Dark skies return for peak of Perseids meteor shower

Alan Pickup

Author: Alan Pickup

The maps show the sky at midnight BST on the 1st, 23:00 on the 16th and 22:00 on the 31st.

We have bright moonlight as the month begins, but an unobtrusive lunar crescent in our morning sky by the 13th when the prolific Perseids meteor shower reaches its peak. With moonlight not an issue, we just need some clear skies to enjoy the Perseids, the so-called Tears of St Lawrence, at their very best.

Perseid meteors caught by an ASE meteor camera on 12 Aug 2022.

The meteoroid particles in the Perseids stream were ejected from Comet Swift-Tuttle, probably many centuries ago, and trace an eccentric orbit around the Sun every 133 years or so, moving from just inside the Earth’s orbit to far beyond the orbit of the outermost planet, Neptune. The comet itself last visited our neighbourhood in 1992 but its entire path around the solar system is peppered with meteoroids and it is some of these that the Earth runs into each year between mid-July and late-August. Encountering the atmosphere at heights above 80 km and at a speed of 58 km per second, they disintegrate in the streak of light we call a meteor which, for the larger and brighter ones, can persist for a second or more as a meteor train. We think that none survive to reach the ground to be collected as meteorites.

The brightest Perseid from 2022 caught by the same ASE camera, leaving a vapour trail

Because the meteors follow parallel paths, perspective means that they appear to radiate from a point, called the radiant. Near the Perseids maximum, this lies in the constellation Perseus, hence the name, though the meteors themselves appear in every part of the sky – it is just their paths that point away from Perseus.

The radiant for the Perseids, plotted on our north star map, lies below the familiar “W” of the constellation Cassiopeia which climbs through our north-eastern and eastern sky overnight to lie overhead at dawn. As the radiant rises, so we meet the meteoroids more head-on and their rates tend to increase. The Earth is predicted to hit the densest part of the Perseids stream during daylight at around 10:00 BST on the 13th, so the preceding night should yield the best display over Scotland, with rates approaching 80 or more meteors per hour for someone enjoying ideal conditions. Rates should still be respectable over the two or three preceding nights, and one or two later.

With the Sun continuing to track southwards, our nights are properly dark again in time for the Perseids. Edinburgh’s sunrise/sunset change from 05:16/21:21 BST on the 1st to 06:14/20:11 on the 31st. Meanwhile, the Moon is full on the 1st, at last quarter on the 8th, new on the 16th, at first quarter on the 24th and full once more on the 31st.

The Moon appears huge as it stands exceptionally low down in our southern sky at the beginning of August, and again at the month’s end. Much of this is due to the “Moon illusion”, the optical illusion which makes the Moon look larger when it is close the horizon. A contributing factor is that the two full Moons this month occur near its perigee, its nearest point to the Earth when it is some 50,000 km closer and up to 14% wider than when it is near its apogee. You can call these full Moons “supermoons” if you wish, and you may hear the second one called a “blue moon”, though the logic behind the latter is contested and has nothing to do with the Moon’s actual colour.

The Summer Triangle, formed by the stars Vega, Deneb and Altair, is crossing the high meridian (due south) at our star map times as the Square of Pegasus is halfway up our sky in the east and the Plough stands a little lower than this in the north-west.

Closeup of constellation Lyra

Use binoculars on the star Epsilon (see map) in Lyra, only 1.7° above-left of Vega, to see it as a pair of stars – indeed, keen-sighted observers may see them with the naked eye. Switch to a telescope, though, and each of these splits in two and we realise that Epsilon deserves its title as the Double Double. A fifth star makes Epsilon a striking multiple stellar system, all locked together by gravity at a distance of 160 light years from the Sun.

Venus sets before Edinburgh’s sunset at the start of the month as it moves to sweep more than 7° south of the Sun at its inferior conjunction on the Sun’s near side on the 13th. Thereafter, it is quick to spring back into view in our pre-dawn sky, rising in the east 30 minutes before the Sun on the 21st and almost two hours before sunrise on the 31st. By then it is unsurpassed at magnitude -4.4 and stands 15° high in the east at sunrise, with binoculars or a telescope showing its disk to be 50 arcseconds wide and 10% illuminated.

Mars and Mercury set too soon after sunset to be seen in our evening twilight even though the latter stands furthest east of the Sun (27°) on the 10th.

Saturn reaches opposition at distance of 1,311m km on the 27th when it stands opposite the Sun in the sky and reaches its highest point in the south, 22° for Edinburgh, in the middle of the night. On that date, telescopes show its globe to be 19 arcseconds wide, framed by rings that span 43 arcseconds and have their north face tipped 9° to the Earth. Our charts show it in the south-east in the constellation Aquarius where is creeping westwards and shines steadily at magnitude 0.6 to 0.4. Catch the Moon below-right of Saturn on the night of the 2nd, to its left on the 3rd and close to the planet again on the 30th.

Jupiter, brighter than any star as it brightens from magnitude -2.4 to -2.6 this month, rises in the east-north-east as our evening twilight ends and climbs to stand high in the south-east by dawn. Edging eastwards below Hamal in Aries, it is prominent near the Moon on the night of the 7th when a telescope shows its disk to be 41 arcseconds wide.

Diary for 2023 August

Times are BST

  • 1st 20h Full Moon
  • 3rd 11h Moon 2.5° S of Saturn
  • 8th 11h Last quarter
  • 8th 11h Moon 2.9° N of Jupiter
  • 9th 14h Moon 1.4° S of Pleiades
  • 10th 03h Mercury furthest E of Sun (27°)
  • 10th 12h Moon 9° N of Aldebaran
  • 13th 10h Peak of Perseids meteor shower
  • 13th 12h Venus in inferior conjunction on near side of Sun
  • 13th 23h Moon 1.7° S of Pollux
  • 16th 11h New Moon
  • 21st 11h Moon 2.5° N of Spica
  • 24th 11h First quarter
  • 25th 03h Moon 1.1° N of Antares
  • 27th 09h Saturn at opposition at distance of 1,311m km
  • 30th 19h Moon 2.5° S of Saturn
  • 31st 03h Full Moon

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