Professor David Rothery, Open University
On 9 May, Mercury will be seen as a black dot silhouetted against the SunÖ but why are scientists so excited about it? David Rothery spoke to Graihagh Jackson about its importance and how to watch the event safely...
David - A transit is when one of the two planets closer to the Sun than the Earth passes directly between the Sun and the Earth, and then we see the planet silhouetted as a tiny black dot against the Sunís disc.
Graihagh - Why is that exciting?
David - Well, particularly for Mercury, Iím working on a space mission to Mercury - itís my planet! And itís very hard to see in the evening or morning sky because itís always so close to the sun and when itís in front of the sun (transiting it) you canít miss it if youíve got the right equipment to look at it. Itís a little bit of a thrill to see this; it doesnít happen very often.
Graihagh - How often does it happen?
David - WellÖ the last one was in November, 2006 but it started just before sunset in the UK so we didnít get much of a chance to see it. Before that it was May, 2003 and it was very early in the morning. But this one on May 9th is perfect timing for us because it starts just after midday (12.12. BST) and it finishes at 19.42 BST so, if there are no clouds, weíll see the whole of it.
Graihagh - So it means you donít have to get up too early and, alas, dress up nice and warm?
David - Itís May in Britain - itíll be sunny, thereíll be a few fluffy clouds, blue sky and a nice sun. But, weíve got to say this - you should not look at the Sun without proper gear to see this. Itís too tiny to see with the unaided eye anyway. Itís 150th the solar diameter when itís sitting in front of the sun. You need a magnified image and what you mustn't do is try binoculars or a telescope or anything like that. You have to project an image onto a card or use a proper solar filter in front of a telescope to cut the light out or go to an event. There are lots of events that you can find or just stay at home and look at it on the internet.
Graihagh - I mean, I think Iíd rather see it in the flesh, even if it is indirectly! You can find out whether there are events near you - www.nakedscientists.com/mercury transit. But if youíre not lucky enough to have some Mercury display happening on your doorstep, all is not lostÖ
David - OK, well this is what I used to do when I was a lad. You focus your telescope on some distant trees just to get it into focus. Then you swing it round until itís pointed at the Sun but you do not look through the eyepiece. You hold a white care a couple of feet behind the eyepiece of the telescope and you will see a projected image of the Sun. But donít look through it because youíll burn your eyes out - itís very, very, dangerous.
Graihagh - OK, Iíll be well behaved. Donít you worry David. And I presume of the projected card you see the sun but then you also see Mercury transiting across the Sun, maybe like a dark patch or something?
David - Mercury will be a tiny black dot. I mean, if the sunís image is 6 cm across, Mercury will be half a millimeter, so itís tiny.
Graihagh - Wow. OK, so it might be quite tricky to see in that caseÖ
David - Look carefully, but it will be there.
Graihagh - OK. Now you said you study Mercury and youíre particularly excited about this transit because it means youíll be able to see it at a more sociable hour, but how much can you really ell?
David - The transit isnít actually useful to us scientifically these days. Thereís nothing we can learn from Mercury but watching it transit but it itís nice to see the planet doing it.
Graihagh - I agree but just because it isnít so scientifically important these days, doesnít mean it wasnít once. Venus was the first transit observed in 1639, just outside of Manchester, by a chap Jeremiah Horrocks.
What I like about Jeremiahís story is that in those days, there wasnít a specialised astronomical telescope market - funny that - and so stargazers had to build their own. Jeremiahís father and uncles were watchmakers and as a result, well practised in the art of precision instruments. By day he worked for them and in return, they helped him build the best telescope they could. And it was with a watchmakers telescope, he indirectly observed Venus transit across the sun - using the method David described before.
Now, letís get to why this is important.
Skip forward 50 or so years and Edmond Halley publishes a paper, describing how if you can observe a transit from multiple places from Earth, using triangulation, you can work out how far the Sun from the Earth is - now known as the astronomical unit.
David - Itís important because, if you cast yourself back to the 17th century, we did not know how big the solar system was. It was late in the 16th, early in the 17th century when opinion was swinging away from the old view of the solar system, which had everything going round the earth, to accepting what Copernicus had said - look, the Sunís in the middle and everything else goes round the Sun. And once that was realised, it became important to work out well how big is all this? I mean you want know how far away things are.
Graihagh - Halley died 19 years before the Mercury Transit in 1942 BUT naturalists from France, Austria and Britain were packed off on various expeditions far and wide across the world to observe the rare phenomenonÖ. As far as Siberia, Norway, Newfoundland and Madagascar. All this wasnít underpinned by our quest to understand the size of our universe though it was good old fashioned competition! Indeed, thatís why Captain James Cooke was packed off to Tahiti on his first tour of the world!
Their findings? Well, what was reported in the Philosophical Transitions journal was:
"the mean distance from the Earth to the Sun (is) 93,726,900 English miles."
We now know today thatís itís 92,955,000 miles. That means they were off just by 0.008% - pretty remarkable really, considering this was the mid 1700s!
David - So this is was a step towards realising the scale of the universe - which is vast!
Graihagh - Back then, it was transiting planets in our solar system, but now scientists watch exoplanets transit - thatís planets orbiting around OTHER stars. And turns out you can tell quite a lot more than just how far away they areÖ
David - There are many exoplanets we are observing in very close orbits around their stars. So the travel faster and a transit may last a matter of minutes or an hour or so but itís never as quick as just being over in seconds.
Graihagh - OK, but what I was keen to know was: what can you find out about a transiting exoplanet in that short window of time?
David - Measuring transits is a way of timing the orbits of planets around other stars and measuring their size. So you can have a go at working out their densities and what their properties might be. But to actually measure something, the thing you can really measure is the planetís atmosphere. If it has a dense atmosphere, the starlight shining through the atmosphere will be affected by that atmosphere, so youíll just get faint lines superimposed on the spectrum of the star corresponding to the gases in that planetís atmosphere. Itís a very, very careful, subtle observation just looking at how the quality of the starlight changes when the planetís in front of it. If the starlight dims because the planetís blotting out a tiny portion of the starís disc and the light that gets to you, a tiny fraction of that is shining through the planetís atmosphere. So itís spectroscopy looking at spectral lines.