The Planets in April and May 2020

Mercury starts this period as a morning object but is very hard to observe for the northern hemisphere, especially from the UK. It rises only 20 minutes before the Sun in early April and is drawing rapidly closer to it as the month progresses. From the 3rd to the 5th Mercury passes little more than a degree below Neptune but the pairing will only be well seen in the southern hemisphere. Mercury passes behind the Sun (Superior Conjunction) on the 4th of May then rapidly moves into the evening sky.

Greatest eastern evening elongation from the Sun is not until early June but the planet is very well placed for UK observation from mid-May onwards, initially sitting mid-way between the Sun and Venus, then rapidly moving closer to that much brighter planet. On the 21st and 22nd the pair are very close with Venus dazzlingly bright at magnitude -4.3. Mercury will be just below Venus on the 21st and just east on the 22nd, at a considerably dimmer magnitude of -0.6. By the end of May the pair have split, with Mercury still at around 10 degrees of elevation in the North-West just as Venus is setting.

Venus itself dominates the western sky early in April, becoming visible shortly after sunset at nearly 40 degrees of elevation. Initially it sits just below the Pleiades star cluster (M45) and the pairing will offer a tempting photographic target as twilight deepens. Since Venus is past its greatest elongation it will move steadily closer to the Sun as the period progresses but is still very well placed for observation from the UK and can just be followed in darkening skies until the last week of May. In this period its apparent size, from pole to pole, will grow from 26 to 57 arc seconds and its illuminated phase falls from 47% to a tiny 1%, but extreme car will be needed to view it in late June due to the dangerous proximity of the Sun. Catch Venus in early April giving a brilliant and fascinating display.

Mars is a morning object, steadily brightening from magnitude +0.8 to magnitude 0.0 in this period and will appear extremely red; at least in part due to its low elevation as seen from the UK; Mars suffers from the low angle the ecliptic makes with the horizon early on spring mornings. On the 1st of April Mars rises in the south-east at around 0340 UT and sits just below Saturn, which is a full magnitude brighter. By sunrise the pair has gained only around 13 degrees of elevation and will only gain 4 more by south-transit in full daylight.

By mid-period Mars is rising around 0240 UT and gains 15 degrees of elevation by sunrise some two hours later. If you can follow it into daylight it improves to over 20 degrees of elevation at south transit, and by the end of May this improves to nearly 30 degrees, transiting at 0630 UT. By this time Mars will have grown from 6 to just over 9 arcseconds in apparent size and will start to show significant detail in good seeing conditions.

Jupiter dominates the morning sky, rising 20 minutes before Saturn in early April (at 0315UT) and 15 minutes before in late May, by which time the pair both rise a little before midnight UT. Like Mars, Jupiter and Saturn gain little elevation by sunrise but are both much easier to follow into full daylight. South transit for Jupiter is at 0712 UT with 16 degrees of elevation in early April, 0525 UT mid period with 17 degrees and 0325 UT by late May, still at 17 degrees of elevation; correct for mid UK latitudes: Saturn follows at similar elevations a few minutes later. Jupiter brightens somewhat, from magnitude -2.1 to -2.6 in the period and grows from 37 to nearly 45 arcseconds in apparent size.

In the same period Saturn brightens a little from +0.7 to +0.6 in magnitude with its famous rings tilted towards us by 20.5 degrees. This is the most ‘closed’ the rings will be for 2020 but they will still clearly show structure. Look particularly around the 21st of April when Saturn is at West Quadrature (that is, the Sun, Earth, Saturn angle is at 90 degrees). At this time the shadow cast by the planet on the section of rings behind it may be particularly obvious.

The outer ‘Ice-Giants’, Uranus and Neptune, are effectively unobservable from the UK with Uranus in conjunction (behind the Sun) on the 26th of April and Neptune lost to morning twilight.

Alan Clitherow