One interesting trend to watch in archaeology is the use of computers to simulate how the past would have looked. I've blogged before about a reconstruction that allows you to take a virtual walk through ancient Rome, and a project to digitally simulate authentic lighting conditions for ancient scenes and buildings.

Now similar technology is being applied to Stonehenge - not so much the famous standing stones themselves, but other monuments and burial mounds in the area. Researchers have long speculated over whether burial monuments from the Early Bronze Age were positioned according to particular geometric patterns, perhaps in a way that reflected the cosmology or belief system of the time. For example, Ann Woodward of the University of Birmingham's Institute of Archaeology and Antiquity has suggested that monuments around Stonehenge seem to be arranged in two roughly concentric circles centred on the standing stones, one with a radius of 0.6-1.2 kilometres, and one with a radius of 2-2.4 kilometres. Such ideas are quite tricky to test, however, especially when you have remains of a variety of different ages and types, positioned on a rolling three-dimensional landscape. So Woodward's colleague Vince Gaffney is leading a 3-year effort to map buried archaeological remains in the terrain around Stonehenge with the latest geophysical imaging techniques.

The Stonehenge Hidden Landscapes Project, which starts this week, will involve taking millions of measurements of the position of remains in the ground. These will be analysed using computer software originally developed for building video games, to produce 2D and 3D images of how the landscape would have looked in Neolithic times. Then researchers can use the reconstructions to test their ideas about how different monuments and burial mounds were arranged.

I've just had a first-hand - and rather disconcerting - insight into how conspiracy theories start. A week or so ago I wrote a light-hearted blog post for New Scientist, looking into a theory that King Tutankhamen might have had a hormonal disorder that among other things causes underdeveloped genitalia.

Researchers analysing Tut's mummy had just ruled out the idea, citing the fact that Tut's penis is "well-developed". Intriguingly, however, their paper said that the penis is no longer attached to the rest of the mummy. On a whim I decided to double check that the organ is definitely Tut's and couldn't have been swapped by ancient embalmers to cover up his condition. Unlikely, perhaps, but surely a question that was begging to be asked.

Egyptologist John Taylor confirmed that the penis is indeed Tut's, as it was attached to the body when the mummy was first unwrapped. It must have broken off in modern times, probably during an early autopsy. So far so interesting - but it was clear that nothing untoward had occurred. I added a couple of tongue-in-cheek digs at conspiracy theorists, and it made a fun little tale for a Friday afternoon.

Unfortunately, some journalists seem to have taken those digs seriously. The story went viral. But it wasn't my story that went viral. In the version rapidly gaining momentum across the internet, I was now reported as claiming that Tut's penis has been stolen in order to cover up its small size.

My original article ruled out the idea that ancient embalmers swapped Tut's penis. And though I mentioned in passing the well-known fact that it was thought missing for a while, I did not suggest that the penis has been switched more recently either - I'm not aware of any evidence for this idea.

The trouble seems to have started with an article on Time's website, written by an intern, who reported that "some, including Marchant...believe that Tut's penis was swapped sometime after his body was embalmed, suggesting a conspiracy to save him from embarrassment". (The words "including Marchant" have since been removed, at my request.)

Other blogs and news sites followed, twisting the story further. According to the New York Daily News, "scientists" think the penis was taken. Meanwhile a Denver blog reports that the penis is still missing.

This reminds me a bit of a news story I once edited for New Scientist, about a physicist who said he didn't think the elusive Higgs boson (nicknamed "the God particle") would ever be found. The news story and accompanying editorial reported that other physicists were confident the particle would be found, explained why the £6-billion Large Hadron Collider (LHC), then being built to look for the Higgs, was still important, and pointed out that it would actually be more exciting for physics if the Higgs particle didn't exist.

Then The Times newspaper followed up the story under the headline "God particle disappears down £6-billion drain", and New Scientist received sackfuls of complaints from physicists around the world. It was the Times headline that really upset them, but they blamed New Scientist. No matter how responsible our own story, they said, we were irresponsible to provide ammunition that might allow others to attack the LHC.

