How to observe elusive Mercury

Many people go through life without ever seeing Mercury, even though it shines as brightly as some of the brightest stars in the sky. That is because it never gets far enough away from the Sun to be seen in a dark sky.

Mercury in evening sky
Mercury in the evening sky, photographed at Sandwich, Kent. Credit: Paul Sutherland

You have to look for it when it gets furthest from the Sun, as seen from the Earth, which means either during dusk in the evenings, or just before dawn when it is in the morning sky. Even at one of it better apparitions, when it gets 28° from the Sun, it will still be low over the horizon, and so may be lost in murk or cloud.

The times to look for Mercury are when it reaches its eastern elongation after sunset or western elongation before sunrise. But one elongation is not the same as another. Best times to look are when the celestial ecliptic – the imaginary line through the sky along which the Sun and planets appear to travel – is steepest to the horizon.

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Even when it does make one of its favourable appearances, at the time of what astronomers call its greatest elongation from the Sun, it will still be close to the horizon so that low cloud or inconvenient buildings can block it from view.

Binoculars will help you to pick it out – but never look with them while the Sun is still above the horizon. Because it zips round the Sun so quickly, Mercury’s favourable appearances do not last long, so spot it while you can! A telescope will show that the planet exhibits Moon-like phases but do not expect to see any surface detail.

Transit of Mercury
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

Very occasionally, Mercury also reveals itself when it makes a transit across the face of the Sun. The usual essential safety precautions must be taken on such occasions and will reveal the planet as a tiny dot against the solar disk. The last Transit of Mercury on May 9, 2016 and the next will be on November 11, 2019.

Transits of Mercury can occur in May or November, when the planes of our two planets’ orbits intersect at what are known as the nodes. Intervals of 13 and 33 years separate May transits, and intervals of 7, 13 and 33 years, November transits.

Mercury is too small to be seen using eclipse glasses, and a telescope will be needed to show its black dot silhouetted against the Sun’s disk. On no account look through a telescope at this, unless it is fitted with a professional-standard filter. Read more about Mercury in our article.

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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.