Elusive Mercury, the nearest planet to the Sun, can now be spotted in the western sky soon after sunset, on its way to greatest eastern elongation (19°) from the Sun on 11 April. This is Mercury’s best evening showing of the year and it joins brilliant Venus, itself in the midst of a marvellous evening apparition that has been wowing observers for some weeks now.
Mercury is never easy to locate, as it doesn’t stray far from the Sun’s glare and is only visible for a short time around dawn or dusk. However, during April Mercury appears about as far away from the Sun in the sky at mid-northern latitudes as it ever gets, meaning there’s a great chance to observe it soon after sunset until about the start of the last week of April.
Mercury now shines at around magnitude –1.1, as bright as it appears this month; the fleet-footed planet always shines at its brightest at the start of evening apparitions. Mercury lies roughly 12° high at sunset, though try looking for the planet about 35 minutes later when the sky has darkened considerably to aid detection. Mercury has an elevation of around 8° above the west to west-northwestern horizon at this time. As April wears on Mercury slowly climbs away from the horizon, though it fades in brightness as it does so.
At around the time of greatest elongation east on 11 April, Mercury achieves its highest altitude to twilight ratio to sit about 17° high at sunset (at about 7.50pm BST and 8.10pm from London and Edinburgh, respectively), though its faded to around magnitude +0.1. It shows a fat, 40.3 per cent illuminated crescent 7.7” in apparent diameter. A small telescope easily shows Mercury’s moon-like phases, which change rapidly as it speeds along its orbit. During any moments of steadier seeing in less-turbulent air, it may be possible to glimpse some vague light grey markings on Mercury’s rocky surface.