2016 April 8
See Mercury in the evening sky
Mercury has a reputation for being a difficult planet to spot with the naked eye and it was said that Copernicus never managed to see it. The difficulty arises not because it is faint, it isn’t, it can be brighter than Sirius, but because being so close to the Sun it is only visible for a limited time and is best seen around greatest eastern or western elongation. On April 18 it reaches greatest eastern elongation for northern hemisphere observers and is therefore at its most prominent in the western sky after sunset for a week or so before and after that date. So, if you have never seen Mercury, now is the ideal time to track it down. Of course you will need a clear horizon around azimuth 281 degrees (west of west-north-west). A clear sky is obviously ideal although Mercury is frequently bright enough to be visible through thin haze. Sweeping around with binoculars can help locate the planet, but do make sure the Sun has fully set before starting your hunt. From mid-UK the Sun sets around 20:00 BST on April 18 with the end of civil twilight 35 minutes later.
It might be thought that the best time to see Mercury would be when it is at its greatest altitude but there are other factors to consider. Although it will then be high in the sky and so clear of possible obstructions, it will not be at its brightest as its phase and apparent diameter change as its distance from Earth varies. For example, at 20:00 BST on April 12 it will shine at mag -0.6 showing a phase through a telescope 59% and a diameter of 6.7arcsec, whereas on April 18, although higher in the sky, it will only shine at mag -0.1 with its phase now reduced to 38% although its diameter has increased to 7.8 arcsec. On April 25, when still at an altitude of 18 degrees, it will only shine at magnitude +1.5 as its phase has now become a thin crescent of 18% although its diameter has increased to 9.5 arcsec.
This means that it will probably – weather and horizon obstructions considered – be easier to spot Mercury before greatest elongation. If you have a telescope it is always fun to watch its phase change during these few days although don’t expect to see surface detail visually unless you have a large telescope and good seeing. Patrick Moore, writing in his book Practical Amateur Astronomy, regarded his 12.5inch reflector as too small to see any detail and thought apertures of at least 18-inches were necessary. A simple fun project, certainly for imagers, would be to record the change in phase and diameter around the period of greatest elongation.
If you fail to spot Mercury this time there will be another good opportunity in the morning sky in the autumn.