Jupiter partners with Saturn in our southern night sky

Alan Pickup

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. An arrow depicts the motion of Mars.

Now that the Sun is edging southwards following our summer solstice, it also begins to dip slightly further below our northern horizon in the middle of the night. One welcome result, at least for astronomers, is that we can look forward to darker nights and the emergence of the fainter stars from Scotland’s night-long twilight.

The Summer Triangle takes its rightful place high in our southern night sky at our star chart times. Its corners are marked by the bright stars Vega in the constellation Lyra, Deneb in Cygnus and Altair in Aquila and it is destined to be a feature of our nights until well into the autumn.

This summer, though, it has a dual purpose in pointing downwards to the Sun’s two largest planets, Jupiter and Saturn. These gas giants lie side by side and come to opposition in the constellation Sagittarius this month when they stand 180° away from the Sun so that they rise at about sunset and set at sunrise. They also reach their highest in the middle of night and, being at their closest for the year, are at their best for viewing through a telescope. Their low altitude, though, does mean that we can’t expect the sharpest of views.

Jupiter is unmistakable, being brighter than any star in our summer night sky and shining at magnitude -2.8 at opposition in Sagittarius on the 14th. Its distance then is 619 million km and telescopes show its cloud-banded disk to be 48 arcseconds broad, around one fortieth of the apparent size of the Moon.

Spots and streaks in its clouds progress from east to west (left to right) as it rotates in just under ten hours. Foremost is the Great Red Spot, an anticyclonic storm that has raged in Jupiter’s southern hemisphere for centuries and was once as large as three Earths. Now it is barely one Earth wide and last year there were fears that it might be in its death throes as some of its material appeared to spin away – a process that seems to have paused for now. Even binoculars are sufficient to give views of the four main Jovian moons which line up to east and west of the disk, changing their configuration as they orbit in periods from 1.8 to 17 days.

For all the fascination of Jupiter, nothing can compare with the beauty of Saturn. The ringed planet shines at magnitude 0.1 and is at opposition at a distance of 1,346 million km on the 20th. Its polar-flattened disk is then 18 arcseconds wide and its rings span 42 by 15 arcseconds, their north face being tipped 22° in our favour. Saturn’s cloud bands are more subdued than those of Jupiter and noticeable spots are rare. Perhaps the most famous is a white spot in Saturn’s northern hemisphere discovered in 1933 by the English comedian Will Hay, who was also a skilled amateur astronomer. A similar disturbance has been sighted several times since, and there are predictions that one will appear in 2020.

Saturn lies 6° to the left (east) of Jupiter as the month begins, a separation that grows slightly as both planets creep westwards at different rates. Look for the Moon below Jupiter on the 5th-6th.
Mars rises in the east just before our map times and is tracking eastwards near the Pisces/Cetus border below the Square of Pegasus. Conspicuous at magnitude -0.5, and brightening to -1.1, its rusty surface lends it an orange hue and, as it approaches from 122 million to 96 million km, its disk swells from 11 to 15 arcseconds.

July sees the intended launch of three more missions to Mars including a Mars orbiter, Hope, built by the UAE and due to be launched on a Japanese rocket on the 14th. China plans to dispatch the Tianwen-1 orbiter, lander and rover on the 23rd while NASA is targeting the 22nd for the launch of its Perseverance rover. The latter carries a small experimental drone helicopter called Ingenuity.

Venus is now established as a brilliant morning star in the east-north-east. Its altitude thirty minutes before sunrise improves from 7° to 21° over the month as its distance grows from 58 million to 91 million km and the diameter of its crescent shrinks from 43 to 27 arcseconds. Catch it just below the waning earthlit Moon on the 17th, with Aldebaran in Taurus to their right. Mercury is just emerging as morning star, lower and to Venus’ left in the twilight, and lies 3° below-right of the Moon on the 19th.

Edinburgh’s sunrise/sunset times for July change from 04:32/22:01 BST on the 1st to 05:16/21:21 on the 31st. The full Moon on the 5th grazes the southern edge of Earth’s outer shadow in yet another penumbral lunar eclipse, the third and least noticeable of four this year. The event is only just underway when the Moon sets and the Sun rises for us. Last quarter occurs on the 12th, new moon on the 20th and first quarter on the 27th.

As I mentioned last time, the Sun’s shallow dip overnight means that conditions are optimum for spotting noctilucent (night shining) clouds. We have been treated to plenty of displays over recent weeks, usually in the form of cirrus-like wisps or rippled banks of electric-blue cloud that are high enough, at around 82 km, to catch the sunlight after our all-too-traditional lower level clouds are in darkness. Comprised of ice freezing around particles of meteor debris, they are Earth’s highest clouds and are usually glimpsed low down between the north-west one hour after sunset to the north-east one hour before sunrise.

Diary for 2020 July

Times are BST

  • 1st 04h Mercury in inferior conjunction
  • 2nd 18h Moon 6° N of Antares
  • 4th 13h Earth farthest from Sun (152,095,296 km)
  • 5th 06h Full moon and penumbral lunar eclipse
  • 5th 23h Moon 1.9° S of Jupiter
  • 6th 10h Moon 2.5° S of Saturn
  • 11th 21h Moon 2.0° S of Mars
  • 12th 08h Venus 1.0° N of Aldebaran
  • 13th 00h Last quarter
  • 14th 09h Jupiter at opposition at distance of 619m km
  • 16th 04h Moon 7° S of Pleiades
  • 17th 03h Moon 4° N of Aldebaran
  • 17th 08h Moon 3°N of Venus
  • 19th 05h Moon 4° N of Mercury
  • 20th 19h New moon
  • 20th 23h Saturn at opposition at distance of 1,346m km
  • 22nd 16h Mercury furthest W of Sun (20°)
  • 22nd 22h Moon 4° N of Regulus
  • 26th 15h Moon 7° N of Spica
  • 27th 14h First quarter
  • 30th 00h Moon 6° N of Antares

This is an extended version, with added diary, of Alan’s article published in The Scotsman on 30 June 2020, with thanks to the newspaper for permission to republish here.