Mars and the Pleiades in closest rendezvous in 32 years

Alan Pickup

Author: Alan Pickup
The maps show the sky at 23.00 GMT on the 1st, 22.00 GMT on the 16th and 21.00 GMT (22.00 BST) on the 31st. An arrow depicts the motion of Mars. Summer time begins at 01.00 GMT on the 28th when clocks go forward one hour to 02.00 BST.

As an exciting fresh era of Martian exploration begins with the arrival of three new missions, the planet still features in our evening sky. This month sees it closer to the Pleiades or Seven Sisters star cluster than during any other approach between 2006 and 2038.

The other major planets are poorly placed, with Venus rounding the Sun’s far side on the 26th and Mercury, Jupiter and Saturn hugging our south-eastern horizon at dawn.

Our charts show Mars sinking in the western sky as it tracks east-north-eastwards in Taurus. As darkness falls on the 1st, though, it stands high in the south-west and 3° or six Moon-breadths below the Pleiades. Shining at magnitude 0.9, it is equal in brightness to the orange giant star Aldebaran which lies 13° to Mars’ left and is seen against the V-formation Hyades cluster of fainter stars. Over the following days, Mars slips to the lower-left of the Pleiades, being nearest at 2.7° on the 4th and directly between the cluster and Aldebaran on the 8th.

On the evening of the 19th, look for the 33% illuminated crescent Moon 2.3° below-right of Mars and just one-third of the way between Mars and Aldebaran. Mars dims to magnitude 1.3 by the 31st when it sits between the horns of Taurus and above Aldebaran. As Mars recedes from 219 million to 263 million km during March, it shrinks between 6.4 and 5.3 arcseconds in diameter, making it too small for useful telescopic study.

The Sun climbs another 12° northwards in March to cross the sky’s equator at 09:37 GMT on the 20th, the moment of our spring or vernal equinox when days and nights are almost of equal length around the Earth. Our days are lengthening at their fastest rate of the year so our view of the stars and constellations at nightfall is also changing quickly.

Take Orion, for example. As nautical twilight ends on the 1st, and the sky becomes effectively dark, Alnilam, the middle star of Orion’s familiar Belt, stands 33° high and due south as seen from Edinburgh. By the equivalent time on the 31st, though, it has dipped to 22° in the south-west and during April it slides into our western evening twilight.

Edinburgh’s sunrise/sunset times change this month from 07:04/17:47 GMT on the 1st to 06:46/19:49 BST on the 31st, after our clocks jump one hour forwards to British Summer Time on the 28th.

The Moon is at last quarter on the 6th, new on the 13th, at first quarter on the 21st and full on the 28th.

As the twilight fades on the 14th there is a slim chance of spotting the very young Moon when it is only 2% sunlit and a mere 4° high in the west at 18:50, but probably only visible through binoculars and provided the horizon is exceptionally clear. It should be much easier on the 15th, when it is 5% illuminated and visible for more than two hours after sunset. On that evening, and for the ones that follow, look for earthshine as the Moon’s darker side is lit by the almost-full Earth in the lunar sky.

Over the succeeding nights the Moon climbs against the star background but never strays far from the ecliptic, the path taken by the Sun during the course of the year. Since the planets orbit the Sun in a similar plane to that of the Earth, they, too, keep close to the ecliptic and it is no surprise that Mars is hugging the ecliptic as it passes between the Pleiades and Aldebaran.

Although it is not plotted, the ecliptic continues across our southern chart to pass through Gemini, Cancer, Leo and Virgo, a path that takes it close to the Praesepe star cluster in Cancer, and the bright stars Regulus in Leo and Spica in Virgo. Praesepe (the Latin for manger) appears as a hazy blob to the naked eye and as a swarm of stars through binoculars, leading to its alternative name of the Beehive. In all, it holds around 1,000 stars at a distance of some 577 light years.

Regulus lies 79 light years away and consists of four stars grouped as two pairs, though only one, a blue-white subgiant star, is obvious. Curling above Regulus is the Sickle of Leo, a reversed question-mark of stars that represent the lion’s head and mane. Here we find Algieba, 130 light years away and one of the finest double stars in the entire sky. We need a telescope to divide its pair of striking golden-yellow stars which appear only 4.7 arcseconds apart and take 510 years to orbit each other.

Leo’s body stretches to the left (east) of Regulus to Denebola, the lion’s tail, and includes the third magnitude star Chertan which is useful as a signpost for locating the brightest of all the asteroids, the 525 km diameter Vesta, as it comes to opposition on the 4th. Vesta moves from 1.6° (3 Moon-widths) left of Chertan late on the 1st to lie 1.3° due north (above) Chertan one week later. At magnitude 5.8, it is an easy binocular object as it approaches within 204 million km of the Earth.

Vesta was orbited by NASA’s Dawn spacecraft in 2011 and 2012 and found to be a remarkable, and perhaps unique, asteroid in that (like the Earth) it is a differentiated world with an iron-rich core overlaid by a rocky mantle and volcanic crust. The early solar system probably had many similar bodies, dubbed protoplanets, that became the building-blocks of the planets, or were smashed to pieces in collisions.

Diary for 2021 March

  • 2nd 07h Moon 7° N of Spica
  • 3rd 10h Mars 2.7° S of Pleiades
  • 5th 07h Mercury 0.3° N of Jupiter
  • 5th 14h Moon 5° N of Antares
  • 6th 01h Last quarter
  • 6th 11h Mercury furthest W of Sun (27°)
  • 11th 00h Neptune in conjunction with Sun
  • 13th 10h New moon
  • 18th 22h Moon 6° S of Pleiades
  • 19th 18h Moon 1.9° S of Mars
  • 19th 22h Moon 5° N of Aldebaran
  • 20th 09:37 Vernal equinox
  • 21st 15h First quarter
  • 23rd 00h Mars 7° N of Aldebaran
  • 23rd 11h Moon 3° S of Pollux
  • 24th 10h Moon 2.9° N of Praesepe
  • 26th 01h Moon 5° N of Regulus
  • 26th 07h Venus in superior conjunction on far side of Sun
  • 28th 01h GMT = 02h BST Start of British Summer Time
  • 28th 19h Full moon
  • 29th 16h Moon 6° N of Spica

This is an extended version, with added diary, of Alan’s article published in The Scotsman on February 27 2021, with thanks to the newspaper for permission to republish here.