Elusive Mercury is second evening star alongside Venus
Orion is striding proudly across the meridian as darkness falls, but, even before the twilight dims, we have our best chances this year to spot Mercury low down in the west and close to the more familiar brilliant planet Venus.
Both evening stars lie within the same field-of-view in binoculars for much of March, so the fainter Mercury should be relatively easy to locate using Venus as a guide. Provided, of course, that we have an unobstructed horizon. Mercury never strays far from the Sun’s glare, making it the most elusive of the naked-eye planets – indeed, it is claimed that many astronomers, including Copernicus, never saw it.
Blazing at magnitude -3.9, Venus hovers only 9° above Edinburgh’s western horizon at sunset on the 1st and sets 64 minutes later. Mercury, one tenth as bright at magnitude -1.3, lies 2.0° (four Moon-breadths) below and to its right and may be glimpsed through binoculars as the twilight fades. Mercury stands 1.1° to the right of Venus on the 3rd and soon becomes a naked eye object as both planets stand higher from night to night, becoming visible until later in the darkening sky.
By the 15th, Mercury lies 4° above-right of Venus and at its maximum angle of 18° from the Sun, although it has more than halved in brightness to magnitude 0.2. The slender young Moon sits 5° below-left of Venus on the 18th and 11° above-left of the planetary pairing on the 19th. Earthshine, “the old Moon in the new Moon’s arms”, should be a striking sight over the following few evenings.
On the 22nd, the 30% illuminated Moon creeps through the V-shaped Hyades star cluster and hides (occults) Taurus’ leading star Aldebaran between 23:31 and 00:14 as they sink low into Edinburgh’s west-north-western sky.
Falling back towards the Sun, Mercury fades sharply to magnitude 1.4 by the 22nd when it passes 5° right of Venus and becomes lost from view during the following week. At the month’s end, Venus stands 15° high at sunset and sets two hours later.
The Sun climbs 12° northwards in March to cross the sky’s equator at the vernal equinox at 16:15 on the 20th, which is five days before we set our clocks forward at the start of British Summer Time. Sunrise/sunset times for Edinburgh change from 07:04/17:47 GMT on the 1st to 06:46/19:49 BST (05:46/18:49 GMT) on the 31st. The Moon is full on the 2nd, at last quarter on the 9th, new on the 17th, at first quarter on the 24th and full again on the 31st.
Orion is sinking to our western horizon at our star map times while the Plough, the asterism formed by the brighter stars of Ursa Major, is soaring high in the east towards the zenith. To the south of Ursa Major, and just reaching our meridian, is Leo which is said to represent the Nemean lion strangled by Hercules (aka Heracles) in the first of his twelve labours. Leo appears to be facing west and squatting in a similar pose to that of the lions at the foot of Nelson’s Column in Trafalgar Square.
Leo’s Sickle, the reversed question mark that curls above Leo’s brightest star Regulus, outlines its head and mane and contains the famous double star Algieba whose two component stars, both much larger than our Sun, take more than 500 years to orbit each other and may be seen through a small telescope. Regulus, itself, is occulted as they sink towards Edinburgh’s western horizon at 06:02 on the morning of the 1st.
Jupiter, easily our brightest morning object, rises at Edinburgh’s east-south-eastern horizon at 00:47 GMT on the 1st and at 23:41 BST (22:41 GMT) on the 31st, climbing to pass around 17° high in the south some four hours later. Brightening from magnitude -2.2 to -2.4, it is slow moving in Libra, being stationary on the 9th when its motion reverses from easterly to westerly. Jupiter is obvious below the Moon on the 7th when a telescope shows the Jovian disk to be 40 arcseconds wide.
If we look below and to the left of Jupiter in the south before dawn, the three objects that catch our attention are the red supergiant star Antares in Scorpius and, further from Jupiter, the planets Mars and Saturn.
Mars lies in southern Ophiuchus, between Antares and Saturn, and is heading eastwards into Sagittarius and towards a conjunction with Saturn in early April. The angle between the two planets falls from 17° to only 1.5° this month as Mars brightens from magnitude 0.8 to 0.3 and its distance falls from 210 million to 166 million km. Mars’ disk swells from 6.7 to 8.4 arcseconds, becoming large enough for surface detail to be visible through decent telescopes. Sadly, Mars (like Saturn) is so far south and so low in Scotland’s sky that the “seeing” is unlikely to be crisp and sharp.
Incidentally, on the morning of the 19th Mars passes between two of the southern sky’s showpiece objects, being a Moon’s breadth below the Trifid Nebula and twice this distance above the Lagoon Nebula. Both glowing clouds of hydrogen, dust and young stars appear as hazy patches through binoculars but are stunning in photographs.
Saturn, creeping eastwards just above the Teapot of Sagittarius, improves from magnitude 0.6 to 0.5 and has a 16 arcseconds disk set within its superb rings which span 37 arcseconds at midmonth and have their northern face tipped towards us at 26°. The waning Moon lies above-left of Mars on the 10th and close to Saturn on the 11th.
Diary for 2018 March
Times are GMT until March 25, BST thereafter.
