Brilliant Venus plunges into the evening twilight
Stargazers will be hoping for better weather as Orion and the stars of winter depart westwards in our evening sky, Venus dives into the evening twilight and around the Sun’s near side, while all the other bright planets are on view too. Indeed, Venus has the rare privilege of appearing as both an evening star and a morning star over several days, provided our western and eastern horizons are clear.
Orion still dominates our southern sky at nightfall as Leo climbs in the east and the Plough balances on its handle in the north-east. The Sun’s northwards progress and our lengthening days mean that by nightfall at the month’s end Orion has drifted lower into the south-west, halfway to his setting-point in the west. He is even lower in the west-south-west by our star map times when it is the turn of Leo to reach the meridian and the Plough to be almost overhead.
Leo’s leading star, Regulus, sits at the base of the Sickle of Leo, the reversed question-mark of stars from which meteors of the Leonids shower stream every November. The star Algieba in the Sickle (see chart) appears as a glorious double star through a telescope. Its components are larger and much more luminous than our Sun and lie almost 5 arcseconds apart, taking some 510 years to orbit each other. The pair lie 130 light years away and are unrelated to the star less than a Moon’s breadth to the south which is only half as far from us.
The Sun travels northward across the equator at 10:28 GMT on the 20th, the moment of the vernal (spring) equinox in our northern hemisphere. On this date, nights and days are of roughly equal length around the globe. Sunrise/sunset times for Edinburgh change from 07:04/17:47 GMT on the 1st to 06:46/17:49 BST (05:46/18:49 GMT) on the 31st after we set our clocks forwards to BST on the morning of the 26th. The lunar phases change from first quarter on the 5th to full on the 12th, last quarter on the 20th and new on the 28th.
Look for the young earthlit Moon well to the left of the brilliant magnitude -4.6 Venus on the 1st when telescopes show the planet’s dazzling crescent to be 47 arcseconds in diameter and 16% sunlit. Venus’ altitude at sunset plummets from 29° in the west-south-west on that day to only 7° in the west on the 22nd as its diameter swells to 59 arcseconds and the phase shrinks to only 1% – indeed, a few keen-sighted people might be able to discern its crescent with the naked eye and this is certainly easy to spot through binoculars.
Venus dims to magnitude -4.0 by the time it sweeps 8° north of the Sun and only 42 million km from the Earth at its inferior conjunction on the 25th. This marks its formal transition from the evening to the morning sky, but because it passes so far north of the Sun as it does every eight years or so, Venus is already visible in the predawn before we lose it in the evening. In fact, it is 7° high in the east at sunrise on the 22nd, and it only gets better as the month draws to its close.
Before Venus exits our evening sky, it meets Mercury as the latter begins its best spell as an evening star this year. On the 20th, the small innermost planet lies 10° to the left of Venus, shines at magnitude -1.2 and sets at Edinburgh’s western horizon 78 minutes after the Sun. By the 29th, it is 10° high forty minutes after sunset and shines at magnitude -0.4, easily visible through binoculars and 8° to the right of the very young Moon.
Mars, near the Moon on the 1st and again on the 30th, dims from magnitude 1.3 to 1.5 this month as it tracks from Pisces into Aries. By the month’s end, it lies to the left of Aries’ main star Hamal and sets at our map times. It is now more than 300 million km away and its disk, less than 5 arcseconds across, is too small to be of interest telescopically.
The Moon has another encounter with the Hyades star cluster on the night of the 4th-5th, hiding several of its stars but setting for Scotland before it reaches Taurus’ main star Aldebaran. The latter, though, is occulted later as seen from most of the USA. The Moon passes just below Regulus on the night of the 10th-11th and meets the planet Jupiter on the 14th.
Jupiter, conspicuous at magnitude -2.3 to -2.5, rises in the east at 21:37 GMT on the 1st and only 31 minutes after Edinburgh’s sunset on the 31st. Now edging westwards above the star Spica in Virgo, it is unmistakable as it climbs through our south-eastern sky to cross the meridian in the small hours and lie in the south-west before dawn. Its disk, 43 arcseconds wide at mid-month, shows parallel cloud bands through almost any telescope, while its four moons may be glimpsed through binoculars as they orbit from one side to the other.
Saturn, the last of the night’s planets, rises in the south-east at 03:44 GMT on the 1st and almost two hours earlier by the 31st. Improving very slightly from magnitude 0.5 to 0.4 during March, it is the brightest object about 10° above the south-south-eastern horizon before dawn. Look for it 4° below-left of the Moon on the 20th.
This is a slightly-revised version of Alan’s article published in The Scotsman on February 28th 2017, with thanks to the newspaper for permission to republish here.
