Mercury, the smallest planet

Mercury, the smallest planet
  • Mercury is also the closest planet to the Sun and is therefore very difficult to spot

Venus has been dazzlingly bright in the evening sky for the last few months but it has now passed in front of the Sun and reappeared in the morning sky, rising before the Sun. In its place now is the tiny planet Mercury.

Mercury is the smallest planet in the solar system, it is also the closest planet to the Sun and, being so close and so small, it is usually very difficult to spot. But tonight, 31 March, and for the next few nights, Mercury will be at its most distant point from the Sun and should be easy to spot if you have a clear view of the western horizon. When Mercury and Venus are at their most extreme point away from the Sun it is known as greatest elongation; when the planet is visible after sunset it is known as greatest eastern elongation; and when it appears before sunrise it is known as greatest western elongation.

Because this greatest eastern elongation of Mercury comes so close to the spring equinox it means that Mercury will set directly above the Sun and will remain visible for almost an hour and a half after the Sun has set. Look towards the western horizon about half an hour after the Sun has gone down and you should be able to spot the tiny pink-coloured 'star'; at this elongation it will be 19° from the Sun (a closed fist at arm's length is about 10°). You can use binoculars to help you find it in the twilight sky and once you have found it you'll be surprised at how bright it can appear - equal to the brightest star Sirius. If you're still having trouble spotting it tonight then find the crescent moon and follow a line to where the Sun sets; you'll pass Mars along the line and Mercury will be at the end of the line. But you will have to be quick, Mercury races around the Sun taking only 88 days to complete one orbit so each evening after tonight it will rapidly get closer to the Sun until it is eventually lost in the glare. It is this rapid movement of Mercury that inspired the Greeks to name the planet after the Greek messenger to the Gods. It travels around the Sun at an incredible 30 miles per second! Interestingly, it takes Mercury 59 Earth days to rotate once on its axis, so all parts of its surface experience periods of intense heat and extreme cold. Although its mean distance from the sun is only 36 million miles, Mercury experiences the greatest range of temperatures: over 480°C on its dayside and -149°C on its nightside.