With the Tut blog post, was I irresponsible for writing something that others could then twist into a conspiracy theory? With hindsight I wish I'd spent more time making sure the article was idiot-proof, though I'd hate to think that journalists can't have fun with stories, for fear of how they might be misconstrued. Either way, I haven't had any angry letters from Egyptologists yet (though one blogger has accused me of "living in the realm of fantasy"). But I've got some sacks ready just in case...

Here's the update I promised in my post on the Dendera Zodiac. If you remember, the zodiac is an ancient Egyptian bas-relief carved onto huge sandstone blocks. It originally formed part of the ceiling of a temple in Dendera, Egypt, but is now on display in the Louvre in Paris.

The design is a representation of the sky. Egyptologist Sylvie Cauville and astrophysicist Eric Aubourg used the positions of the star constellations and planets to date the Zodiac to between 15 June and 15 August in the year 50 BC, during the period between the death of Cleopatra's father, Ptolemy Auletes, in 51 BC, and the establishment of joint rule between Cleopatra and Caesarion (her 5-year-old son by Julius Caesar), in 42 BC.

Two eclipses - the solar eclipse of 7 March 51 and the lunar eclipse of 25 September 52 - are represented on the skyscape in the locations where they would have occurred.

But why would the Egyptians have wanted to commemorate this particular moment? I emailed Cauville and she says her hypothesis is that the total solar eclipse coincided in Alexandria with the death of Cleopatra's father. "She [Cleopatra] may have wanted to inscribe for eternity the passing of power from King-Rê to herself, the female sun."

I'm so glad I asked! Rê is another name for the sun god, Ra, by the way. The pharaohs, including those of Cleopatra's dynasty, often claimed that they were sons and daughters of Ra.

Dendera would have been chosen because the temple there was dedicated to female royalty. (The temple at Edfu, where two similar zodiacs are located, is dedicated to the male royalty.) The Dendera zodiac was on the ceiling of one of the temple's two chapels dedicated to Osiris, the god of eternal return. There's more about all this in Cauville's 1997 book, Le Zodiaque d'Osiris.

"It's a shame that so many fanciful things have been written about the Zodiac," adds Cauville. "The astronomical reality is so much more beautiful." Couldn't agree more.

[Cauville's comments have been translated from French]

I've had fun today looking into the latest arguments over the mummy of Tutankhamun. The pharaoh died young, aged around 19, so there's been plenty of speculation over what killed him since his mummy was first unwrapped in 1922.

Murder - by a blow to the back of the head or by poison - has been suggested. And signs of broken leg led to the idea that he fell from his chariot during a hunting accident.

Egypt's chief archaeologist Zahi Hawass and colleagues recently carried out an extensive scientific investigation of Tut's mummy as well as several others from his family (all captured on film of course for an accompanying TV documentary). They reported in JAMA earlier this year that he probably died from a debilitating bone disorder, combined with an attack of malaria.

But a letter just published in JAMA suggests that sickle cell disease is a more likely explanation. You can read more about this in this news story I've written for New Scientist today.

This letter was just one of several comments published this week on Hawass's work. One of them urges caution over the DNA results from the mummies, suggesting that contamination could have crept in. Another challenges the identification of mummy KV55 as Akhenaten, Tut's father.

But the letter that I was most intrigued by suggests that Tutankhamun and family, including Akhenaten, may have suffered from a hormonal disorder that given them elongated skulls, and caused the men to develop breasts. This would explain why artwork of the time depicts Tutankhamun, Akhenaten and others with long heads and feminine figures (just type "Akhenaten" into a Google image search and you'll see what I mean).

A key piece of evidence in this debate is King Tut's penis, which it turns out has been broken away from his body. It's also impossible to check whether Tut had breasts, because the front of his chest is missing. Is this a cover-up? You can read more on what I found out in a blog post here...

Embedded in the ceiling of a small room in the Egyptian antiquities section of the Louvre, Paris, are two huge sandstone blocks. Cut into them is a dramatic circular pattern of figures and hieroglyphs - the Dendera zodiac.

I've reviewed a book about this fascinating bas-relief in today's New Scientist. The Zodiac of Paris, by historian Jed Buchwald and writing professor Diane Greco Josefowicz, discusses the stormy debate triggered when the zodiac was first discovered in 1790 by French scientists attached to Napoleon's campaign in Egypt.

