Nightly planetary procession ends with Venus at brightest before dawn

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.

With the exception of Mars, all the planets are on display in our September night sky. We need to rise early, though, to catch Venus at its brightest and Mercury which makes its best pre-dawn appearance of the year.

The Summer Triangle, formed by the bright stars Vega in the constellation of Lyra, Deneb in the tail of Cygnus the Swan and Altair in Aquila the Eagle, stands high on the meridian as darkness falls and is beginning to topple westwards by our map times. On dark moonless nights we can glimpse the Milky Way flowing up from Sagittarius and through the Triangle before plunging down in the north-east through the “W” of Cassiopeia and Perseus towards Capella in Auriga.

As the Sun tracks southwards, the centre of its disk crosses the sky’s equator at 07:50 BST on the 23rd to mark the autumnal equinox in our northern hemisphere. Edinburgh’s sunrise/sunset time change from 06:16/20:08 BST on the 1st to 07:13/18:51 on the 30th. The Moon is at last quarter on the 6th, new on the 15th, first quarter on the 22nd and full, the Harvest Moon, on the 29th.

Jupiter blazes low down in the east at our map times and lies below and right of the Moon on the 4th. The Moon sits just below the Pleiades a day later as the rising of Taurus heralds the appearance of the Orion and his spectacular retinue of winter constellations. Indeed, by the time the dawn twilight arrives on the 1st, Orion stands below Taurus and well clear of our south-eastern horizon, while by the month’s end it is resplendent in the southern predawn sky.

Jupiter Mike Christie 20 Aug 2023

Jupiter 8 August 2023, Mike Christie

As the most conspicuous planet for most of the night, Jupiter climbs through the east and south-east after our map times to stand almost 50° high in the south during morning twilight at present. Moving hardly at all against the stars of Aries, it brightens from magnitude -2.6 to -2.8 this month and appears 46 arcseconds wide in the middle of the period. Telescopes provide fascinating and ever-changing views of its four main moons and the dark and light bands of cloud, called belts and zones respectively, that run parallel to its equator. Spots and streaks also come and go, though its famous red spot, an anticyclonic storm in its southern hemisphere, has endured for centuries.

The planet Uranus lies 8° to the east of Jupiter and almost midway between Jupiter and the Pleiades. Shining at magnitude 5.7, it is visible through binoculars and shows a 3.7 arcseconds disk through a telescope.

Our eastern morning sky, though, belongs to Venus which rises two hours before the Sun on the 1st and climbs to stand 16° high in the east at sunrise. Brilliant at magnitude -4.4, and reaching its greatest brilliancy at magnitude -4.5 on the 18th, it rises earlier each morning until it rises for Edinburgh at 03:05 BST by the 30th, more than four hours before sunrise, and is 33° high at sunrise. With such brilliance, Venus is capable of casting a shadow under a dark sky, and we should be able to follow its climb into the sky well beyond sunrise.

Venus over Loch Venachar, 18 Apr 2023 by Andrew Farrow,

Venus over Loch Venachar, 18 Apr 2023 by Andrew Farrow,

Against the stars, Venus tracks eastwards from southern Cancer to lie 8° above and right of Regulus in Leo at the month’s end. Viewed through a telescope, or even binoculars, Venus appears as a spectacular crescent, 50 arcseconds across and 11% sunlit on the 1st. As its distance grows from 50 million to 77 million km during the month, it shrinks to 32 arcseconds and 36% illuminated by the morning of the 30th.

Look for Venus below and right of the waning earthlit crescent Moon on the 11th and to the Moon’s right on the 12th. On the 13th, the Moon stands alongside Regulus and 9° above the planet Mercury as the latter emerges from inferior conjunction on the near side of the Sun a week earlier. At magnitude 2.2, Mercury is a challenging target on that morning. Between the 17th and the 30th, though, it improves between magnitude 0.8 and -1.0 to be visible through binoculars and perhaps the unaided eye as it stands 4° to 6° high and almost due east one hour before sunrise.

Our southern sky chart shows Saturn in the south-south-east on its way to passing 22° high and due south less than two hours later. It lies against the stars of Aquarius the Water-bearer and 12° below the quartet of stars which traditionally represent Aquarius’ Water Jar, though their formation has led to them being dubbed the Steering Wheel.

