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Wednesday, 20 June 2012

Ogling at Venus

In the last few weeks, astronomers around the world have been getting hot under the collar about the beautiful planet Venus. Because Venus orbits the Sun inside the orbit of the Earth, every now and then we form a straight line - something astronomers call a "conjunction". This means that we can watch as Venus transits across the face of the Sun, and this is what happened on the 5-6 June.

Within hours of the transit finishing, the internet was awash with hundreds of images of a small black dot against the surface of the Sun. These were perhaps not the most awe inspiring of astronomy images out there, so why were astronomers so excited about it?

First of all, this is a very rare occurrence. Although conjunctions between the Earth and Venus happen every 19-20 months, the planets have different inclinations. In other words, we see Venus passing by a little bit above, or a little bit below the Sun. Only when all the conditions are exactly right can we see Venus crossing the face of our parent star, and that only happens every 243 years (in pairs separated by 8 years). The last transit was 8 years ago, so we won't see another one until December 2117!

Historically speaking, transits of Venus have been a boon to astronomers. In 1769, one of the biggest Things We Didn't Know of the time was the size of our solar system. Although we knew the relative spacing between the inner planets (we hadn't discovered the outer planets yet!) we had no idea what the actual distances were. How far was the Earth from the Sun? Watching Venus cross the face of the Sun would answer that, but there was one big problem - they couldn't just watch it from Europe.

None other than the famous Captain Cook set sail for Tahiti, in the middle of the South Pacific ocean, a year ahead of the transit. This was a voyage into the unknown, before the discovery of Australia (by Cook in 1770), and was both very expensive and incredibly dangerous for the crew. But Cook, and the Royal Academy who paid for it, felt it was worth it. Today, we could fly there in a day. It took Cook and his crew 8 months.

As Cook went to Tahiti, Jean-Baptiste Chappe went to Mexico - and of his team just one man survived the trip. Maximilian Höll went to Norway.

Together, the astronomers' observations were an resounding success, and helped arrive at a value just 2.5% from the actual value we know today (150 million km). In 1874 and 1882 (the next transits to occur) astronomers got this down to less than half a percent, and today we know the distance to within tens of meters.

For many astronomers, reliving such a rare and momentous event was reason enough to watch, but the science isn't over yet.

Light is made up of a continuous spectrum of wavelengths, or different colours. If the wavelengths are long (or short) enough, we can't see them anymore, but we do have equipment that can. When light passes through a planet's atmosphere, some of those wavelengths are absorbed (soaked up) by the different gases present. Different gases absorb different wavelengths, so by carefully monitoring which wavelengths have been absorbed they can determine which gases are present.

© All rights reserved by Nick Howes, image reproduced with permission.
So when Venus passed across the surface of the Sun last week, scientists were busy doing just this, to improve our understanding of what's happening on a planet millions of miles away. They even used the famous Hubble Space Telescope to do it.

As well as telling us more about our planetary neighbour, astronomers are hoping this will help us in our quest to understand the planets around other stars: exoplanets.

The first exoplanet was discovered in 1988, and the rate of discovery has positively exploded in recent years. Today, we know of hundreds of exoplanets - and NASA's Kepler spacecraft has identified thousands of potential candidates awaiting confirmation. To date, we have been unable to image them directly - we can't actually see them because of the distances involved. But when the planets pass in front of their star, they block a little bit of the light, and this we can detect. Their atmospheres also absorb some of the starlight, just like Venus did with the light from our Sun - so if astronomers can get high quality information about Venus, the same method can be used to help them in their search for and categorisation of exoplanets.

Next week, astronomer and TWDK contributor Nick Howes will explain how he captured the stunning image shown above in our first guest post.