‘Transit of Venus: the biggest ruler in the world’
On the first floor of Sydney Observatory is a small display about this rare astronomical event – developed by the Observatory in the lead up to the transit of 2004.

The transit of Venus exhibit at Sydney Observatory

On the afternoon of Tuesday 8 June 2004, the planet Venus crossed in front of the Sun. When Venus’s dark silhouette moved across the disc of the Sun – a transit of Venus – we witnessed one of the rarest and most famous events in astronomy. This was the first transit of the planet in more than 120 years.

The cover of the 'Transit of Venus' book, produced by Sydney Observatory in 1874

Transits of Venus were important in past centuries because they provided the opportunity to measure the distance of the Earth from the Sun and hence determine the scale of the solar system. It was Edmond Halley, of Halley’s Comet fame, who pointed out the feasibility of using transits for this purpose.

The most celebrated transit, in June 1769, was observed from many places. One of the observers was Lieutenant James Cook who sailed to Tahiti with the astronomer Charles Green in the ship HMS Endeavour. After observing the transit, Cook opened sealed orders from the Admiralty to search for the unknown southern continent. He did not find this mythical land, but did claim New Zealand and New South Wales for the British Crown.