Saturn 14 Aug Radim Stano

Saturn with Titan, Dione, Enceladus, Rhea (from left to right), Radim Stano 14 Aug 2023

The ringed planet stood at opposition, directly opposite the Sun in the sky, on August 27 and is at its best at magnitude 0.4 in our evening sky. Indeed, it is a decade since it stood as high in our midnight sky although its altitude will continue to grow year on year over the coming decade. This means that telescopic views of its clouds, rings and moons are gradually improving as we see it through less of our atmosphere. In mid-September, its disk is 19 arcseconds wide while its rings span 42 arcseconds and have their north face tipped at 10° to our view. Look for it above and left of the Moon on the 26th.

Some 15° left of the Jar or Steering Wheel is another asterism, the Circlet, which represents one of the pair of fish in Pisces. Around 7° below the Circlet is the Sun’s farthest planet, Neptune, which reaches opposition at a distance of 4.3 billion km on the 19th. At magnitude 7.8, it is only just visible through binoculars while telescopes rarely show more than a tiny blue dot, only 2.3 arcseconds wide this month. Clouds, both light and dark, have been glimpsed on its surface and research reported in the past month links their frequency to the 11-years cycle of sunspot activity – certainly, with the Sun roaring towards sunspot maximum, Neptune’s clouds seem to have all-but-vanished for now.

There is speculation that the new Comet Nishimura may reach naked eye brightness this month, though only in our twilight. Discovered on August 11, it is the third comet to be discovered by the Japanese amateur astronomer Hideo Nishimura and has been given the official name C/2023 P1 (Nishimura). It reaches perihelion, less than 34 million km from the Sun, on September 17.

Initially seen as an 11th magnitude telescopic object in the predawn sky, the comet has brightened quickly and is probably a 7th magnitude binocular object in Cancer, 14° above-left of Venus and 4° above-left of the Praesepe or Beehive star cluster, before dawn on the 1st. It may be near the 5th magnitude when it enters the top of the Sickle of Leo on the morning of the 7th and perhaps the 4th magnitude as we lose it the twilight well to the left of Regulus by the 11th or so.

Scotland is far enough north that we may then have a brief window of opportunity to glimpse it as a binocular object in our western evening twilight as it passes north-east of the Sun and reaches its brightest. Forty minutes after sunset on the 13th, it stands 6° high and 13° right of the sunset position and may be as bright as the 2nd magnitude. Any tail will be pointing upwards, away from the Sun. It could remain visible in our evening twilight as it moves to the left and gradually lower over the following evenings, but we are likely to lose it by the 19th.

Being a comet, Nishimura could, of course, disappoint. Indeed, there are some predictions that it could disintegrate on its way to perihelion. On the other hand, its orbit must have brought it to perihelion a few hundred years ago, so it certainly survived that previous encounter with the Sun. We must wait and observe.

Diary for 2023 September

Times are BST

  • 4th 21h Moon 3° N of Jupiter
  • 4th 22h Jupiter stationary (motion reverses from E to W)
  • 5th 21h Moon 1.2° S of Pleiades
  • 6th 12h Mercury in inferior conjunction on near side of Sun
  • 6th 18h Moon 9° N of Aldebaran
  • 6th 23h Last quarter
  • 10th 05h Moon 1.5° S of Pollux
  • 11th 07h Moon 4°N of Praesepe
  • 11th 14h Moon 11° N of Venus
  • 13th 04h Moon 4° N of Regulus
  • 13th 19h Moon 6° N of Mercury
  • 15th 03h New Moon
  • 18th 13h Venus at brightest (magnitude -4.5)
  • 19th 12h Neptune at opposition at distance of 4,324m km
  • 21st 09h Moon 0.9° N of Antares
  • 22nd 14h Mercury furthest W of Sun (18°)
  • 22nd 21h First quarter
  • 23rd 07:50 Autumnal equinox
  • 27th 02h Moon 2.6° S of Saturn
  • 29th 11h Full Moon (Harvest Moon)

This is an extended version, with added diary, of Alan’s article published in The Scotsman on 31 August 2023, with thanks to the newspaper for permission to republish here.