Science & Environment

Planet Mercury passes across the face of the Sun

Mercury transit Image copyright NASA
Image caption Mercury appears in silhouette against the bright surface of the Sun

Mercury has made a rare transit, where the planet passes across the face of the Sun as seen from Earth.

During the transit, Mercury appeared as a dark silhouetted disc against the bright surface of our star.

There are 14 transits in this century; the last before this was in 2016, but the next event will not occur until 2032.

The transit began at 12:35 GMT, when the edge of Mercury appeared to touch the curvature of the Sun.

Image copyright Getty Images
Image caption Transits of Mercury are relatively rare (this image shows a previous, complete transit)

It ended at 18:04 GMT when the edge of the silhouetted planet appeared to leave the Sun's disc.

Professor Mike Cruise, president of the UK's Royal Astronomical Society (RAS), said: "This is a rare event, and we'll have to wait 13 years until it happens again. Transits are a visible demonstration of how the planets move around the Sun."

The entire event was visible from the eastern US and Canada, the south-western tip of Greenland, most of the Caribbean, Central America, the whole of South America and some of West Africa.

In Europe (including the UK), the Middle East, and most of Africa, the Sun set before the transit ended, and so the latter part of the event was not visible.

In most of the US and Canada, and New Zealand, the transit was in progress as the Sun rose. Observers in eastern Asia, southern and south-eastern Asia, and Australia were not able to see the transit.

Image copyright NASA/JHUAPL/Carnegie Inst Washington
Image caption Mercury is the smallest and innermost planet in the Solar System

Mercury is the smallest and innermost planet in the Solar System.

It completes each orbit around the Sun every 88 days, and passes between the Earth and Sun every 116 days. As the orbit of Mercury around the Sun is tilted compared with the orbit of the Earth around the Sun, the planet normally appears to pass above or below our nearest star.

A transit can only take place when the Earth, Mercury and the Sun are exactly in line in three dimensions.