The planets Mercury and Venus are the only two planets to transit the Sun, when viewed from the Earth, with the latter, the transits of Venus, occurring in pairs that are more than 100 years apart.
What is a transit?
A transit occurs when a planet or moon passes between the Sun and the viewer.
Transits of the Moon are known as a 'solar eclipse'.
Transits of Mercury and Venus appear as small dark spots against the disc of the Sun instead of covering the whole disc, as the Moon does in an eclipse. This is because the planets are much more distant than the Moon.
Transits of Venus are more famous than the relatively common transits of Mercury. Scientists in the 18th and 19th centuries used them to establish the scale of the solar system and measure the distance from the Earth to the Sun.
The most celebrated transit of Venus is that of 3 June 1769, which was observed from many places. One observer was Lieutenant James Cook who had sailed under British Admiralty orders to the newly discovered island of Tahiti with the astronomer Mr Charles Green in the ship HMS Endeavour in order to observe the transit.
How does a transit occur?
A transit occurs when, as seen from Earth, a planet appears to move across the disc of the Sun.
Only our two inner planets, Mercury and Venus, pass between the Earth and the Sun.
However, a transit of Venus or Mercury does not occur each time the planets they pass between the Earth and the Sun. Most times, the planet, Earth and the Sun are not aligned when viewed 'from the side', resulting in the planet appearing to pass above or below the Sun when viewed from Earth.
Find out more about History of the transit of Venus.
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