Looking Forward to the Planets in April and May 2021

Mercury starts April moving towards superior conjunction behind the Sun, reaching this point on the 19th. It can best be seen in the dark morning skies of the southern hemisphere before conjunction but afterwards moves on to the best evening apparition of the year for the north. Greatest western elongation from the Sun is on the 17th of May and on that day Mercury will start to be visible at some 15 degrees of elevation in the west-north-west from around 20.15 Universal Time (UT). It may be hard to pick out in the light sky of early evening but Venus will be easy to find close by at the same time; scan on a 45 degree angle up and left from Venus to find Mercury. This advice and timing holds good from early May through until the 28th of May when Mercury sits just to the left of Venus, still more than 10 degrees above the horizon from mid-UK latitudes; a very photogenic conjunction.

Venus itself is best seen in May as it is too close to the Sun in early April for safe observation. By late April it starts to put on a good evening show for the southern hemisphere but stays quite close to the sinking evening ecliptic from the UK. In late May Venus passes Mercury and is very obvious as a low evening ‘star’ in the north-west after sunset.

Mars is best seen early in the period, starting April in the horns of Taurus above Aldebaran. Visible from around 19.15 UT Mars will initially be some 40 degrees above the horizon, a little south of due-west, thereafter setting in the north-west after midnight. The eastward, prograde, motion of Mars against the background stars puts it in Gemini by early May and helps to maintain its visibility, appearing from 20.15 UT but around 10 degrees lower than early April. Mars sets after midnight UT until mid-May and will appear as a bright red spark fading slightly from magnitude +1.3 to +1.7 in the period; it shrinks below 5 arcseconds in size in mid April.

Jupiter is an early morning object for the UK, following Saturn across the eastern sky in twilight. Twice in its slow orbit around the Sun, roughly every 5 years, the orbits of the Moons of Jupiter are seen exactly edge on from Earth. This will be the case in April and May leading to a series of ‘mutual events’ when one moon is eclipsed by the shadow of another or is directly occulted by it. A list of these events can be found at:-
https://www.cambridge.org/turnleft/pages/whats_up_tonight/the_best_2021_galilean_satellite_mutual_events
It must be said the UK is not well placed to see them; try looking around 03.27 UT on May 6th when Io eclipses Ganymede and 03.44 UT on May 14th when Io eclipses Europa. Both events happen low in the south-east. Jupiter rises around 04.30UT in early April, 02.45UT in early May and at 00.50UT by the end of the period, by which date it will gain 20 degrees of altitude above the horizon by sunrise.

Saturn rises some 35 minutes ahead of Jupiter but gains slightly less altitude; like Jupiter it will be much better placed for southern observers but both planets do gain more height above the horizon for northern observers than last year and are well worth seeking out. Saturn is at west quadrature on May 3rd when the Saturn – Earth – Sun positions form a right-angle. Its beautiful rings close to a minimum for the year on the 20th of May, tilted towards us by 16.7 degrees, and then open slightly as the year progresses.

Uranus starts this period only 15 degrees from the Sun and may just be glimpsed in early evening twilight in early April. Conjunction with the Sun happens on April 30th and thereafter Uranus is lost to morning twilight for the rest of May. Neptune will be past solar conjunction but is little better placed than Uranus. In late May in might just be visible in early morning twilight, rising at 01.30UT, but is a more realistic target later in the year.

Alan Clitherow.