Venus ends its evening spell with a steep fall in the west

Alan Pickup

Author: Alan Pickup
The maps show the sky at 01:00 BST on the 1st, midnight on the 16th and 23:00 on the 31st.

Given the lockdown, it is too much to hope that we could all enjoy clear views of the western and northwestern sky this month. If we do, then we can follow the dazzling evening star Venus as it plunges towards its inferior conjunction on the Sun’s near side on June 3. On the way, Venus and Mercury have a brief encounter, with Venus serving as the perfect guide to spotting its fainter sister planet.

Less ideal for stargazing in Scotland is the fact that Sun climbs 7° northwards during May to within 1.4°, or less than three Sun-breadths, of its latitude at our summer solstice. The result is that our nights contract and night-long twilight increasingly swamps the fainter stars. Edinburgh’s sunrise/sunset times change from 05:28/20:53 BST on the 1st to 04:36/21:46 on the 31st. The Moon is full on the 7th, at last quarter on the 14th, new on the 22nd and at first quarter on the 30th.

The Moon is close to first quarter on April 30, lying high in the south-west as the twilight fades and less than 4° above-left of the Praesepe (Beehive) star cluster in Cancer which is best viewed through binoculars. May 1 sees it above-right of Regulus, the leading star of Leo the Lion which squats high on the meridian (in the south) at nightfall as the Plough sails overhead. On the 5th, look for the Moon above-left of Spica in Virgo while the 8th finds it above-left of the strikingly red supergiant Antares in Scorpius, deep in our southern sky.

Our charts, for the period around midnight, show Leo dipping into the west as Antares gimmers low in the south-south-east. The brightest star visible at the time is Arcturus in Bootes, high in the south, which precedes the distinctive arc of Corona Borealis the Northern Crown. The latter is shaped more like a half-crown or diadem, with its main jewel sometimes called Alphecca or, more aptly, Gemma.

Venus is brilliant at magnitude -4.5 in our evening sky where it lies a few degrees below and right of Elnath, the star at the tip of the northern horn of Taurus. From 30° above Edinburgh’s western horizon at sunset on the 1st, it sinks to set at our northwestern horizon more than four hours later at 01:17. However, as it moves around the near side of its orbit, it stands lower each evening and sets earlier. Its altitude at sunset drops to 15° by the 20th and as little as 3° by the 31st when it is half as bright at magnitude -3.8 and all-but-disappeared into the Sun’s glare in the north-west.

Its distance, meantime, falls from 63 million to 43 million km, its disk inflates from 39 to 58 arcseconds in diameter, and the illuminated portion of its disk narrows from 24% to the merest sliver of 0.3%. Binoculars already show it as a crescent and, no doubt, some sharp-eyed individuals may claim to see this with their naked eyes before the month is out.

As it emerges from the Sun’s far side, Mercury lies 14° below and right of Venus on the 15th and climbs to shine at magnitude -0.6 when it passes 1.4° below Venus on the 21st. On the 24th, Mercury sits 5° above-left of Venus and the same distance to the right of the extremely slender earthlit Moon.

When Venus last made a precipitous exit from May’s evening sky, in 2012, Mars stood to the left of Regulus and Saturn was above-left of Spica. Now both Mars and Saturn are morning objects, albeit very low in our south-east where they are outshone by the giant world, Jupiter.

The latter rises in the south-east less than two hours after our map times and is conspicuous (magnitude -2.3 becoming -2.6) but only 10° high before dawn. It lies in Sagittarius while Saturn, fainter at magnitude 0.6 to 0.6, is 5° to its east (left) in Capricornus. Both are edging eastwards against the stars but, as they are overtaken by the swifter-moving Earth, they appear to reverse their progress at so-called stationary points on the 11th for Saturn and 14th for Jupiter. The waning Moon lies below-right of Jupiter on the 12th.

Mars rises in the east-south-east for Edinburgh at 03:48 on the 1st and by 02:26 on the 31st as its distance to the east of Jupiter increases from 25° to 45°. Although no higher than Jupiter before dawn, it does brighten this month from magnitude 0.4 to 0.0.

A month ago, we were getting somewhat excited by the prospect that Comet ATLAS, the final comet to be discovered in 2019, might become a spectacular object as it approached perihelion, its closest point to the Sun, on May 31. I warned, though, that it might disintegrate on the way and it is disappointing to report that it is doing so. As its fragments disperse, the comet has already fallen below easy binocular visibility.

Let us hope that the same fate does not befall Comet SWAN which was discovered by an instrument on the solar-observing SOHO spacecraft on March 25, although it was mid-April before we could be reasonably sure about its path. It reaches perihelion on May 27 when it is near Mercury’s orbit and 65 million km from the Sun. Becoming visible in our east-north-eastern predawn sky in mid-May, I suspect it may reach the fourth magnitude as it sweeps close to Algol in Perseus on the night of the 20th-21st and lies 2° right of Capella in Auriga as the month ends. Our maps show Algol and Capella low in the north but, even with binoculars, we may be lucky to spot the hazy smudge of Comet SWAN against the twilight.

Diary for 2020 May

Times are BST

  • 2nd 04h Moon 4° N of Regulus
  • 4th 23h Mercury in superior conjunction
  • 5th-6th Peak of Eta-Aquarids meteor shower
  • 7th 12h Full moon
  • 8th 23h Moon 6° N of Antares
  • 11th 10h Saturn stationary (motion reverses from E to W)
  • 12th 11h Moon 2.3° S of Jupiter
  • 12th 19h Moon 2.7° S of Saturn
  • 14th 15h Last quarter
  • 14th 19h Jupiter stationary (motion reverses from E to W)
  • 15th 03h Moon 2.8° S of Mars
  • 22nd 09h Mercury 0.9° S of Venus
  • 22nd 19h New moon
  • 24th 04h Moon 4° S of Venus
  • 24th 12h Moon 2.8° S of Mercury
  • 27th 20h Moon 2.0° N of Praesepe
  • 29th 10h Moon 4° N of Regulus
  • 30th 04h First quarter

This is an extended version, with added diary, of Alan’s article published in The Scotsman on 30 April 2020, with thanks to the newspaper for permission to republish here.