Transit of Mercury

May 7, 2003

Starting just after 6am on May 7, 2003 a transit of Mercury was visible from London. 

A Transit of Mercury happens when the planet Mercury passes between the Earth and the Sun.  It is visible, only through telescopes with proper filters, as a little black dot.   On this occasion , the little dot took about 5 hours to crawl across the face of the Sun (see picture, courtesy BBC).

A group of stalwart professional and amateur astronomers gathered at the Royal Observatory Greenwich to watch the transit from 6am (I arrived about 10am—well, it was visible until after 11am!)

A Transit of Mercury happens about 13 times a Century.  It is only visible via a telescope and so was first observed by Gassendi in 1631.    In 1677 Edmund Halley (second Astronomer Royal, and of Halley’s Comet fame) realised that by timing the transit from two different places on Earth it would be possible to calculate the distance from the Earth to the Sun.   Halley calculated that observation of a Transit of Venus would be better for this than Mercury.  Halley’s recommendation resulted in the expeditions in 1761 and 1769 to observe the Venus Transit.  Transits of Venus are rare.  No living person has observed one but the next is due on June 8, 2004.