Mercury joins two brightest planets in evening sky
Spring may have arrived, but the leading constellation of our winter sky, Orion, is still on view in our early evening sky, if not for very much longer. Look for it in the south-west at nightfall, with the three stars of his Belt lying almost parallel to the horizon. Stretch their line to the left to reach Sirius in Canis Major, our brightest nighttime star, and to the right towards Taurus, with its bright star Aldebaran and the Pleiades star cluster. High in the south-south-west is Gemini, with the twins Castor and (slightly brighter) Pollux.
To the south of Gemini is Procyon, the Lesser Dog Star in Canis Minor. Together with the true Dog Star, Sirius, and the distinctive red supergiant Betelgeuse at Orion’s top-left shoulder, Procyon completes an almost-isosceles triangle which we dub “The Winter Triangle”. At present, in fact, it forms a similar but smaller triangle with Pollux and the conspicuous planet Jupiter which dominates our southern sky at nightfall. Leo stands to the left of Jupiter with its leading star, Regulus, in the handle of the Sickle.
As April’s days lengthen, our whole sky-scape shifts further westwards each evening until, by the month’s end, Orion is setting in the west as the sky darkens
The Sun climbs more than 10° northwards during April and sunrise/sunset times for Edinburgh change from 06:44/19:50 BST on the 1st to 05:32/20:49 on the 30th. Meanwhile, the duration of nautical twilight at the start and end of the night stretches from 84 to 105 minutes.
The Moon is full on the 4th when a total lunar eclipse is visible from the Pacific and surrounding areas but not from Europe. In fact, totality, with the moon just inside the northern part of the Earth’s dark umbral shadow lasts for a mere 4 min 34 sec centred at 13:00 BST, making this the briefest total lunar eclipse for 486 years. By comparison, totality lasts for 72 minutes during the next total lunar eclipse which is visible from Scotland on the morning of 28 September this year. The Moon’s last quarter on the 12th is followed by new moon on the 18th and first quarter early on the 26th.
By nightfall on the 26th, that first quarter Moon lies 6° below Jupiter in the south-south-west. Jupiter, itself, dims a little from magnitude -2.3 to -2.1 and is slow-moving in Cancer 5° to the left of the Praesepe cluster in Cancer. The star cluster is best seen through binoculars which also show the changing positions of the four main Jovian moons as they swing from side to side of the planet. The giant planet progresses into the south-west by our star map times and sets in the north-west more than five hours later.
Even though Jupiter is twice as bright as Sirius, it pales by comparison with the evening star Venus which blazes brilliantly in the west at nightfall, and sets at Edinburgh’s west-north-western horizon by 23:38 BST on the 1st and in the north-west as late as 01:08 on the 30th. This month Venus approaches from 180 million to 150 million km and swells in diameter from 14 to 17 arcseconds, its dazzling disk appearing gibbous through a telescope as its phase changes from 78% to 67% illuminated.
Venus also speeds eastwards during the period, moving from Aries to Taurus where it passes 2.7° south of the Pleiades on the 11th to end April 3° south of Elnath at the tip of the Bull’s northern horn. There should be an impressive sight on the evening of the 21st when Venus lies 7° above-right of the earthlit crescent Moon which, in turn, is 2.5° above-left of Aldebaran.
Mars, magnitude 1.4, is low and hard to spot in our western evening twilight, becoming lost from view later in the month as it tracks towards the Sun’s far side. However, after passing beyond the Sun at superior conjunction on the 10th, Mercury emerges in our twilight to begin the innermost planet’s best evening apparition of the year.
On the 19th, Mercury shines at magnitude -1.3 and stands 4° high in the west-north-west forty minutes after sunset. Mars lies 2.8° above and to its left while the sliver of the earthlit Moon is 12° high and to their left. By the month’s end Mercury is 10° high forty minutes after sunset and shines at magnitude -0.4 22° below and to the right of Venus and only 1.7° below-left of the Pleiades. Binoculars may help to pick it out at first but it should emerge as a naked-eye object as the twilight fades and it sinks to the north-western horizon by 23:00.
Saturn is on show during the second half of the night though it does not climb far above our horizon so is not well placed for the sharpest views of its stunning ring system. For Edinburgh, it rises in the south-east at 00:46 on the 1st and by 22:40 on the 30th, reaching its highest point of 15° in the south four hours later before dawn.
Shining at magnitude 0.3 to 0.1, Saturn lies in Scorpius where it creeps 1.5° westwards above the double star Graffias. As it lies below and left of the Moon on the 8th, a telescope shows the planet’s rotation-flattened disk to be 18 arcseconds wide, within rings that span 41 arcseconds and have their northern face tilted Earthwards at 25°. Although they show an amazing complexity on the small scale, the main rings, dubbed A and B, are separated by the relatively empty dark arc of the Cassini Division. B, the brightest of the rings, has A outside it and the dusky C ring within.