Mars grows in prominence as the Mars-fleet departs

Alan Pickup

Author: Alan Pickup
The maps show the sky at 23:00 BST on the 1st, 22:00 on the 16th and 21:00 on the 30th.

Some clear nights would make for a pleasant change, although our sky at nightfall is itself altering only slowly. Jupiter, always brighter than any star, stands alongside Saturn low in the south while above them looms the large Summer Triangle, formed by Vega, Deneb and Altair, the leading stars in the constellations Lyra, Cygnus and Aquila respectively.

By our map times, the Summer Triangle is toppling westwards as it gives way to the smaller Square of Pegasus, climbing in the south-east. Whereas the Triangle is awash with stellar interest, the Square is relatively barren and may appear empty of stars under moonlit or light-polluted skies – indeed, some observers use the number of stars they can count within it as a test of the sky’s darkness and clarity.

A third planet, Mars, lies below the Square and is growing ever more prominent as it approaches its nearest point to the Earth and opposition in October. It will then be 62 million km away, making it closer and brighter than it will be again until 2035. To take advantage of the optimum time to embark on the journey, September has seen the successful launch of three new missions to the Red Planet – the UAE’s Hope orbiter, China’s Tianwen-1 orbiter, lander and rover and NASA’s Perseverance rover – all seem to be going well and all are due to arrive next February.

Mars rises in the east before our evening twilight has faded and it climbs through the south-east to pass more than 40° high on Edinburgh’s meridian before dawn. It surpasses Jupiter in brightness this month as it improves from magnitude -1.8 to -2.5. Swelling in diameter from 19 to 22 arcseconds, it is large enough for telescopes to show dusky marking on its rusty surface, along with its white southern polar ice cap which is tipped 18° towards us at present.

As Mars begins to be overtaken by the Earth, it slows to a stationary point against the stars of Pisces on the 9th when its motion reverses from easterly to westerly. The planet lies 3° left of the Moon on the evening of the 5th and is an impressive sight less than a degree above the Moon as that night ends.

Jupiter dims only slightly this month from magnitude -2.6 to -2.4 and lies in Sagittarius where it, too, is stationary on the 13th, although this time for its progress to switch from westerly to easterly. Saturn is 8° to Jupiter’s left, rather fainter at magnitude 0.3 to 0.5 and itself stationary on the 29th. Look out for the Moon below-right of Jupiter on the 24th and below Saturn on the 25th.

A telescope shows the four main moons of Jupiter to either side of its cloud-striped globe, around twice as wide as Mars and 42 arcseconds across in mid-month. Saturn’s rings have their north face tipped 23° towards us and span 40 arcseconds around the planet’s 18 arcseconds wide globe.

The Sun travels 11.5° southwards during September to cross the equator at our autumnal equinox at 14:31 BST on the 22nd. Edinburgh’s sunrise/sunset times change from 06:18/20:06 BST on the 1st to 07:14/18:50 on the 30th. The Moon is full on the 2nd, at last quarter on the 10th, new on the 17th, at first quarter on the 24th and full again on 1 October, which, since it is the full moon closest to the equinox, will make it 2020’s Harvest Moon.

Neptune, visible through binoculars at magnitude 7.8 in Aquarius, reaches opposition at a range of 4,327m km on the 11th while Uranus, in Aries and around 13° east of Mars, is a little brighter at magnitude 5.7. Venus, in contrast, blazes as a magnitude -4.1 morning star in the east for four hours before sunrise. Now showing a dazzling gibbous disk through a telescope, it is 17 arcseconds wide, 65% sunlit and 4° below-right of the waning earthlit Moon on the 14th, with the Praesepe star cluster in Cancer within the same binocular view.

The Milky Way, the merged light of countless dim and distant stars in the plane of our Galaxy, arches overhead at our map times as it flows from Sagittarius and Scorpius on the south-western horizon to Auriga low in the north-east. On its way, it passes through the Summer Triangle and close to Deneb before plunging down through Cepheus, Cassiopeia and Perseus. If we wait until the Moon is out of the way, and can escape the blight of street lights, then binoculars reveal numerous fainter stars along it, some of them double or grouped into clusters.

One such cluster is Messier 11, or M11, which lies below and right Aquila in the minor constellation of Scutum the Shield. We can find it 4° west of Lambda Aquilae (see chart) where binoculars show it as a hazy mass about half as wide as the Moon. Telescopes reveal its brighter component stars, some of them in a V-formation which has led to its popular name of the Wind Duck Cluster from their likeness to a flock of flying ducks. M11 is some 6,000 light years distant and contains thousands of stars that formed together about 300 million years ago.

Another attractive target is the so-called Coathanger, a pattern of stars 1.5° wide that resembles an upside-down coat-hanger. Easily found using binoculars, it lies about one third of the way from Altair to Vega and, like M11, is very close to the centre-line of the Milky Way. Just 8° north of it is Albireo, the star at Cygnus’ beak. A small telescope or good binoculars show it as a striking double star, composed of contrasting orange and blue-green components.

Diary for 2020 September. Times are BST

  • 2nd 06h Full moon
  • 6th 06h Moon 0.03° N of Mars
  • 8th 20h Moon 6° S of Pleiades
  • 9th 19h Mars stationary (motion reverses from E to W)
  • 9th 20h Moon 4° N of Aldebaran
  • 10th 10h Last quarter
  • 11th 21h Neptune at opposition at distance of 4,327m km
  • 13th 01h Jupiter stationary (motion reverses from W to E)
  • 13th 17h Venus 2.3° S of Praesepe
  • 14th 05h Moon 2.2° N of Praesepe
  • 14th 06h Moon 4° N of Venus
  • 15th 17h Moon 4° N of Regulus
  • 17th 12h New moon
  • 22nd 12h Moon 6° N of Antares
  • 22nd 14:31 Autumnal equinox
  • 24th 03h First quarter
  • 25th 08h Moon 1.6° S of Jupiter
  • 25th 22h Moon 2.3° S of Saturn
  • 29th 04h Saturn stationary (motion reverses from W to E)

This is an extended version, with added diary, of Alan’s article published in The Scotsman on August 31 2020, with thanks to the newspaper for permission to republish here.