The Transit of Venus

June 8, 2004

In the late 17th/early 18th Century, measuring the size of the solar system accurately was one of the great challenges for science.  Kepler’s equations had determined relative dimensions for the size and period of the orbits of the planets, and the orbit periods were known.  The challenge was to make an accurate measurement of any one orbit size, from which all the others could be calculated.  In 1677, when he was on St. Helena, Edmund Halley had observed a transit of Mercury and realised that simultaneous measurements of a transit from different places on the earth could be used to calculate the size of the orbit.  He had the insight that transits of Venus might be more suitable for this.  Halley saw that because the transits were rare it would be necessary to plan in advance and be properly organised to take the opportunity to measure the next which were due in 1761 and 1769, when he would very likely be dead.   (Halley was a dab hand at predicting events to happen after he was gone!)   In 1716 he read his famous Admonitions (pdf) paper to the Royal Society, setting out the importance of organising expeditions and describing the technique for using the measurements of the transit.  Halley became the 2nd Astronomer Royal in 1720.

During a Transit of Venus, the planet can be seen from the Earth crossing the face of the Sun.  Transits of Venus are quite rare.  They occur in pairs eight years apart,  with an interval of over a century between pairs.  Only three pairs have occurred since the invention of the telescope.  The last occurred in 1874 and 1882, so a transit has not been witnessed by any living person.   This century transits will occur on June 8, 2004 and June 6, 2012.   The Flamsteed Astronomy Society, with the ROG, is planning projects and events to mark the 2004 transit.   Eddie Yeadon is leading the FAS program and details will appear here as they are finalised.

“We therefore recommend again and again, to the curious investigators of the stars to whom, when our lives are over, these observations are entrusted, that they, mindful of our advice, apply themselves to the undertaking of these observations vigorously. And for them we desire and pray for all good luck, especially that they be not deprived of this coveted spectacle by the unfortunate obscuration of cloudy heavens, and that the immensities of the celestial spheres, compelled to more precise boundaries, may at last yield to their glory and eternal fame."       Edmund Halley (1656—1742)

Photographic plate from the 1882 transit (USNO)

Where Cook’s expedition observed the 1769 transit -  Point Venus (R) and Matavai Bay, Tahiti  (Photo Eric Schreur, Kalamazoo Valley Museum)

Several famous characters were mixed up in expeditions to measure the transits.  For the 1761 transit Nevil Maskelyne voyaged to St Helena where he was entirely defeated by the weather!  On the trip, however, Maskelyne tested Meyer’s lunar tables and used the lunar distance technique to find accurate longitudes.  On his return he promoted the technique by publishing the tables which became the Nautical Almanac after he was appointed 5th Astronomer Royal.  Maskelyne’s commitment to lunar distances led to his confrontations with John Harrison... but that’s another story!   Also for the 1761 transit a pair of astronomers set off for Sumatra but were forced to turn back when their ship was attacked by a French frigate.  Prodded back to sea by the Royal Society, Mason & Dixon made it to the Cape of Good Hope in time for the transit.  They later did the famous survey of the boundary between Pennsylvania and Maryland, the Mason-Dixon line (hence ‘Dixie’)

Kylie Minogue and the Transit of Venus

Maskelyne was Astronomer Royal in time for the 1769 transit.  He prepared a meticulous set of Instructions (pdf) for the astronomers despatched to the eight corners of the world.  The Royal Society and Royal Navy organised an expedition to Tahiti under the leadership of Captain James Cook.   The Navy took the opportunity to instruct Cook to look for the Great South Continent after the transit observations were complete.  Rather than the transit observations, Cook’s first voyage is now, of course, remembered almost totally for gifting to the world Foster’s Lager,  Dame Edna Everage, and Kylie Minogue.

1882 transit -  expedition members in a pre-fab hut (despite Rob Warren’s musings it is unlikely that this hut was just dropped from a helicopter)

Never look directly at the Sun or via a telescope without a proper filter !!