Mercury puts on a good show in the evening sky

Mercury, the closest planet to the Sun, is well placed for northern hemisphere observers at the moment and can be seen above the western horizon after sunset.

Mercury and Mars
Mercury is the bright object below centre, just above an anvil-shaped thunder cloud, in this photo, taken on 1 April, 2017, from near Faversham, Kent, in the UK. Mars is visible too, top left. Image credit: Paul Sutherland
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And while Mercury is never easy to spot, because it can never stray too far in the sky from the Sun, and so is always in a bright sky, it is higher than it would be at other times of the year. That is because the ecliptic – the imaginary line in the sky that marks the orbital plane of the planets – is at a particularly steep angle from northern latitudes at this time of year.

The moment when Mercury is at its farthest apparent distance from the Sun in the sky is termed an elongation. In the evening sky, it is called an eastern elongation, because the planet lies to the east of the Sun. Greatest elongation east occurs on 1 April, when Mercury will lie 19° from the Sun and shine at around magnitude 0. The elongation is not as great as some, because Mercury has an eccentric orbit around the Sun and is physically closer to it than at other times. However, as the sky darkens, you may see it as much as 10° above the horizon as the twilight fades.

Mercury, Mars and the Moon
How Mercury, Mars and the Moon will line up in the western sky on the evening of 31 March, 2017, from mid-northern latitudes. Image credit: Skymania using Stellarium

As March comes to an end, the young Moon has been passing close by. Tonight, on the 31st of March, it can be found to the upper left of Mercury, as shown in our main image. Between the two you will find another planet, Mars, now shining at just magnitude 1.5, a far cry from the time of opposition last May when it was a brilliant magnitude -2.2.

More about Mercury

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By Paul Sutherland

Paul Sutherland has been a professional journalist for nearly 40 years. He writes regularly for science magazines including BBC Sky at Night magazine, BBC Focus, Astronomy Now and Popular Astronomy, plus he has authored three books on astronomy and contributed to others.

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