Brilliant Venus chases Mars in our western evening 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 31st. Arrows depict the motions of Venus and Mars.

As the days lengthen, Scotland’s nights are increasingly twilit and, unless they are avid solar observers, astronomers here may be frustrated if their interests lie among the fainter objects, or even aurorae of which we have enjoyed some spectacular recent displays.

Venus remains a striking evening object in the west at nightfall. True, it does lose a few degrees in altitude, but it also brightens by 22%, from magnitude -4.1 to -4.3, remaining brilliant until the early hours when it sets in the north-west. Viewed through a telescope, it shows a gibbous disk, 66% illuminated and 17 arcseconds across on the 1st, though this swells to 23 arcseconds by the 31st when it is 52% sunlit. Dichotomy, when it appears exactly D-shaped, comes a few days later on 4 June.

Earthlit crescent Moon, Mark Phillips on 21 April 2023

The 1st finds Venus 3° below-left of Elnath, the star at the tip of the northern horn of Taurus. Arrows on our chart show Venus chasing Mars through the constellation of Gemini. Venus stands 1.7° above-right of the star cluster M35 (binoculars help) on the 9th and 0.7° above-right of the star Mebsuta on the 16th. Passing below Castor and Pollux later in the period, it is visited by the earthlit crescent Moon which stands 4° above-left of Venus and 3° below Pollux on the 23rd.

Mars lies 6° below Pollux on the 1st and is similar in brightness at magnitude 1.3. However, the planet fades to magnitude 1.6 as it tracks 17° eastwards to stand in line to the left of Castor and Pollux around mid-month and only a degree west (right) of the Praesepe star cluster in Cancer on the 31st. Praesepe translates as Manger, though we also know it as the Beehive. Binoculars show its individual stars which lie some 610 light years away and formed together about 600 million years ago.

Saturn is the only other bright planet in our May night sky though we must look low down in the east-south-eastern morning twilight to see it. At magnitude 1.0 to 0.9, it rises for Edinburgh at 04:10 BST on the 1st and at 02:15 on the 31st. It lies 8° above-left of the Moon on the 13th. Both Mercury and Jupiter lie deeper in Scotland’s dawn twilight where they are unlikely to be seen.

Occultation of Jupiter, 17 May 2023, 14:45 BST (Stellarium)

At magnitude -2.1, though, Jupiter is bright enough to be visible in daylight against a clear blue sky, provided we know just where to look. This means that Scotland has a chance of seeing the rare occultation of Jupiter by the Moon on the afternoon of the 17th.

As seen from Edinburgh the occultation lasts from 14:45 until 15:03 BST when Jupiter slips behind the southern part of the Moon’s thin crescent, at roughly the 8 o’clock position as we see them in the sky. At the time, the Moon is 30° high in the west-south-west and 26° below-right of the Sun. The first challenge will be to locate the Moon itself since it is a mere 5% illuminated. Thereafter, Jupiter might be seen through binoculars, though it is certainly best to use a telescope to view the event. The occultation is deeper and longer for observers north of Edinburgh, while observers near a line from Ayr to Berwick-upon-Tweed see Jupiter just graze the lunar limb. Watch from further south, and we miss the occultation altogether.

The Sun climbs another 7° northwards during May to within 1.5°, or 3 Sun-diameters, of its high point at the summer solstice on 21 June. Edinburgh’s sunrise/sunset times change from 05:30/20:51 BST on the 1st to 04:37/21:45 on the 31st.

When the Moon reaches full phase on the 5th, it slides through the outermost southern part of the Earth’s shadow, resulting in a penumbral lunar eclipse which concludes only minutes before moonrise for Britain. The Moon reaches last quarter on the 12th, is new on the 19th and at first quarter on the 27th.

The Plough stands just west of the zenith at our star map times, the curve of its handle extending to the orange giant star Arcturus in Boötes, conspicuous high in our southern sky. Leo is sinking in the west, its leading star, Regulus, being below and left of the Moon on the 26th. Spica in Virgo, lower in the south, is below the Moon on the 3rd and has a second encounter when it is passed by the Moon as the month ends. The red supergiant Antares, glowering low in the south-south-east, hovers below-left of the Moon on the 6th.

