March Night Sky

March provides the best opportunity this year to see Mercury in the evening sky for us northerners. Mercury is never very far away from the sun in the sky, so for evening appearances you have to catch it after the sun has set, but before Mercury does. The geometry of our position on the earth’s surface, and how that relates to the ecliptic (the plane in which  that the planets of the solar system orbit) means that the best opportunity for this is around March. This is when the sun is “moving” northwards in our sky at the maximum rate, and a planet setting after the sun is therefore approximately where the sun will be in a few weeks time, and therefore further north. That means it will be that much higher in our sky when the sun sets. For similar reasons, the best time to see Mercury at its morning appearances is in September. This is all much easier in the Tropics, where the Sun sets, and rises, more or less vertically. (That is why night falls so quickly in these parts of the world.) Mercury has been known in European cultures since antiquity because it is easy enough to spot at most apparitions around the Mediterranean Basin. It would be interesting to know if any cultures at around our latitude discovered it independently!

The last week of March is the best time. The chart below shows the Western Horizon on 29 March just after sunset, at about 7.45 BST. Mercury is quite bright in Magnitude terms (-0.1 – brighter than any star apart from Sirius), but is also in a bright sky, so will be difficult to spot. The attraction of looking on that day is that a very young moon will be showing a very slender crescent a few degrees to the South. (This, too, will be hard to spot in the bright sky.) But as the sky darkens, both should reveal themselves over the next half an hour or so.

Mercury in March 2017



Uranus is not far away, but will be too faint to see in the bright sky. Mars should be visible, however, as shown on the map.

By the very end of March, Venus, which has been shining brightly in our evening sky, will finally have overtaken earth in its orbit to reach “inferior conjunction”, when it passes broadly between the earth and the sun. On this occasion, it will pass a long way north of the sun, and this has an intriguing effect. For about the middle two weeks of March, Venus will be visible both in the evening sky after sunset, and in the morning sky before sunrise. If you get a chance to look at it with any kind of optical aid, even binoculars, you will see a very slender crescent. Only to be tried after sunset or before sunrise!