Watch as Mercury meets Venus and Mars before dawn

One of the best opportunities to see Mercury from mid northern latitudes, including Europe and North America, occurs over the next few days. You will also be able to see Venus and Mars nearby.

A view of the planetary gathering as it appeared on the morning of 11th September, 2017, from Walmer, UK. Venus is top right, Mercury below centre, under the star Regulus. Mars might just be glimpsed to Mercury’s lower left. Image credit: Paul Sutherland

The innermost planet will be as far from the Sun as it can appear – known as greatest elongation. And it will appear higher in the sky than at other times because the plane of the planets’ orbits, known as the ecliptic, is steeply inclined to the horizon in September.

It means that Mercury can get as much as 10° above the eastern horizon before the sky becomes too bright to observe it.

Greatest elongation west, as a morning extreme is known, occurs on 12 September, 2017, when the planet will be nearly 18° away from the Sun. After that, it begins steadily to close back in towards the Sun as it zips around it in its 88-day orbit.

Mercury will be just brighter than zero magnitude at best, which is as bright as some of the brightest stars, such as Vega and Capella. It would be easier to see if it were high up in a dark sky rather than low down in the twilight.

To find Mercury, you will need a good eastern horizon, unobscured by trees, hills or buildings. Start looking at around 5.30 am local time. A guide to help you will be the much more brilliant second planet, Venus, which will lie about 10° above, and slightly to the right, of Mercury.

How Mercury, Mars and Venus will appear before dawn on the morning of 14th September. Image credit: Skymania using Stellarium

Binoculars will help you to locate Mercury, especially if there is any murk close to the horizon. But in a clear sky, you should then be able to see it with just your eyes once you know where to look.

As well as being reasonably near Venus in the sky, Mercury will also make some very close encounters with another planet, Mars, and the first magnitude star Regulus in the constellation of Leo.

On the morning of Sunday 10th March, 2017, Mercury will pass just 39 arc minutes (or a little over a Moon’s width) from Regulus. You should see them both easily with binoculars or in the field of view of a telescope with a low power eyepiece.

This close-up shows Mercury below the bright star Regulus, upper right, and Mars at centre, above the anchored ship off Walmer, UK. Image credit: Paul Sutherland

On the 16th and 17th September, Mercury comes even closer in the sky to planet Mars. This is purely a line-of-sight effect, of course, as they are distantly separated in the Solar System.

On Saturday 16th Mars passes incredibly close to Mars, which will be shining at a much fainter magnitude 1.8. At closest, they will be just 3.5 arc minutes apart, a phenomenon best seen from the mid Pacific! Before dawn that day from the UK, the separation will have been nearly 30 arc minutes, or a Moon’s width.

They will be a lot closer by dawn over the east coast and then the west coast of the USA. Next morning from the UK, on the 17th, the separation will have reached 20 arc minutes and be steadily increasing again.

Another nice photo opportunity comes a day later, on the morning of 18th September, when the waning crescent Moon will appear in line with Mercury, Mars and Venus.

Related: How to observe elusive Mercury

Related: Mercury – closest planet to the Sun

Related: What’s in the night sky this month


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