The zodiac's original home was the ceiling of an ancient temple near the village of Dendera, on the west bank of the Nile. It was taken to France in 1821 by engineer Jean Lelorrain - the book has a vivid description of his epic if politically incorrect expedition. He used gunpowder to blast the blocks from their original home, before using a sledge on rollers to drag them to the Nile (it took 16 days to cover the 4 miles). The river was at its lowest ebb so he also had to build a 60-foot earth ramp to slide the stones down to the water's edge, not to mention the fact that the boat he was trying to load them onto nearly sank under the weight.

The book focuses, however, on efforts of scientists and intellectuals of the time to date the zodiac. Scientists believed that the zodiac, along with three others found - one more at Dendera and two at Esneh - was intended to represent the state of the sky when it was produced. By deciphering the symbols, therefore, they could match up the zodiac to a particular date and work out how old the temple was. Each scholar seemed to have his own method of interpreting the symbols but most believed they were thousands of years old, perhaps dating to as far back as 15,000 BC.

All this went down extremely badly with religious conservatives, as according to the Bible the world itself was only six thousand years old, and humanity had been all but destroyed in a catastrophic flood in around 2000 BC. As the book's blurb says, the resulting row wasn't just about the zodiac, but "the merits of scientific calculation as a source of knowledge about the past". With today's arguments over intelligent design, it's a useful reminder that clashes between scientists and religious fundamentalists over the validity of different sources of knowledge are nothing new.

The book ends with the young Egyptologist Jean-Francois Champollion, best known for deciphering hieroglypics, who concluded in the early 19th century that the astronomical symbols in the zodiac formed part of a text, not an image of a sky, and dated it (much less controversially) to Roman times.

What really interests me about the zodiac, however, is what we know now about how old it is, and what the design represents - something that Buchwald and Josefowicz don't address. So I dug around a little bit and the latest interpretation is that it dates from around 50 BC. The key is an empty cartouche on a part of the temple's ceiling that was left in Egypt. A cartouche is a hieroglyph that looks like a little box and usually it would contain the name of the current ruler. Because it's empty, historians think the temple was built during the gap between death of Cleopatra's father, Ptolemy Auletes, in 51 BC, and the establishment of joint rule between Cleopatra and Caesarion (her 5-year-old son by Julius Caesar), in 42 BC.

Sylvie Cauville of the Centre for Computer-aided Egyptological Research (CCER) based at Utrecht University is the latest scholar to tackle the zodiac itself. She believes that does represent a map of the sky after all.

According to the summary on the Louvre website: "The vault of heaven is represented by a disc, held up by four women assisted by falcon-headed spirits. Thirty-six spirits or "decans" around the circumference symbolise the 360 days of the Egyptian year. The constellations shown inside the circle include the signs of the zodiac, most of which are represented almost as they are today. Aries, Taurus, Scorpion and Capricorn, for example, are easily recognisable, whereas others correspond to a more Egyptian iconography: Aquarius is represented as Hapy, the god of the Nile flood, pouring water from two vases."

Cauville has identified the symbols for the five planets known at the time, and says they are located on the map within certain signs of the zodiac. For example Venus is behind Aquarius, Jupiter is near Cancer, Mars is directly above Capricorn. This particular configuration of the planets among the constellations only happens about once every thousand years, and it occurred between 15 June and 15 August in the year 50 BC - matching perfectly the date suggested by the empty cartouche.

One lovely detail is that the map shows two eclipses, in the part of the sky where they would have been seen. Again from the Louvre website: "The solar eclipse of 7 March 51 is depicted as the goddess Isis holding a baboon (the god Thoth) by its tail, signifying her attempt to stop the moon from hiding the sun. The lunar eclipse of 25 September 52 is represented by an udjat-eye (the "whole one") because a lunar eclipse only occurs when the moon is full."

I'm still intrigued by why the Egyptians would have wanted to represent the sky at this particular point in time. The chapel that originally held the the bas-relief was intended to celebrate the resurrection of the god Osiris, so perhaps that's a clue. If I find out I'll post again...

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.