Transits of Venus are historically important as it was realised by Edmund Halley that, by observing the start (or end) time from various points on the Earth’s surface, parallax could be used to determine the distance of the Earth to the Sun (known as the Astronomical Unit) and therefore the true scale of the Solar System could also be determined. Perhaps the most famous expedition has to be the one that took in the Transit of Venus in 1769 that involved HMS Endeavour, Capt James Cook, astronomer Charles Green and the Lincolnshire astronomer and naturalist Joseph Banks (not forgetting Joseph Gilbert from Wrangle, Boston, Lincs. Observing from the South Sea Isle of Tahiti they successfully recorded the event and the data combined with observations of the 1761 Transit put the Sun - Earth distance at 95 Million miles - impressive considering the average distance today is given at 93½ Million miles!

Attention was also drawn to the ‘black drop effect’ where once the Venusian disk was just inside that of the solar disk there lingered a sliver of black connecting the planets disk to the limb of the Sun. The expedition hoped that with their much superior instruments that the effect would be greatly reduced helping to improve the accuracy of their timings but even so it was still noticeable. This is something that to my observation  was not clearly apparent during the 2004 Transit perhaps due to using the webcams (see my simulation of the effect at left). The effect is due to the fact that Venus has an atmosphere so its disk is not as sharply defined. The Solar disk itself is not as sharply defined either due to its gaseous nature so the solar limb is also slightly blurry - put the two together and you can see why getting an accurate measurement of ‘first (and last) contact’ is actually quite a challenge. Note that in my simulation the atmosphere of Venus is greatly exaggerated to help show the effect. As with the Mercury transit, Venus was jet black against the disk of the Sun and so was unlikely to be confused with any sunspots that may be visible. Sunspots are great magnetic storms on the surface of the photosphere and often have a dark umbra surrounded by a lighter penumbra.

Interestingly at least three different motions are occurring whilst we observed this event. Firstly the Sun got higher in the sky as it was rising due to the turning of the Earth (which gives us day and night). Secondly the Sun was moving slowly along the ecliptic from right to left on the above chart due to the motion of the Earth orbiting the Sun and thirdly there was the actual motion of Venus in its own orbit about the Sun as it moves from left to right across the solar disk! It's an 8 year wait until the next and final transit of Venus in our lifetime! The second Transit is only partially seen from the UK and Ireland so I reckon that now is the time to start saving up for a very special holiday in 2012!