Iceland spar, or how Vikings used sunstones to navigate

Cross post from the EGU Blog.

Nowadays, we can rely on GPS receivers or magnetic compasses to tell us how to reach our destination. Some 1000 years ago, Vikings had none of these advanced navigation tools. Yet, they successfully sailed from Scandinavia to America in near-polar regions where it can be hard to use the Sun and the stars as a compass. Clouds or fog and the long twilights characteristic of polar summers complicate direct observations of these celestial bodies. So how did they find their bearings? A new study published in Proceeding of Royal Society A shows that they probably used Iceland spar, a “sunstone”.

Centuries-old Viking legends tell of glowing sunstones that navigators used to find the position of the Sun and set the ship’s course even on cloudy days. In 1967, a Danish archaeologist named Thorkild Ramskou speculated that the Viking sunstone could have been Iceland spar, a clear variety of calcite common in Iceland and parts of Scandinavia.

This crystal has an interesting property called birefringence: a light ray falling on calcite will be divided in two, forming a double image on its far side. (This double image is easily seen by placing transparent calcite on printed text.) Further, the Iceland spar is a polarising crystal, meaning the two images will have different brightnesses depending on the polarisation of light.

<img src=“; alt="" title=“800px-3310.calcite_(Iceland_Spar)_birefringence” width=“300” height=“199” class=“mt-image-center” style=“text-align: center; display: block; margin: 0 auto 20px; /><div style=” text-align:="" center;“=”" />Birefringence of Iceland Spar seen by placing it upon a paper with written text. Source: Wikimedia Commons.

Light is made up of electromagnetic waves with component electric and magnetic fields. If these components have a specific orientation, the light is said to be polarised, while in unpolarised light the orientation of these fields has no preferred direction. Calcite can appear dark or light depending on the polarisation of light that falls upon it.

Sunlight becomes polarised as it crosses the Earth’s atmosphere, and the sky forms a pattern of rings of polarised light centred on the Sun. Changing the orientation of calcite as light passes through it will change the relative brightness of the projections of the split beams, even when the Sun is hiding behind clouds or just below the horizon. The beams are equally bright when the crystal is aligned to the Sun.

It can be hard to determine when exactly these split beams have equal brightness. But the new study, led by Guy Ropars at the University of Rennes 1 in France, suggests Vikings could have built a simple device to better use the sunstone.

The technique consists in covering the Iceland spar with an opaque screen with a small hole in its centre and a pointer. As light passes through the hole onto the crystal, a dark surface below it receives the projection of the double image for comparison.

<img src=“; alt="" title=“device” width=“300” height=“220” class=“mt-image-center” style=“text-align: center; display: block; margin: 0 auto 20px; /><div style=” text-align:="" center;“=”" /> The authors of the Proceedings of the Royal Society study believe Vikings could have used a device like this to navigate. The crystal is inside, and the projection of a double image is seen below it. Credit: Guy Ropars. Source: ScienceNOW.

By rotating the apparatus and determining the direction at which the two images were equal in brightness, the team managed to pinpoint the Sun’s position on a cloudy day with an accuracy of one degree on either side. Researchers also conducted tests when the Sun was largely below the horizon. “We have verified that the human eye can reliably guess clearly the Sun direction in dark twilights, even until the stars become observable,” Ropars’ team writes in the paper.

Although archaeologists have not yet found Iceland spar among Viking shipwrecks, the new study adds credence to the idea that Viking seafarers used the crystal in their travels.

Further, the recent finding of a calcite crystal on a sixteenth century Elizabethan ship shows that navigators could have used Iceland spar even after the appearance of the magnetic compass. Cannons on ships could perturb a magnetic compass orientation by 90 degrees, so a crystal serving as an optical compass could be crucial in avoiding navigational errors and get sailors to a safe port.

EGU Geosciences Communications Fellowship

Maybe some of the readers of this blog know of journalists interested in applying for a generous fellowship from the European Geosciences Union?

The European Geosciences Union (EGU) is offering fellowships for journalists to report on ongoing research in the geosciences. Successful applicants will receive up to €5k to cover expenses related to their projects, including following scientists on location.

Keep reading on the EGU website.

Portrait of an Imperfect but Beautiful Spiral

Some of the stuff I wrote while working for the European Southern Observatory (ESO) is only now being released. The text of today’s ESO Picture of the Week is one of my favorites because it goes with a gorgeous image of a cute galaxy. Enjoy!


Credit: ESO/Oleg Maliy

Not all spiral galaxies have to be picture-perfect to be striking. Messier 96, also known as NGC 3368, is a case in point: its core is displaced from the centre, its gas and dust are distributed asymmetrically and its spiral arms are ill-defined. But this portrait, taken with the FORS1 instrument on ESO’s Very Large Telescope, shows that imperfection is beauty in Messier 96. The galaxy’s core is compact but glowing, and the dark dust lanes around it move in a delicate swirl towards the nucleus. And the spiral arms, patchy rings of young blue stars, are like necklaces of blue pearls.

Messier 96 lies in the constellation of Leo (The Lion). It is the largest galaxy in the Leo I group of galaxies; including its outermost spiral arms, it spans some 100 000 light-years in diameter — about the size of our Milky Way. Its graceful imperfections likely result from the gravitational pull of other members in the group, or are perhaps due to past galactic encounters.

A multitude of background galaxies peers through the dusty spiral. Perhaps the most striking of these objects is an edge-on galaxy that — because of a chance alignment — appears to interrupt the outermost spiral arm to the upper left of Messier 96’s core.

