The three stellar lighthouses in our twilit midsummer nights
Author: Alan Pickup
The maps show the sky at 01:00 BST on the 1st, midnight on the 16th and 23:00 on the 30th.
Given that our nights will soon be at their briefest and most twilit of the year, and the planets are visible only low down before dawn, we might be forgiven for taking a temporary break from stargazing this month.
The Sun remains open for study (safely, of course) and is farthest north at 10:14 BST on the 21st, marking the summer solstice in our hemisphere and the time when it is overhead at the so-called Tropic of Cancer, 23.44° north of the equator. In fact, at that moment the Sun is placed not against the constellation of Cancer but some 30° to its west and near the Gemini/Taurus border – the discrepancy is due to precession, the 26,000 years wobble in the direction of the Earth’s axis in space.
Edinburgh’s sunrise/sunset times change from 04:35/21:47 BST on the 1st, to 04:26/22:03 on the 21st and 04:31/22:02 on the 30th. The Moon is at first quarter on the 7th, full on the 14th, at last quarter on the 21st and new on the 29th.
Clouds thwarted many of our attempts to view the total eclipse of the Moon before dawn on 16 May. Observers in the Americas fared better and tell us they saw one of the darkest lunar eclipses for years. Could it be that dust in the Earth’s atmosphere from December’s titanic eruption of a submarine volcano in Tonga absorbed some of the sunlight that would otherwise have penetrated around the Earth to illuminated the eclipsed Moon?
While the persistent twilight swamps many of the stars at our map times, three well-separated bright ones stand out and may serve as starry lighthouses as we navigate the fainter stars and constellations between.
Foremost, and the brightest star north of the sky’s equator, is the orange giant Arcturus in Bootes which blazes in the middle of our south-western sky as the night begins. Shining just brighter than magnitude 0.0, it is only slightly more massive than our Sun and some 2.5 billion years older. Unusually, it is speeding through space as it passes the solar system at 122 km/sec and a distance of 37 light years (ly).
Next comes Vega in Lyra, higher still towards the south-east and perhaps most famous as the brightest star in the Summer Triangle it forms with Altair in Aquila and Deneb in Cygnus. While 7% fainter than Arcturus, it is closer at 25 ly, twice as massive and much younger, having formed “only” some 500 million years ago. An encircling disk of dust hints that it might have an embryonic planetary system. Our old friend precession dictates that Vega will become Earth’s pole star around the year AD 13,700.
In line between Arcturus and Vega are the constellations of Corona Borealis, the Northern Crown with its jewel Gemma, and Hercules and its Keystone asterism, a quadrilateral of stars (see chart) named for the locking stone at the summit of an arch. Near the western edge of the Keystone is M13, the Great Globular Cluster of hundreds of thousands of ancient stars at a distance of 22,200 ly. Appearing through binoculars as a fuzzy ball, it is twice as bright as another globular, M92, which sits 6° north of the top-left star of the Keystone. At 26,700 ly, M92 is similar in distance to the supermassive black hole at the heart of our Milky Way galaxy whose image was released on 12 May.
Only 5% fainter than Vega is Capella in Auriga, standing 12° above our northern horizon and twinkling strongly. A little older than Vega, it is a multiple star consisting of four stars in tight formation at a distance of 43 ly.
In the depths of our winter’s nights Capella passes almost overhead as Orion stands in the south. During the summer, though, it is a marker for the direction we are most likely to see noctilucent (night-shining) clouds, or NLCs. Formed when ice freezes onto dust particles, NLCs are often silvery-blue in colour and have a rippled or striated appearance like cirrus clouds. Confined to a narrow altitude around 82 km, they are high enough to catch the sunlight after lower-level louds are in darkness, say from one hour after sunset until one hour before sunrise.
Sometimes NLCs can blanket the entire sky but they are more usually confined to low altitudes towards the north-west after nightfall, shifting to the north-east before dawn – just the path taken by Capella during the night so that the star frequently gatecrashes images of them.
If we travel further south, we lose sight of Capella and NLCs altogether, but our view of the planetary line-up in the east before dawn improves enormously. As it is, the planets lie relatively flat against Scotland’s predawn sky with the brightest, Venus, rising in the east-north-east only 62 minutes before sunrise on the 1st. At magnitude -4.0, it is forty times brighter than Arcturus and bright enough to be visible 8° high in the east as the Sun rises. By the 30th, it rises 101 minutes before the Sun and is 12° high at sunrise.
Jupiter lies higher to Venus’ right and is a fifth as bright at magnitude -2.3. Rising in the east at 02:44 BST for Edinburgh on the 1st, and as early as 00:55 on the 30th, it is now in Pisces, below the Square of Pegasus, and stood 0.6° above the much fainter Mars on Sunday. Mars is magnitude 0.7 and lies 1.7° below-left of Jupiter on the 1st, a separation that grows to 19° by the 30th. The waning crescent Moon lies 7° below-left of Jupiter and 8° right of Mars before dawn on the 22nd and just 2° above Venus (and 4° below the Pleiades) on the 26th.
Saturn, also magnitude 0.7 and slow-moving in Capricornus, rises in our east-south-east at 01:50 on the 1st and 23:51 on the 30th climbing to stand some 16° high in the south-south-east before dawn where we see it above-left of the Moon on the 18th.
Diary for 2022 June
Times are BST
- 3rd 07h Moon 2.1° S of Pollux
- 4th 09h Moon 4° N of Praesepe
- 5th 15h Saturn stationary (motion reverses from E to W)
- 6th 04h Moon 5° N of Regulus
- 7th 16h First Quarter
- 10th 08h Moon 5° N of Spica
- 11th 14h Venus 1.6° S of Uranus
- 13th 15h Moon 3° N of Antares
- 14th 13h Full Moon
- 16th 16h Mercury furthest W of Sun (23°)
- 18th 13h Moon 4° S of Saturn
- 21st 04h Last Quarter
- 21st 10:14 Summer solstice
- 21st 15h Moon 2.8° S of Jupiter
- 22nd 04h Venus 6° S of Pleiades
- 22nd 19h Moon 1.0° S of Mars
- 25th 23h Moon 3° S of Pleiades
- 26th 09h Moon 2.7° N of Venus
- 29th 04h New Moon
This is an extended version, with added diary, of Alan’s article published in The Scotsman on 31 May 2022, with thanks to the newspaper for permission to republish here.