks had been allowed to build on their achievements, he said, they could have reached the moon by 400 AD, and by now, we'd presumably be exploring other stars.

Inspiring as this idea is, the answer is clearly more complicated than that. It takes more than an invention to change the world - all sorts of social and political factors have to be in place for the revolution to happen.

Robin McKie's review of The Most Powerful Idea in the World: A Story of Steam, Industry and Invention pinpoints this nicely. The book is by William Rosen and the powerful idea in question is James Watt's invention of the separate condenser for the steam engine, which he came up with while strolling through Glasgow Green in 1765.

The steam engine that Watt was working on at the time (see pic) was a clunky, inefficient beast. A build-up of steam forced a piston through a cylinder. Spraying cooling water into the cylinder then caused the steam to condense, sucking it back to its original position. Repeating this cycle meant that the whole apparatus had to be heated and cooled over and over, which wasted huge amounts of energy. On his fateful walk, Watt realised that having a separate condenser would still create a vacuum, but allow the engine itself to be kept at a constant temperature.

Rosen describes this as "one of the best recorded, and most repeated, eureka moments since Archimedes leaped out of his bathtub". Cue the Industrial Revolution. "Within a few decades," says McKie, "webs of railways, factories and mines were spreading across the nation."

Yet we know that other inventors through history were tinkering with ideas not so far removed from Watt's. The engineer Hero of Alexandria, for example, demonstrated a spinning steam-powered contraption called an "aeolipile" in the first century AD.

Rosen's point is that transforming industry took far more than Watt's brilliant idea. Watt's breakthrough would likely have come to nothing if the ground hadn't been prepared by other technological advances, such as the ability to make parts of industrial machinery with precision, and developing a better understanding of the basic science behind steam engines. Just as crucial were intellectual and legal changes - such as new patent laws - that "rewarded both the inventor and society for making and accepting change".

Makes you wonder what other world-changing inventions are out there, just waiting for society to become ready for them. I've a feeling that Clarke, if he was still with us, would have some ideas...

We tend to think of obesity as being a particularly modern disease, but a new "biography" of obesity by cultural historian Sander L. Gilman points out that people have been trying to lose weight ever since ancient times.

So if you're fed up of modern diets, why not try a regime that's thousands of years old?...

The Greek physician Hippocrates (c. 440-370 BC) believed that a person's health depended on having the correct balance of humors in the body - these were blood, yellow bile, black bile and phlegm. Someone who had too much phlegm was likely to be fat (as well as being pale, lazy and cool in character).

So Hippocrates recommended hot, dry foods to help a person to lose weight. He also instructed dieters to eat after "exercise and while still panting from fatigue and with no other refreshment before meals except wine, diluted and slightly cold". For those who wanted to take stricter measures, he suggested that they eat only once a day, take no baths, sleep on a hard bed and "walk naked as long as possible".

The Romans seem to have had a similar hard-work attitude to losing weight - the encyclopaedist Aulus Cornelius Celsus (c. 25 BC - c. 50 AD) recommended tepid saltwater baths, hard exercise, austere food and restricted sleep. But by the 11th century, in the Islamic world, the concept of quick-fix diets had taken hold: the Arabic physician Ibn Sina (980-1037 AD), also known as Avicenna, prescribed an appetite suppressant made of almonds, beef suet, marsh-mallow root and oil of violets, to be taken for 10 days. Can't be any worse than cabbage soup!

Gilman's book is called Obesity: The biography. You can read my review of it in this week's New Scientist.

Archaeologists excavating Egypt's Avenue of Sphinxes announced this week that they have uncovered the remains of a nilometer, used to monitor the water level of the Nile.

The Avenue of Sphinxes, built by the 30th Dynasty king Nectanebo I (380-362 BC), was one of the most important processional routes in ancient Egypt. At 2700 metres long and 76 metres wide, it ran between the Luxor and Karnak temples, and was lined with statues in the shape of sphinxes.

Strange place for a depth gauge? Not really. The Egyptians were dependent on water and silt from the Nile's annual flood for their agriculture, and monitoring the river's level told them whether to expect famine, bountiful crops, or devastating floods in the months ahead. They used various devices to do this for more than 5000 years (until the Aswan Dam rendered the practice obsolete in the 20th century).

The first nilometers were simple marks on the riverbank, or on a column or post placed in the river. Later the Egyptians used steps leading down into the water, with depth markings alongside (see video). Then they got more sophisticated, with a channel leading away from the river into a dedicated well or cistern. Any knowledge about what the Nile was going to do was incredibly valuable, so these wells were often located inside temples, accessible only to priests or rulers.

The nilometer found at the Avenue of Sphinxes seems to be of this last type - it is a 7-metre-diameter cylindrical well dug into sandstone, encircled by a spiral staircase (see image). Clay vessels found at the bottom have helped the archaeologists to date it to the New Kingdom (1569-1081 BC).

The team has also found a 1600-year-old Christian church. It was built with limestone blocks recycled from ancient temples that stood along the route - the blocks are still decorated with scenes of Ptolemaic and Roman kings offering sacrifices to ancient Egyptian gods.

