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The transit of Venus, 8 June 2004
A transit of Venus occurs when the planet passes directly between the Earth and the Sun. They are very rare – no living person has ever seen one. The last occurred in 1882. The next transit of Venus takes place on 8 June 2004, followed by another in 2012 and 2117.
Safety
Do not look directly at the Sun, as this can lead to permanent damage to your eyes. Safe observations of the transit can be made at the Royal Observatory, Greenwich through one of our solar telescopes or with a pair of solar viewers. Solar viewers are worn like spectacles. Please follow the safety instructions that accompany them. Find out more about viewing the transit at the Observatory.
What you will see
Venus will appear as a black disc against the bright surface of the Sun. Viewed from London, it will start to move across the disc of the Sun a few seconds before 06.20. It will travel slowly across the surface and have fully left the Sun's disc by 12.24.
As well as Venus, sunspots may also be visible on the Sun's surface. They are regions of the Sun's surface that are a little cooler than the rest and appear as black spots. Although they vary in size, a typical sunspot will look around the same size as Venus. Unlike Venus, they are uneven in shape and have less clearly-defined edges.
Viewing the transit at the Observatory
The Observatory courtyard will be open to the public from 06.00 on 8 June to enable members of the public to safely view this historic event for themselves under the expert guidance of the Observatory’s astronomers and members of the Museum’s Flamsteed Astronomy Society.
What is the significance of a transit?
In 1716 the astronomer Edmond Halley (Astronomer Royal at Greenwich 1720–42) realised that the distance of the Earth from the Sun could be calculated from measurements of the duration of a transit of Venus made from places widely separated in latitude. This distance, the Astronomical Unit, was only approximately known at the time.
Attempts to make these measurements during the transit in 1761 were unsuccessful due to unfortunate weather and underestimating the difficulties of getting to remote observing sites. Consequently, for the 1769 Transit of Venus, the Royal Society mounted a special expedition under the command of Captain James Cook to make the observations from Tahiti in the Pacific Ocean. Although not completely successful astronomically, this led directly to the discovery of the East coast of Australia on the way back to England.
Our observers in the courtyard, along with others around the world, will be timing the transit on 8 June and repeating the historic calculations. Using radar, the value of the astronomical unit has been determined to an accuracy of a few metres. It is equal to approximately 150 million kilometres.
Warning - never look at the sun through a telescope or binoculars without a proper solar filter as permanent eye damage can result.Useful links
- See our online exhibition - 'Transit of Venus: the expeditions to measure the solar system'
- For a more detailed look at the transit of Venus and images from the 1874 observations, visit the PortCities website.
