New space observatory, Euclid, to probe the cosmic dark side

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.

Even though the Sun has turned southwards, Scotland’s night-long twilight still hinders our view of all but the brighter stars and planets. The brightest of all, Venus is at its brightest on the 9th, but it is also sinking rapidly in our western evening sky and will soon be lost from view.

Noctilucent Clouds June 2023, John Briggs

Noctilucent Clouds June 2023, John Briggs

On the other hand, Saturn and Jupiter are improving later in the night and we may be lucky to catch more of those noctilucent clouds I mentioned last time. Look for their silvery wisps low in the northern quarter of the sky between one hour after sunset and one hour before sunrise.

After the astounding successes of the James Webb Space Telescope, launched two Christmases ago, it is time for it to be joined by another astronomical observatory, called Euclid, near its so-called L2 position, some 1.5 million km from the Earth in the direction opposite the Sun. Prior to the invasion of Ukraine, the Euclid spacecraft has been due to launch aboard a Russian rocket. The mission has been switched to one of Elon Musk’s Falcon 9 rockets which, as I write this, is due to lift off from Cape Canaveral on July 1 to boost Euclid on its month-long cruise to L2.

Euclid mission poster

Image credit: ESA

Named for the Greek mathematician who founded the discipline of geometry in around 300 BC, Euclid was developed by the European Space Agency to investigate the geometry of the universe. To do this, it has a 1.2 m mirror and instruments working at visible and near-infrared wavelengths to make a three-dimensional map of the universe. During its planned six years of operation, it may scan as many as 10 billion objects, mainly galaxies, out to a distance of 10 billion light-years and across more than a third of the entire sky – steering clear of the regions obscured by the gas, dust and stars of the Milky Way.

The research is important to investigate how galaxies have been distributed and have moved together over time, and how the expansion of the expansion of the universe has changed. And, in particular, we should get much-needed clues as to the nature of those two astronomical enigmas, dark matter and dark energy. In both cases, “dark” equates to “mysterious” and, though we have only the sketchiest ideas about what these are, the evidence suggests that together they account for 95% of the universe. Incredibly, normal matter, the stuff that makes up everything we see from the Earth to the farthest galaxies, adds up to the remaining 5%.

The Earth stands at aphelion on the 6th when we lie further from the Sun (152 million km) than at any other time this year.

The Sun sinks 5° southward in our sky during July as sunrise/sunset times for Edinburgh change from 04:31/22:02 BST on the 1st to 05:13/21:25 on the 30th. Full moon on the 3rd is followed by last quarter on the 10th, new moon on the 17th, first quarter on the 25th and full moon again on August 1.

Our charts show Vega in Lyra to the south of overhead as the Summer Triangle it forms with Deneb in Cygnus, to its left, and Altair in Aquila, below, begins to cross the meridian. The twilight means that we probably need to wait another month before Milky Way becomes visible in a dark sky, arching high across the south-east from Sagittarius in the south, through the Triangle and onwards to Cepheus, Cassiopeia and Perseus in the north-east.

The supernova in the galaxy M101, near the end of the Plough’s handle, has faded steadily since it peaked near magnitude 11.0 in late May and is probably close to magnitude 12.0 as July begins.

Venus, still blazing at an unsurpassed magnitude -4.5 in the west at nightfall, stands 13° high for Edinburgh at sunset on the 1st and appears as a crescent through telescopes, 34 arcseconds across and 31% illuminated. It gets lower with each day, though, and is only 4° high, and 7° below the very thin crescent Moon, at sunset on the 20th. By then it is 46 arcseconds wide and 15% sunlit, but it may be hard to spot in the twilight unless our horizon is clear. Now on the near side of its orbit of the Sun, its distance during this interval falls from 74 million to 54 million km and it will be 43 million km away when it rounds the Sun’s near side, passing 8° south of the Sun, at inferior conjunction on August 13.

Mars lies a few degrees above and to the left of Venus but, at magnitude 1.7, is some 300 times fainter and becoming increasingly swamped by the twilight. The planetary pair are 3.6° apart and at their closest on the 1st but slowly separate to 9° by the 20th. We’ll likely need binoculars to spot Mars 0.6° above Leo’s leading star, Regulus, on the evening of the 10th and 7° below-right of the young Moon on the 21st.

Mercury emerges from beyond the Sun to become a very difficult evening object after mid-July. It lies to the right of Venus in Scotland’s evening twilight in the west-north-west where it sets some 50 minutes after the Sun

Saturn rises in the east-south-east less than one hour before our star map times and is 23° high when it crosses the meridian around dawn. Shining at magnitude 0.8 to 0.6 in Aquarius, it is above-left of the Moon on the morning of the 7th when telescopes show its disk to be 18 arcseconds wide with 41 arcseconds rings tipped at 8°.

Jupiter, conspicuous at magnitude -2.2 to -2.4 as it edges eastwards in southern Aries, rises in the east-north-east some 100 minutes after our map times and is well up in the south-east before dawn. Its cloud-banded disk appears 38 arcseconds wide when it stands just to the right of the Moon on the morning of the 12th.

Diary for 2023 July

Times are BST

  • 1st 06h Mercury in superior conjunction on far side of Sun
  • 1st 08h Venus closest to Mars (3.6°)
  • 1st 09h Moon 1.5° N of Antares
  • 3rd 13h Full Moon
  • 6th 21h Earth furthest from Sun (152,093,251 km)
  • 7th 04h Moon 2.7° S of Saturn
  • 9th 19h Venus at its brightest (magnitude -4.5)
  • 10th 03h Last quarter
  • 10th 09h Mars 0.7° N of Regulus
  • 11th 22h Moon 2.2° N of Jupiter
  • 13th 08h Moon 1.7° S of Pleiades
  • 14th 06h Moon 9° N of Aldebaran
  • 17th 20h New Moon
  • 20th 10h Moon 8° N of Venus
  • 20th 15h Moon 4° N of Regulus
  • 21st 05h Moon 3° N of Mars
  • 25th 05h Moon 2.8° N of Spica
  • 25th 23h First quarter
  • 28th 19h Moon 1.3° N of Antares

This is an extended version, with added diary, of Alan’s article published in The Scotsman on 30 June 2023, with thanks to the newspaper for permission to republish here.