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 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.

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.

Mercury photographed low over Ripple mill, UK, in February, 2019. Image credit: Paul Sutherland

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.

Here is more information on how to see a transit of Mercury. You can also read more about Mercury in our article.

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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