Mercury is a planet that many people never see, despite being bright, because, being the innermost planet, it rarely escapes the Sun’s glare. But the run-up to Christmas presents an excellent opportunity to see it in the morning sky before sunrise.
The planet reaches one of its so-called elongations on December 23 when it appears at its furthest from the Sun due to the geometry of its orbit and the Earth’s. On this date, Mercuy will lie 22° west of the Sun and it will rise nearly two hours before it from northern countries such as much of the US, the UK and Europe.
Due to the long nights in mid-winter, many of us will be getting up while it is still dark or dawn is beginning to break, so do take the opportunity to look out to the south-east before about 7am local time to find this elusive planet.
You will need a clear horizon, so low cloud will easily mask it or you could find houses, trees or other obstructions in the way. But if you have an unrestricted view and good weather, Mercury should not be difficult to spot, shining at a healthy magnitude -0.4 in the constellation of Scorpius. (You probably will not see this constellation’s stars!)
I got my first glimpse easily from south-east England on the morning of December 18, and the sky chart here, produced using Stellarium, shows how it will appear, with the Moon not too far away on the morning of December 21. 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 the planet’s Moon-like phases but do not expect to see any surface detail.
Mercury is the closest planet to the Sun, at an average distance of 58 million km, and it is a rocky world like Earth. Its surface is mountainous and heavily cratered, like the Moon, because the planet has only a hint of an atmosphere.
In the evening sky, you can now find the second innermost planet, Venus, which is steadily moving away from the Sun. For more details of what is on view in the sky right now, see our monthly sky map.