Mercury is visible now in the early evening sky, offering sky-watchers in the UK and at other mid-northern latitudes their best chance of the year to spot the innermost planet. Furthermore, on the early evening of 13 May there’s the added bonus of a skinny crescent Moon on the scene just below and to the left of Mercury, sporting a mere 3.7 percent-illuminated phase. A cherry on this topping is provided by dazzling Venus, presently hovering a few degrees above the west-north-western horizon.
Though Venus will perceptively improve in visibility as the month wears on, this apparition of Venus will remain a poor one from UK shores.
One important caveat is that to get a good view of Mercury and its companions you’ll need to view across a reasonably unobstructed horizon. Mercury, the most elevated of the trio, sits at an altitude of just 17 degrees at sunset (at about 8.40pm BST from London), sinking to around 10 degrees by the end of civil twilight (when the Sun lies at an altitude of less than six degrees) at about 9.25pm BST. There’s probably not an insignificant percentage of star-gazers who have yet to glimpse the shy and elusive planet, though Mercury can get surprisingly bright and can easily be seen without optical aid.
Since the beginning of May, Mercury has been gradually emerging from the Sun’s glare and will reach greatest elongation (22°) east of the Sun on 17 May; around this date, the planet will be at its highest for the month in the west-north-western sky.
Soon after sunset on 13 May, Mercury shines at magnitude +0.09 and from a clear western vista can be located, hopefully without too much difficulty, at around 10 degrees up (hold your fist held out at arm’s length; across your knuckles measures 10 degrees) in the north-north-west (use your smartphone’s compass to line up Mercury at an azimuth of around 296 degrees). A pair of 10×50 binoculars could come in very handy if sky conditions are a little hazy or there’s thin cloud around. Remember, NEVER sweep for Mercury (or indeed any astronomical body lying close to the Sun) with binoculars or a telescope until you are absolutely certain that the Sun is still below the horizon or has set.
A small telescope can show Mercury’s moon-like phases; how clearly depends largely on prevailing seeing conditions. Mercury’s phase is a first-quarter mimicking 45.5 per cent. By the evening of greatest elongation, the planet will have faded to around magnitude +0.5 and exhibit a fat, 35-per cent-illuminated crescent phase. Try using an orange or a light-red filter (Wratten 21 or 23A), the former of which has 50 per cent light transmission and produces a brighter though less-effective view. Red or deep-red filters (Wratten 25 or 29) are more useful for large-aperture telescopes.
Mercury and Venus can be imaged through moderate- to large-aperture telescopes (say, above 150mm [six-inches] in aperture) when equipped with either an infrared (IR) or ultraviolet (UV) filter. These will help improve the visibility of dark markings on both planets (surface detail on Mercury and transient atmospheric [cloud] structure on Venus) and reduce the adverse effects of poor seeing.