This image was processed by ESO using the observational data found by Oleg Maliy from Ukraine, who participated in ESO’s Hidden Treasures 2010 astrophotography competition 1, organised in October-November 2010, for everyone who enjoys making beautiful images of the night sky using astronomical data obtained with professional telescopes. The image was made with data taken at visible and infrared wavelengths through B, V, and I filters.

1 ESO’s Hidden Treasures 2010 competition gave amateur astronomers the opportunity to search through ESO’s vast archives of astronomical data, hoping to find a well-hidden gem that needed polishing by the entrants. To find out more about Hidden Treasures, visit

The meteorite that hit the Comette house

In my new job, I often find myself wondering about what is and what isn’t newsworthy. Is the story I have to tell worthy of a press release? What is it about it that can make it sell?

There are criteria that help press officers decide which science stories have potential to hit the headlines, and which are likely to be read only by the selected few that check an organisation’s website. “Timing” is often number one on the list: the story is news if the event it describes has just happened. If the issue has direct influence on people’s lives (“significance”, think material damage and loss of human lives), has profound consequences (“implications”), such as the revision of a widely accepted scientific theory, or describes a “major discovery”, it may also make it into mainstream newspapers. As a rule, you know a story is newsworthy if one or more of these criteria is satisfied.

But as with every rule, there are exceptions. Some stories make it to the newspapers simply because they allow journalists and editors to come up with titles like this one: “Comette family home damaged by egg-sized meteorite.” Go ahead, read the Guardian article, you know you are curious. A meteorite fell on the Comette home, what are the odds? There it is: the certain je ne sais quoi that makes this a newsworthy story.

If you do read the article, you’ll understand that is pretty much all there is to it. The rock hit the family home “some time over the summer”, or one to two months ago; you can cross “timing” off the list. The meteorite broke one roof tile, so not much material damage to account for; there goes “significance”. I also don’t think an egg-sized rock falling in Paris has any profound “implications”, and it definitely doesn’t qualify as a “major discovery”.

OK, you could argue, as the journalist who wrote the piece does, that meteorite falls in France are not that common (there is a certain “mystery” to the story). The article quotes a mineral expert saying that there have only been 50 or so in the past four centuries, and none in Paris. But this information is buried somewhere in the middle of the article: the journalist knows that that’s not what makes the editor select the piece for today’s newspaper.

So, really, this story is newsworthy simply because a tiny meteorite happened to fall on the house of a family named Comette. But that’s enough to sell it.

One post for a double celebration

It has been a year. On 23 September 2010, I posted for the first time in Dinner Party Science, Nature Network edition. This is also the 50th time I’m writing, so I have two reasons to celebrate.

In commemoration, I’ve decided to share with you some numbers and facts about the blog that marked the past year:

1. On average, I wrote 4 entries a month in the past twelve months.
2. My posts got a total of some 160 comments.
3. Since I started counting, about three months after the blog was born, over 9,000 people visited the site.
4. Most visits came from the United States, but people from all over the world dropped by.

Screen shot 2011-09-18 at 5.49.51 PM.png

Location of blog’s visits. Anyone in Greenland keen to stop by? Source: Google Analytics.

5. Dinner Party’s most loyal visitors are Londoners, who read the blog almost 500 times last year (more than in any other city) and stayed for an average of 7 minutes/visit.
6. Search keywords that brought people to this blog include: “afraid of large universe”, “animal sex in rubber boots”, “decision pee”, “are socks good for us” and “fellacio contagious”.
7. My most successful post was “Tycho’s drunken moose and other stories”, with over 3,800 page views (roughly 25% of the total).
8. Tycho’s post was picked up by Stumbleupon on the 4th of July and the blog had a peak in visits that day (over 1,800).

Thank you to all of those who read, edited or commented on my posts. And a special thanks to the reader who “stumbled upon” Tycho’s drunken moose 8 months after I wrote the post!

Not sure what else I should do to celebrate this blog’s first anniversary… Am I allowed to ask for presents?

The other drunken moose

I did not expect to write twice about drunken moose in less than a year. But here it is. With over a week of delay because my new job doesn’t let me blog as frequently as I would like, but here it is nonetheless.

About ten days ago, the Swedish edition of The Local, reported that a moose (elk, for the British readers) had been found stuck in a tree in western Sweden. It is not everyday that moose make the news but, as it turns out, this particularly one was drunk.

Per Johansson was arriving home after work in Särö, south of Gothenburg, when he heard a deep roaring coming from the neighbors’ garden.

“It was raining really bad. In the wind I heard something screaming with a very dark voice,” Johansson told in an interview to CNN. “At first I wondered if it was the crazy neighbors, but then I heard it again and went and checked. I saw something really big up in a tree in my neighbors’ yard and it was a moose. It must have been drunk after eating fermented apples and as it was reaching out for more fruit it must have slipped and fallen into the tree.”

Removing the animal from its wooden prison wasn’t an easy task. Johansson tried to free the moose by himself, but quickly gave up because it kicked ferociously as he approached.

Johansson and his neighbors managed to saw down some of the tree branches, but it wasn’t until a fire brigade arrive on the scene that the animals was freed. They had to bend the tree to the point where the animal could slide out by itself.

Once freed, the moose collapsed near the tree, inebriated and exhausted. It took it a full night to recover from the episode, but the beast managed to drag itself into the woods the next morning.

Curiously enough, it is not unusual to see moose drunk in Sweden this time of year. Anders Gardhagen, spokesman at the Gothenburg Fire and Rescue Services, said to CNN that “moose are attracted by the apple trees, and in the autumn when the apples have fallen off the trees we normally have at least one of these cases of intoxication.”


Source: BBC. Credit: Gustav Johansson/Reuters.