Watch Mercury Glide Across the Sun in Rare Transit

A photograph of the transit of Mercury of 9 May 2016. Mercury is seen in silhouette as the dark dot below left of centre. The other dark features are sunspots.
Credit: Wikipedia / Elijah Mathews (CC BY-SA 4.0)

A rare transit of Mercury will take place on Monday, November 11, 2019, when the smallest planet in our Solar System will pass directly between the Earth and the Sun. The last time this happened was recent, in 2016, but the next won’t be until 2032. During the transit, which takes place in the morning in the United States and in the afternoon in the UK and Europe, Mercury will appear as a dark silhouetted disc set against the bright surface of the Sun.

The transit begins at 1235 GMT, when the edge of Mercury appears to touch the edge of the Sun, and ends at 1804 GMT when the edge of the silhouetted planet appears to leave the Sun. Observers in different locations will see the transit taking place up to 2 minutes before or after these times, as the planet will appear to take a slightly different path across the Sun.

On the morning of November 11, UK amateur astronomical societies and public observatories will be running events where members of the public can safely enjoy the transit, as well as live webcasts of the spectacle. The Royal Astronomical Society will be supporting a (free) event run by the Baker Street Irregular Astronomers in Regent’s Park, central London, where members of the public can book places to come and view the transit using appropriate equipment at no cost.

Professor Mike Cruise, President of the Royal Astronomical Society, is keen for people to experience the transit for themselves. “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, and everyone with access to the right equipment should take a look, or go to an organised event if the weather is clear, or alternatively follow one of the live webcasts. I do want to stress though that people must follow the safety advice – looking at the Sun without appropriate protection can seriously damage your eyes.”

The entire event is visible from the eastern United States 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 will set before the transit ends, and so the latter part of the event will not be visible. In most of the United States and Canada, and New Zealand, the transit will be in progress as the sun rises. Observers in eastern Asia, southern and south-eastern Asia, and Australia will not be able to see the transit.

Mercury 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.

There are 13 or 14 transits of Mercury each century, so they are comparatively rare events, though each one can typically be seen over a large area of the Earth’s surface. A transit was first seen in 1631, two decades after the invention of the telescope, by French astronomer Pierre Gassendi.

At any time, Mercury blocks out no more than a tiny part of the light from the Sun. This means that the event should NOT be viewed with the naked eye. Looking at the Sun without appropriate protection, either during the transit, or at any other time, can cause serious and permanent damage to the eyes.

The Society for Popular Astronomy has an online guide on how to safely view the transit, for example by projecting the solar image with binoculars or a telescope. Mercury is too small to be visible using the pinhole projectors that worked successfully in the solar eclipse in March 2015, and similarly cannot be seen by using ‘eclipse glasses’ with solar filters.

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