1st 06h Moon occults Regulus (disappears at 06:02 for Edinburgh)
2nd 01h Full moon
4th 14h Neptune in conjunction with Sun
5th 18h Mercury 1.4° N of Venus
7th 07h Moon 4° N of Jupiter
9th 10h Jupiter stationary (motion against stars reverses from E to W)
9th 11h Last quarter
10th 01h Moon 4° N of Mars
11th 02h Moon 2.2° N of Saturn
15th 15h Mercury furthest E of Sun (18°)
17th 13h New moon
18th 01h Mercury 4° N of Venus
18th 18h Moon 8° S of Mercury
18th 19h Moon 4° S of Venus
20th 16:15 Vernal equinox
23rd 00h Moon occults Aldebaran (23:31 to 00:14 for Edinburgh)
24th 16h First quarter
25th 01h Start of British Summer Time
27th 02h Moon 1.8° S of star cluster Praesepe in Cancer
31st 14h Full moon
This is a slightly revised version, with added diary, of Alan’s article published in The Scotsman on February 28th 2018, with thanks to the newspaper for permission to republish here.
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.
Venus highest and brightest as evening star
If you doubt that February offers our best evening sky of the year, then consider the evidence. The unrivalled constellation of Orion stands astride the meridian at 21:00 GMT tonight, and two hours earlier by February’s end. Around him are arrayed some of the brightest stars in the night sky, including Sirius, the brightest, and Capella, the sixth brightest which shines yellowish in Auriga near the zenith. This month also sees Venus, always the brightest planet, reach its greatest brilliancy and stand at its highest as an evening star.
By our map times, a little later in the evening, Orion has progressed into the south-south-west and Sirius, nipping at his heel as the Dog Star in Canis Major, stands lower down on the meridian. All stars twinkle as their light, from effectively a single point in space, is refracted by turbulence in the Earth’s atmosphere, but Sirius’ multi-hued scintillation is most noticeable simply because it is so bright. On the whole, planets do not twinkle since their light comes from a small disk and not a point.
I mentioned two months ago how Sirius, Betelgeuse at Orion’s shoulder and Procyon, the Lesser Dog Star to the east of Betelgeuse, form a near-perfect equilateral triangle we dub the Winter Triangle. Another larger but less regular asterism, the Winter Hexagon, can be constructed around Betelgeuse. Its sides connect Capella, Aldebaran in Taurus, Rigel at Orion’s knee, Sirius, Procyon and Castor and Pollux in Gemini, the latter pair considered jointly as one vertex of the hexagon.
Aldebaran, found by extending the line of Orion’s Belt up and to the right, just avoids being hidden (occulted) by the Moon on the 5th. At about 22:20 GMT, the northern edge of the Moon slides just 5 arcminutes, or one sixth of the Moon’s diameter, below and left of the star. Earlier that evening, the Moon occults several stars of V-shaped Hyades cluster which, together with Aldebaran, form the Bull’s face.
Sunrise/sunset times for Edinburgh change from 08:07/16:46 on the 1st to 07:06/17:45 on the 28th. The Moon is at first quarter on the 4th and lies to the west of Regulus in Leo when full just after midnight on the night of the 10th/11th. It is then blanketed by the southern part of the Earth’s outer shadow in a penumbral lunar eclipse. The event lasts from 22:34 until 02:53 with an obvious dimming of the upper part of the Moon’s disk apparent near mid-eclipse at 00:33. This time, the Moon misses the central dark umbra of the shadow where all direct sunlight is blocked by the Earth, but only by 160 km or 5% of its diameter.
Following last quarter on the 18th, the Moon is new on the 26th when the narrow track of an annular solar eclipse crosses the south Atlantic from Chile and Argentina to southern Africa. Observers along the track see the Moon’s ink-black disk surrounded by a dazzling ring of sunlight while neighbouring regions, but not Europe, enjoy a partial eclipse of the Sun.
Venus, below and to the right of the crescent Moon as the month begins, stands at it’s highest in the south-west at sunset on the 11th and 12th and blazes at magnitude -4.6, reaching its greatest brilliancy on the 17th. It stands further above-and to the right of the slim impressively-earthlit Moon again on the 28th.
Viewed through a telescope, Venus’ dazzling crescent swells in diameter from 31 to 47 arcseconds and the illuminated portion of the disk shrinks from 40% to 17%. Indeed, steadily-held binoculars should be enough to glimpse its shape. This month its distance falls from 81 million to 53 million km as it begins to swing around its orbit to pass around the Sun’s near side late in March.
Mars stands above and to the left of Venus but is fainter and dimming further from magnitude 1.1 to 1.3 during February. It appears closest to Venus, 5.4°, on the 2nd but the gap between them grows to 12° by the 28th as they track eastwards and northwards through Pisces. Both set before our map times at present but our charts pick them up at midmonth as they pass below-left of Algenib, the star at the bottom-left corner of the Square of Pegasus.
Mars shrinks below 5 arcseconds in diameter this month so few surface details are visible telescopically. This is certainly not the case with Jupiter, whose intricately-detailed cloud-banded disk swells from 39 to 42 arcseconds. We do need to wait, though, for two hours beyond our map times for Jupiter to rise in the east and until the pre-dawn hours for it to stand at its highest in the south. Second only to Venus, it shines at magnitude -2.1 to -2.3 and lies almost 4° due north of Virgo’s leading star Spica where it appears stationary on the 6th when its motion switches from easterly to westerly. Look for the two below-left of the Moon on the 15th and to the right of the Moon on the 16th.
Saturn is a morning object, low down in the south-east after its rises for Edinburgh at 05:25 on the 1st and by 03:48 on the 28th. At magnitude 0.6 to 0.5, it stands on the Ophiuchus-Sagittarius border where it is below-right of the waning Moon on the 21st. It is a pity that telescopic views are hindered by its low altitude because Saturn’s disk, 16 arcseconds wide, is set within wide-open rings which measure 16 by 36 arcseconds and have their northern face tipped 27° towards the Earth. Mercury remains too deep in our south-eastern morning twilight to be seen this month.