Ancient monuments from Stonehenge to Egypt's great pyramids are carefully aligned to astronomical events, telling us that from the very beginnings of civilisation, people have been fascinated by the heavens above us. Throughout history this has been a driving force for technology, such as the intricate cogwheels and pointers of the Antikythera mechanism, the astrolabe, equatorium, and telescope, not to mention mechanical clocks, which started off as huge astronomical displays.

And it is still going on today - some of the biggest, most sophisticated (and expensive) machines in the world are being built in order to make sense of the cosmos. Whereas the ancient Greeks, say, wanted to know things like how the solar system was arranged, when the next eclipse would be, or the distance from the Earth to the Sun, today we have different questions. Why is the universe expanding at an ever faster rate? What is the nature of the ‘dark' matter that makes up almost a quarter of the universe? Why does the universe appear fine-tuned for life? To probe such huge questions, physicists have had to build their machines literally at the ends of the Earth - from the coldest deserts and highest mountains to the deepest mines.

I'm reading a book at the moment called The Edge of Physics, published last month, in which science journalist Anil Ananthswamy (disclaimer: he's a friend of mine) visits today's cutting-edge physics experiments and tells their stories, I guess it's a cross between a science book and a travelogue. Scientists are so often portrayed as weedy white-coated geeks in labs, but the ones that Anil meets are more likely to be tough adventurers, willing to endure all kinds of physical hardship for their science. My favourite chapter so far is when Anil visits the vast frozen expanse that is Lake Baikal in Siberia. Hidden beneath more than a kilometre of ice is a neutrino telescope. The scientists who tend the telescope live for months in the blasting cold, sleeping in tiny wooden cabins perched on the creaking ice.

What I love about the book is that it portrays a real sense of wonder, a sense of our tiny place in an awesome universe. I think that would have been very much in tune with how ancient civilisations saw things too.

I find it amazing to look at artefacts that survive from the ancient world - just to think about all the centuries of history that they have existed through. But a lovely piece in the today's Observer goes one better, pointing out that plenty of organisms that were alive thousands of years ago are still going strong today. They range from the 2000-year-old welwitschia plant, a conifer that produces just two leaves in its entire lifetime, to actinobacteria that have been frozen in Siberian permafrost for a mind-boggling half a million years.

Photographer Rachel Sussman aims to photograph as many of these organisms as possible. To qualify, they have to be at least 2000 years old, meaning they would all have been around in the time of the ancient Greeks. She tells the Observer that the project has a two-fold message - "a humble, existential aspect in which the entirety of human history feels dwarfed by the longevity of life around us" - and an environmental caution. "We have these organisms that have quietly persevered for an unfathomable amount of time but which are now in jeopardy," she says. "The Siberian actinobacteria are half a million years old and live in the permafrost. If the permafrost isn't permanent, the oldest living things on the planet will die."

The picture above is of a brain coral, which can live for up to thousands of years. It isn't one of Sussman's, but you can see some of her photos on her blog.

It's nice to see that the Antikythera mechanism is finally getting the recognition it deserves. This 2000-year-old collection of gearwheels and pointers is one of the most impressive artefacts that survives from the ancient world, yet historians of science and technology have consistently failed to take it seriously (claims that it was made by aliens haven't helped...)

However I've just been watching BBC2's breathless yet beautiful new series, The Story of Science. The first episode focuses on the history of astronomy and it features the mechanism prominently as a demonstration of how ancient Greeks viewed the cosmos. There's an interview with Michael Wright, who has reconstructed the device, along with some nice footage of his model (rather more professionally shot than my own attempt!) If you missed tonight's programme you can check it out on iplayer here.

This may be the first of a spate of TV appearances for the mechanism. As well as a film being made by Tony Freeth of the Antikythera Mechanism Research Group, I hear that National Geographic is planning a documentary about the device.

In the December issue of Scientific American there's a lengthy feature on the Antikythera mechanism by film-maker Tony Freeth, driving force behind the Antikythera Mechanism Research Project. He gives some background to this 2000-year-old artefact and its significance as well as talking through his team's research on it and their results.

It's fitting that Scientific American has returned to the mechanism, as the magazine published one of the most important early articles on this ancient Greek device - written by science historian Derek de Solla Price, 50 years ago in 1959 (vol 200, no 6, pp 60-67). In the article, he laid out his theory that the battered pieces once formed part of a surprisingly sophisticated "calendar computer", calculating the movements of the Sun and Moon. He was right of course, but he didn't know the half of what it could do. It would be many years before researchers realised that the device was calculating eclipses too, and probably the epicyclic motions of the planets. Price did fully grasp its importance, however. "Nothing like this instrument is preserved elsewhere," he wrote. "On the contrary, for all we know of science and technology in the Hellenistic Age we should have felt that such a device could not exist."

While on the subject of Price's early work, I also love this quote from an article he wrote in the British science magazine Discovery, around the same time: "If it is genuine, the Antikythera machine must entail a complete re-estimation of ancient Greek technology. Its discovery 55 years ago... was as spectacular as if the opening of Tutankhamen's tomb had revealed the decayed but recognisable parts of an internal combustion engine."

Disappointingly, Freeth's latest article offers nothing new for those who have been following the subject. But it's a good opportunity to read the Antikythera story as told by one of the main protagonists. You need a subscription to access it on the Scientific American website but you can read it free here.

In case you didn't look yet at my article on drowned cities, New Scientist has added pictures now, so it looks a lot better! You can read it here.