The Transit of


Transcribed from “Horizons” Issue 13, Spring 2004

On Tuesday 8 June this year there will be a transit of Venus, when the planet will appear to move across the face of the Sun.  It isn’t often that we have the opportunity to celebrate a single event with such great significance in both astronomical and maritime history.


In 1716 Edmund Halley, the second Astronomer Royal, .proposed using a transit of Venus in a method to measure the distance from the Earth to the Sun, the Astronomical Unit.  Halley’s method was based on parallax, but required only timing the duration of the transit from points of different latitude.  Timing to a second or so over the six-hour transit was possible, while precise measurement of the small angles was not.


Transits of Venus are very rare, coming in pairs only every 120 years or so.  The next transits suitable for Halley’s method would have been in 1761 and 1769.  Attempts in 1761 failed due to bad luck with the weather and to having underestimated the difficulty of mounting expeditions to suitable remote locations.  Planning for the 1769 transit was taken more seriously, and after pressure by the Royal Society and the Astronomer Royal Nevil Maskelyne, the Navy mounted a scientific expedition under the command of Lt James Cook, to make observations from somewhere in the South Pacific.  He chose Tahiti, and set up an observatory at Point Venus.

Past Chairman of the Flamsteed Astronomy Society, Friend Dr Eddie Yeadon, presents a rare phenomenon that links ROG and NMM ever more closely.


Transits of Venus in 1874 and 1882 were also the subject of expeditions mounted from Greenwich, but there were no transits of Venus during the 20th Century.  The transit on 8 June will be visible in its entirety (weather permitting) from about 06:00 to 11:30.  There will be another transit of Venus in 2012, but only the end of this will be visible from Europe.  There will be no other transit of Venus until 2117.


Tuesday 8 June is also World Ocean Day, so take this opportunity to celebrate the work of Greenwich Astronomers and the wonderful voyages of Cook by coming along to the Observatory during the morning to see the transit for yourself.  Telescopes for public viewing will be available and members of the Flamsteed Astronomy Society will be on hand to help and explain.  The disc of Venus should be visible to the naked eye but, as with an eclipse, you must use a solar viewing filter.  Never look at the Sun through a telescope or binoculars without a proper filter to protect your eyes.

Tahiti from Point Venus: painting by Hodges

Astronomer Royal Edmund Halley:

portrait by Sir Godfrey Kneller