The Summer Triangle formed by Vega, Altair and Deneb is climbing in the east. We can expect the variable star Chi Cygni, in the neck of Cygnus the Swan, to brighten above the 6th magnitude in May as it approaches a peak in brightness around early June. Chi is a so-called long-period variable star, a red giant that pulsates is size and brightness in a period of about 409 days on average. Check the web for its position and the stars with which to compare its brightness.

Chi Cygni lightcurve from BAA data

Chi was last at maximum brightness at about magnitude 4.7 in April last year, making it an easy naked-eye object. Since then, it faded to become a dim telescopic object near the 13th magnitude last November and is now well on its way to recovery – but will it match last year’s effort or even approach its record brightness at magnitude 3.3? Of this type of variable, only Mira or Omicron Ceti can be brighter, having touched magnitude 2.0.

Telescopes show Porrima in Virgo (see map) to be a fine but very close double star. The modestly named Zubenelgenubi in Libra is another much wider double, easy through binoculars and even visible to a good naked eye although not when the full Moon in nearby as it is on the 5th.

The bright Moon will also swamp views of the peak of the Eta-Aquarids meteor shower, expected within a day or so of the 6th. Its fast meteors diverge from a point that lies a few degrees above Saturn as it climbs low into the east-south-east during our pre-dawn twilight. While poorly placed for us, the shower is seen much better from the southern hemisphere where it is one of the finest of the year.

Last month I mentioned that the spring is sometimes called “Galaxy Season”, because of the many galaxies that pepper the region of sky from Virgo northwards. We might just as well refer to “Satellite Season”, too, because of the numerous satellites crossing our sky throughout the night at present, and visible as moving points of light against the stars.

Satellite pollution captured by one of our meteor cameras

Of course, the satellites are there during our winter nights, too, but, except for a period around dusk and dawn, they are usually deep in the Earth’s shadow and invisible. Now, though, the Sun dips no more than a few degrees below our northern horizon overnight where it can still illuminate satellites a few hundred kilometers above our heads, rendering them visible.

The brightest is the International Space Station, the ISS, which can outshine every star as it sweeps eastwards and up to 40° high across Edinburgh’s southern sky during the night. It is just beginning a series of passes in our predawn sky, but these shift earlier each night, eventually petering out at dusk by the 24th. We can get customized predictions using several smartphone apps and also via the website.

Much to the consternation of observers, and especially astrophotographers, satellite numbers have exploded over recent years, particularly with the launch of a multitude of Starlink satellites by Elon Musk’s SpaceX corporation. There were 3,903 Starlinks in orbit as of a few days ago, and although a handful orbit too far south to trouble us, the majority are brighter than the naked-eye limit and hard to avoid as their unwanted light “photobombs” images of distant stars and galaxies.

Diary for 2023 May

Times are BST

  • 2nd 00h Mercury in inferior conjunction on near side of Sun
  • 4th 03h Moon 3° N of Spica
  • 5th 19h Full Moon and penumbral lunar eclipse
  • 6th Peak of Eta-Aquarids meteor shower
  • 7th 14h Moon 1.5° N of Antares
  • 9th 21h Uranus in conjunction with Sun
  • 10th 21h Mars 5° S of Pollux
  • 12th 15h Last quarter
  • 13th 14h Moon 3° S of Saturn
  • 17th 02h Venus 0.7° N of Mebsuta (Epsilon Geminorum)
  • 17th 15h Moon occults Jupiter in daylight
  • 19th 17h New Moon
  • 23rd 13h Moon 2.2° N of Venus
  • 24th 03h Moon 1.6° S of Pollux
  • 24th 19h Moon 4° N of Mars
  • 25th 05h Moon 4°N of Praesepe
  • 27th 01h Moon 5° N of Regulus
  • 27th 16h First quarter
  • 29th 07h Mercury furthest W of Sun (25°)
  • 30th 17h Venus 4° N of Pollux
  • 30th 22h Mars furthest from Sun (249m km)
  • 31st 12h Moon 3° N of Spica

This is an extended version, with added diary, of Alan’s article published in The Scotsman on 29 April 2023, with thanks to the newspaper for permission to republish here.