The transit of Mercury – here’s all you need to know

A rare transit of Mercury will occur on November 11th, 2019. Mercury will pass directly in front of the Sun, and the planet will appear as a tiny dot silhouetted against the face of the Sun.

Mercury appeared as a tiny dark dot on the face of the Sun during the transit of May 9, 2016. The other blotch is a sunspot. Image credit: Paul Sutherland

Usually, you only get to see Mercury just after sunset or before sunrise because it lies so close to the Sun. Another opportunity can come during a total eclipse of the Sun when the phenomenon darkens the sky for a few minutes.

A transit of Mercury is so-called because Mercury transits, or passes in front of, the Sun as viewed from Earth. Only Mercury and Venus can do this because they are the only two worlds between us and the Sun.

Though Mercury orbits the Sun once every 88 days, a transit occurs only every few years. That is because Mercury’s orbit is not level with our own orbit around the Sun, but is tilted to it by 7°. So on every one of Mercury’s orbits, when it lies at “inferior conjunction” between us and the Sun, it actually passes above or below the Sun as seen from Earth.

Key times for the Transit of Mercury of 2019 at a glance

Transit begins: 12.35 07.35 06.35 05.35 04.35
Transit ends: 18.04 13.04 12.04 11.04 10.04
A diagram, not to scale, demonstrating how transit of Mercury can only be seen where our orbits intersect. Image credit: ESO

A transit of Mercury can only occur when Mercury and Earth are lying at one of the two points where the plane of one planet’s orbit crosses the plane of the other’s orbit. For Mercury and Earth, these points are reached in May or November. The crossing points are known as nodes.

May events happen at intervals of 13 and 33 years and November transits at intervals of 7, 13 and 33 years. The last was on May 9, 2016. There are more November transits than May ones because Mercury has a very eccentric (non-circular) orbit and it near its closest point to the Sun (perihelion) in November. You can find a full list of dates of recent and upcoming transits of Mercury and Venus here.

Transits of Venus are much rarer. There were two within an eight-year interval in June 2004 and June 2012, but there won’t be another until December 11th, 2117!

A photo from the transit of Venus in 2004 shows how much bigger that planet appears against the Sun’s disk than tiny Mercury. Image credit: Paul Sutherland

What will happen during the transit?

The transit of Mercury can been seen from any part of the world with the Sun in the sky between the start and end times. The entire transit will be visible from the Eastern USA and all of Central and South America.

From Europe and Africa, the early stages of the transit will be observable, but the Sun will set before the event has ended. From most of the USA and Canada, the transit will already have begun before the Sun rises.

No part of the transit will be visible from Australia, Indonesia, Japan and most of Asia because it will be night time with the Sun beneath the horizon. New Zealand will catch the final hour or so of the transit after sunrise.

Mercury moves onto the Sun’s disk just after 12:35 UT, which is the same as GMT). It then takes nearly five and a half hours to cross the face of the Sun before moving off it at around 18:04 UT.

How to see the transit of Mercury for myself?

WARNING: Before going any further, we must issue a familiar but important warning. The Sun is an incredibly bright object, so on no account should anyone attempt to look directly at it without appropriate safety equipment to protect the eyes. Otherwise you could be blinded! If you are observing at a group event, make sure that no one else attempts to use your telescope without supervision.

Mercury is a small rocky planet that is around twice as far away as Venus when this side of the Sun. Its silhouette is therefore a lot smaller than the sizeable one that Venus showed during its own transits.

Mercury will be only 10 arcseconds in diameter, whereas the Sun is around half a degree in diameter against the sky. You could fit 180 Mercurys in a line from one edge of the Sun to the other! This means the planet will be far too small to be seen with the unaided eye, so solar eclipse glasses will not help you see it. You will need a telescope, or powerful binoculars. For your telescope, pick an eyepiece that magnifies at least 50x.

The best method of viewing the transit is to cover the front end of your telescope with a suitable solar filter. You can buy filters designed to attach to different sized telescopes, or you can make one from a flexible sheet of film. This should be acquired from a reputable telescope dealer. You can find a range of different solar filters to buy in the USA, or a similar range of filters if you are in the UK.

On no account try to make your own filter from an inappropriate material, such as a CD disk, coloured or silver wrapping paper. The wrong filter can let through invisible infrared radiation that can seriously damage your eyes.

If you are making your own solar filter from proper film, such as supplied by Thousand Oaks Optical in the US, or from Baader if you are in the UK or Europe, follow their instructions to make sure it completely covers the front end of your telescope tube and is firmly fixed so that there is no danger of it blowing away.

Your telescope may have a front lens cap that has a smaller, capped opening within it. If so, you can use less solar film and just cover that opening within the lens cap.

A composite image from NASA’s Solar Dynamics Observatory, showing the track of Mercury across the Sun in May 2016. Image credit: NASA’s Goddard Space Flight Center/SDO/Genna Duberstein

Don’t forget to cover the front of any finderscope that you may have attached to your telescope to prevent it letting through light to burn a hole in your clothing while observing, and to prevent anyone trying to look through that too.

If you are using powerful binoculars, which are really just a pair of telescopes, make sure that you cover both front lenses with filters. Or you can put a lens cap on one and the filter on the other to observe with one eye.

The usual way to make sure that your telescope is pointed directly at the Sun when the filter is in place is to watch the telescope’s shadow on the ground. Point the scope towards the Sun and turn the telescope until the shadow reduces to a minimum.

Another traditional way to observe the Sun, including during partial eclipses and transits, has been to project its light through a telescope onto a sheet of white or black card. However this was during the days when telescopes and eyepieces were made of metal parts. Today many have plastic interiors and the magnified heat of the Sun could melt these, so we can no longer recommend this method.

If you are unable to view the transit from your own location for any reason, there will be some live streaming of the event, for example this one from the Griffith Observatory, Los Angeles.

We also have advice on how to observe Mercury at times other than during a transit.

Related: What to see in the night sky

Related: Ten tips for using a new telescope

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