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.
Jupiter conspicuous at opposition in Leo
The Sun, now climbing northwards at its fastest pace for the year, crosses the equator of the sky at 04:30 GMT on the 20th, the time of our vernal equinox. It then rises due east and sets due west, and days and night are equal in length around the globe.
The Sun’s progress means that our nights are falling rapidly later, an effect that appears to enjoy a step-change when we set our clocks forward to British Summer Time on the 27th, though, in this instance, the daylight we gain in the evening is lost in the morning. It is noticeable, too, that the stars at nightfall are shifting quickly to the west. Orion, for example, dominates in the south as darkness falls at present, but has tumbled well over into the south-west by the month’s end.
The Plough is nearing the zenith at our map times and it is the squat figure of Leo the Lion and the prominent planet Jupiter that dominate our southern sky. Jupiter is edging westwards beneath Leo’s hindquarters and passes just below the fourth magnitude star Sigma Leonis over the first few days of the month. Above and to its left is Denebola, the Lion’s tail, while further west (right) is Leo’s leading star Regulus in the handle of the Sickle. Algieba (see chart) appears as a glorious double star through a telescope.
Jupiter comes to opposition on the 8th when it stands opposite the Sun so that it rises in the east at sunset and is unmistakable as it climbs through our south-eastern evening sky to pass 40° high on Edinburgh’s meridian in the middle of the night. Eleven times wider than the Earth and yet with a day lasting under ten hours, it is 664 million km distant at opposition and shines at magnitude -2.5, more than twice as bright as any star other than the Sun.
View Jupiter through binoculars or a telescope, and the fun really begins. Binoculars show its four main moons, Io, Europa, Ganymede and Callisto, which change their relative positions to east and west of the planet’s disk from night to night as they orbit almost directly above the equator. Were it not for Jupiter’s glare, we could see all four of these with the naked eye.
With numerous sulphurous volcanoes, Io is the most geologically active body we know, while Europa is the only one of the four to be smaller than our Moon and is thought to harbour a deep ocean of water beneath its icy crust. This makes it so irresistible as a potential home for life that the US Congress has urged NASA to add a lander craft to a planned mission to Europa over the next decade.
The Jovian disk appears 44 arcseconds wide when we view it through a telescope at present. Even a small telescope shows its main cloud belts but the smaller cloud features that indicate Jupiter’s rotation are more of a challenge. The famous Great Red Spot in the southern hemisphere is a storm that has raged for at least 185 years but is now shrinking noticeably.
By the time Jupiter is sinking in the west before dawn, the two brightest objects low in the south are Mars and Saturn. Mars stands 18° to the right of Saturn and is slightly the brighter of the two at present – their magnitudes being 0.3 and 0.5 respectively, with both of them outshining the red supergiant star Antares in Scorpius which lies more than 5° lower and between them. The Moon stands above-left of Mars on the 1st, above Saturn on the 2nd, and above and between them both on the 29th.
This month Saturn improves only slightly to magnitude 0.4 and hardly moves in southern Ophiuchus, being stationary in position on the 25th. Mars, tracking eastwards from Libra to Scorpius, more than doubles in brightness to magnitude -0.5 as it approaches from 161 million to 118 million km. It also swells in diameter from 9 to 12 arcseconds and telescopes are starting to show surface features, including its north polar cap. There is no comparison, though, with the beauty of Saturn whose superb rings have their north face tipped Earthwards at 26°, near their maximum tilt, and stretch across 38 arcseconds. Saturn’s disk is 17 arcseconds wide and has much more subdued cloud belts than Jupiter.
Although Venus is brilliant at magnitude -3.9, we have slim hopes of seeing it deep in our south-eastern twilight for just a few more mornings. Mercury, already lost from view, reaches superior conjunction on the Sun’s far side on the 23rd.
The sunrise/sunset times for Edinburgh change from 07:03/17:48 GMT on the 1st to 06:45/19:50 BST (05:45/18:50 GMT) on the 31st. The Moon is at last quarter on the 1st, new on the 9th, at first quarter on the 15th, full on the 23rd and at last quarter again on the 31st.
New moon on the 9th brings the first and best of this year’s four eclipses when a total eclipse of the Sun occurs along a path that travels eastwards across Indonesia before swinging north-eastwards over the Pacific to end to the north of Hawaii. Surrounding areas enjoy a partial eclipse but there is nothing to see from Europe. The Moon slims the outer and lighter shadow of the Earth during a penumbral lunar eclipse on the 23rd. Also best seen over the Pacific, it is partly visible from most of the Americas and eastern Asia, but only a minor fading of the southern part of the Moon may be expected.