Saturday, December 10, 2011

The history of Venus transit

Venus transits occurred throughout history. It is not known however, if someone actually saw such a transit before 1639. It is possible to see the transit with the naked eye if the sun is rising or setting, or if there is a hazy dusty air. It is probable that someone in the early past saw it. The first modern astronomer who calculated the transits was Kepler who predicted  the transit of 1631. Sadly, Kepler's predictions were not accurate enough, the transit was invisible from Europe and no one saw it. Kepler predicted the next transit to be only in 1761. However, a very young English astronomer, Jeremiah Horrocks, calculated from Kepler's Rudolphine Tables, that a transit should happen on the 24th of November, 1639. Sadly, Horrocks reached this conclusion less than a month before the transit. The transit was fully visible from America, but there was no time to send a message to the new colonies. So the first observation of a Venus transit was by only 5 people. Horrocks himself and his colleague William Crabtree and together with Crabtree's family we reach a count of 5 people in all. Horrocks and Crabtree utilized the transit to measure Venus's apparent size compared to the size of the sun and used this data to estimate the distance to the sun. Horrocks was among the first to see that the universe is much larger than ever thought before and estimated the earth-sun distance to be 100 Million kilometers. Although this value is still 50% wrong, it was 4.5 times more than the value Kepler gave, and more or less within the correct magnitude.
Horrocks was the first significant English astronomer and he is often called the Father of British Astronomy. He was both a great observer and a great theoretician, a rare combination, suppressing Tycho Brahe, Copernicus, Kepler and Galileo. Sadly, Horrocks died unexpectedly (the reason of his death is unknown) at the young age of 22.
The next pair of transits were in the years 1761-1769. The measurements and calculations from these transits were used to more accurately determine the distance between the Earth and the sun which by than was called Astronomical Unit (AU). Observations were made from many places, the most famous was in Tahiti by no less than the legendary Captain Cook.  From these observations, a better value which is very similar to the present value was calculated.
The next pair was in 1874 and 1882, and gave another chance to refine the AU distance.
The last transit was in 2004, a marvel to watch, and now we are waiting for the 2012 transit, not to be repeated again before 2117.
Jeremiah Horrocks observing the Venus Transit
Jeremiah Horrocks observing the Venus Transit by Eyre Crowe. The painting name is The founder of English Astronomy. Painted in 1891

For further reading please refer to the following Venus Transits books.
Historical observation data can be found here.