Giant planets return to evenings as summer twilight subsides
Author: Alan Pickup
The maps show the sky at 01:00 BST on the 1st, midnight on the 16th and 23:00 on the 31st.
The Sun is furthest away for the year when the Earth stands 152.1 million km from it at aphelion on the 5th, some 5 million km further than we were at perihelion in January. The Sun has already turned southwards, so we can look forward to the return of darker nights over Scotland. At present, though, twilight persists even during Edinburgh’s two darkest hours which tonight commence at 00:17 BST.
Gleaming through the gloaming, though, we can still expect to see those high-altitude noctilucent or night-shining clouds that I mentioned here last time. Usually appearing as tenuous, cirrus-like bluish wisps, they are high enough to catch and reflect the Sun’s light when normal lower-level clouds are in darkness during the night. Normally we sight them low down in the northern part of our sky, but there have been outbreaks over recent weeks that have extended over the whole of the UK and southwards into France.
They form after ice crystals condense around dusty particles in the upper atmosphere, but the origin of those particles is far from clear – are they the dust from meteors, or volcanoes, or man-made pollution? One surprising fact is that there seem to be no confirmed records of them prior to 1885.
The sunrise/sunset times this month change from 04:32/22:01 BST on the 1st to 05:15/21:22 on the 31st. By the month’s end, the Sun is sweeping more than 12° below our northern horizon in the middle of the night so we can anticipate almost four hours of effective darkness. Meanwhile, the Moon reaches last quarter tomorrow, is new on the 10th, at first quarter on the 17th, full on the 24th and back at last quarter on the 31st.
Our charts show Auriga’s bright star Capella low in the north, roughly in the favoured direction for noctilucent clouds. It rivals Vega in Lyra, a little south of overhead, while both are surpassed only marginally by Arcturus in Bootes which is declining in the west. Vega completes the celebrated Summer Triangle with Deneb in Cygnus, very high in the east, and Altair in Aquila in the heart of our south-south-eastern sky.
The largest planets, Saturn and Jupiter, now rise before midnight BST to be prominent in our lower south-eastern sky at our map times where the bright Moon lies below-left of Saturn on the 24th and below-right of Jupiter on the 25th.
Jupiter outshines every star in the sky and brightens a little this month from magnitude -2.6 to -2.8 as it creeps more than 2° (four Moon-widths) westwards against the stars of Aquarius.
Also edging westwards, and due at opposition in Capricornus on 2 August, is Saturn which is fainter at magnitude 0.4 to 0.2. It lies some 20° west (right) of Jupiter and can also be found by extending a line from Vega through Altair.
Both Jupiter and Saturn are excellent telescopic targets. Jupiter’s disk is 47 arcseconds wide at midmonth, its light and dark cloud bands running parallel to its equator and bearing spots and streaks that betray the planet’s rapid rotation in under ten hours. Stable binoculars are all we need to spy its four main moons as they change their configuration on either side of the disk from night to night.
Several moons of Saturn may be glimpsed telescopically, but here the main attraction is the superb ring system. In mid-July, this spans 42 arcseconds around the 18 arcsecond disk and has its north face tipped at 18° towards the Earth.
The other bright planets may be a struggle for observers in Scotland. The most brilliant, Venus at magnitude -3.9, hovers low in our bright evening twilight where its position at sunset changes from almost 10° high in the west-north-west on the 1st to 8° in the west on the 31st. Setting little more than one hour later, it may be elusive unless our horizon is clear. Mars lies 7° left of Venus on the 1st and only 0.5° below Venus on the 13th, but at magnitude 1.8 is too dim for naked-eye visibility in the twilight. The young earthlit Moon stands 5° above-left of the pair on the 12th.
Mercury rises in the north-east more than 75 minutes before the Sun from the 8th to the 20th and might just be glimpsed through binoculars between these dates as it brightens from magnitude 0.1 to -1.1. The incredibly thin crescent of the waning Moon stands just 3° above-left of Mercury on the 8th.
The binocular-brightness nova that erupted in Cassiopeia in March continues to surprise observers by remaining visible near its discovery brightness instead of fading rapidly. Indeed, during May it rose to better than magnitude 5.5 so that it was briefly a naked-eye object. A different nova flared in the south-eastern corner of the constellation Hercules on 12 June and quickly attained a peak magnitude of 6.4, visible easily through binoculars. As most novae do, though, this one has dimmed sharply and is probably a telescopic object around the 12th magnitude as you read this.
Finally, astronomers have spotted what may be the largest comet ever seen. The nucleus of Comet C/2014 UN271 (Bernardinelli-Bernstein) may be 100 km wide, making it perhaps a thousand times larger than Comet Halley’s icy core. Approaching from a distance of more than half a light year, it apparently originated within the Oort Cloud of icy bodies at the far edge of our solar system. Currently 3 billion km away, as distant as Uranus, it already appears comet-like with a surrounding hazy cloud or coma. Perihelion, its closest point to the Sun, is not due until January 2031 when it lies at 1.6 billion km, just outside Saturn’s orbit, but may still be a dim telescopic object.
Diary for 2021 July
Times are BST
- 1st 22h Last quarter
- 3rd 09h Venus 0.4° N of Praesepe
- 4th 21h Mercury furthest W of Sun (22°)
- 5th 23h Earth furthest from Sun (152,100,527 km)
- 8th 06h Moon 4° N of Mercury
- 10th 02h New moon
- 12th 10h Moon 3° N of Venus
- 13th 01h Mars furthest from Sun (249m km)
- 13th 07h Moon 5° N of Regulus
- 13th 08h Venus 0.5° N of Mars
- 17th 06h Moon 6° N of Spica
- 17th 11h First quarter
- 20th 14h Moon 5° N of Antares
- 21st 20h Venus 1.2° N of Regulus
- 24th 04h Full moon
- 24th 18h Moon 4° S of Saturn
- 26th 02h Moon 4° S of Jupiter
- 29th 17h Mars 0.7° N of Regulus
- 31st 14h Last quarter
This is an extended version, with added diary, of Alan’s article published in The Scotsman on June 30 2021, with thanks to the newspaper for permission to republish here.