Starwatch: as the sun passes solstice elusive Mercury comes into view

Mercury will be as far to the east of the sun as it ever gets, and can be found low in the western sky just after sunset

The summer solstice for northern observers occurs on 21 June at 16:54 BST. This is the moment at which the sun will be at its highest position in the sky, giving the longest period of daylight. In the southern hemisphere, the opposite will be true. The sun will reach its lowest altitude and give the shortest period of daylight. Also this week, keep an eye open for inner-planet Mercury. The latter half of June offers a good chance of seeing this tricky-to-spot planet because it is as far to the east of the sun as it can get. It is visible for about 90 minutes following sunset. The chart shows the situation looking west at 22:00 BST on 23 June. You will need a very good horizon. A pair of binoculars might also help locate the planet, and may show you faint Mars too, but remember never use binoculars if the sun is still visible above the horizon. Permanent eye damage can occur.