Mercury at its evening best as JUICE sent to Jupiter

Alan Pickup

Author: Alan Pickup

The maps show the sky at midnight BST on the 1st, 23:00 on the 16th and 22:00 on 30th. Arrows show the motions of Venus and Mars.

April is our last full month with true nighttime darkness before the fainter stars are swamped by the persistent twilight that extends though Scotland’s summer. Meantime, our favourite winter constellation of Orion makes a speedy exit from the western evening sky where the smallest planet, Mercury, is joining Mars and the brilliant Venus. The latter blazes at magnitude -4.0 to -4.1 and stands at its highest for the year at nightfall.

Mercury is making its best evening appearance of 2023, so this is our chance to spot this elusive world – something even many astronomers have yet to do. As viewed from Edinburgh one hour after sunset it stands 4° high in the west-north-west on the 1st and 8° high between the 8th and the 15th. This puts it around 20° below and to the right of the much more conspicuous Venus. Mercury is a naked-eye object as the month begins but catch it early for it dims from magnitude -1.1 on the 1st, to 0.1 on the 11th and 2.2 by the 21st when even binoculars may struggle to show it 5° below-right of the very slender young Moon.

JUICE spacecraft Image: European Space Agency

After its passing dalliance with Venus a month ago, Jupiter has now dived into our evening twilight on its way to conjunction beyond the Sun on the 11th. Two days later, at 13:15 BST on the 13th, the Jupiter Icy Moons Explorer (JUICE) mission is set to be launched by an Ariane 5 rocket from Europe’s Spaceport at Kourou in French Guiana.

Built by the European Space Agency, JUICE should reach Jupiter-orbit in 2031 to begin its exploration of the three largest Jovian moons which are thought to have extensive subsurface oceans of water making them potentially habitable. Indeed, Ganymede, the largest moon in our solar system, may hold more water than all the oceans of Earth put together.

The plan calls for JUICE to enter orbit around Ganymede in 2034, becoming the first craft to orbit a planetary moon other than our Moon. In 2035 it is due to shrink its orbit to allow a final close inspection from 500 km before ending its life in a suicidal crash into the moon’s icy crust. JUICE’s mission should overlap with NASA’s Europa Clipper probe to be launched next year with arrival at Jupiter in 2030.

Edinburgh’s sunrise/sunset times change from 06:44/19:50 BST on the 1st to 05:32/20:49 on the 30th. Full moon on the 6th is followed by last quarter on the 13th, new moon on the 20th and first quarter on the 27th. A solar eclipse on the 20th, not visible from Europe, occurs along a path that stretches between the southern Indian and central Pacific oceans. Along some of this length, the Moon appears too small to hide the Sun completely, resulting in an annular or ring eclipse. Only for part of the path north-west of Australia is the Moon large enough for a brief total eclipse to be witnessed.

As darkness falls on the 1st, Orion stands in the south-west, the three stars of his Belt pointing left to Sirius in Canis Major. Above Orion is the planet Mars, though now much fainter than it was at opposition in December.

April sees Mars dim further from magnitude 0.9 to 1.3 as it tracks eastwards from near the feet of Gemini to lie close to the star Mebsuta on the 14th and 7° below Castor and Pollux as the month ends. Use binoculars to spy the star cluster M35 1.7° below Mars on the 1st and remember to catch the crescent Moon 4° to the right of Mars on the 25th.

Our star maps show Orion disappearing below our western horizon as Castor and Pollux stand side by side some 30° above it. Regulus in Leo has a similar altitude in the south-west while the Praesepe star cluster in Cancer sits about 60% of the way from Regulus to Pollux.

Markarian’s Chain, galaxies in Virgo by Ramsay McIver

To the left (east) of Leo’s tail star, Denebola, is the Virgo cluster of galaxies which leads this time of year to be dubbed Galaxy Season by some observers and astrophotographers. We need a telescope and a dark sky, and much more detailed charts than I can provide here, to identify them and the other galaxies that proliferate in Leo as well as northwards as far as Ursa Major overhead. Ironically, the brightest external galaxy, M31 in Andromeda, is as poorly placed as it could be at our map times, hovering near our northern horizon below Cassiopeia.

The often-overlooked Lyrids meteor shower is active during the second half of the month, with most of its medium-speed meteors arriving within a day to two of its expected peak on the morning of the 23rd. Meteor rates are not high, perhaps some 15 per hour at best, but with no moonlight interference this time conditions are very favourable. Lyrid meteors diverge from a point that lies about 8° to the right of the bright star Vega in Lyra which climbs though the north-eastern and eastern sky overnight.

From some 30° high in the west at sunset, Venus sinks to the north-western horizon by about 23:45 BST tomorrow and 01:10 at the month’s end. Its path against the stars takes it from Aries to Taurus where it passes below-left of the Pleiades, lying within 4° of the cluster from the 8th to the 13th. The planet tracks onwards to stand between the horns of Taurus and above Aldebaran later in the period. The crescent Moon lies between Venus and the Pleiades on the 22nd and 3° above-left of Venus on the next evening.

Similar in size to Earth, Venus is blanketed by clouds and a deep carbon dioxide atmosphere above a baking-hot surface of lava plains and volcanoes, though whether the latter are still active has been a mystery. Scientists have now reexamined radar scans taken by NASA’s Magellan orbiter in the 1990s and realised that at least one volcanic vent was definitely active since its appearance changed dramatically over an eight-month period. The inference is that, rather than being frozen in time, Venus must still be geologically active.

Diary for 2023 April

Times are BST

  • 2nd 09h Moon 5° N of Regulus
  • 6th 06h Full Moon
  • 6th 18h Moon 3° N of Spica
  • 10th 07h Moon 1.5° N of Antares
  • 10th 22h Venus 2.6° S of Pleiades
  • 11th 23h Mercury furthest E of Sun (19°)
  • 11th 23h Jupiter in conjunction with Sun
  • 13th 10h Last quarter
  • 13th 13:15 Scheduled launch of JUICE mission to Jupiter
  • 14th 20h Mars 0.15° S of Mebsuta (Epsilon Geminorum)
  • 20th 05h New Moon and hybrid (annular/total) solar eclipse
  • 20th 22h Venus 8° N of Aldebaran
  • 21st 08h Moon 1.9° S of Mercury
  • 23rd 06h Peak of Lyrids meteor shower
  • 26th 16h Moon 1.8° S of Pleiades
  • 22nd 11h Moon 9° N of Aldebaran
  • 23rd 14h Moon 1.3° N of Venus
  • 26th 03h Moon 3° N of Mars
  • 26th 19h Moon 1.5° S of Pollux
  • 27th 22h First quarter
  • 27th 21h Moon 4°N of Praesepe
  • 29th 17h Moon 5° N of Regulus

This is an extended version, with added diary, of Alan’s article published in The Scotsman on 31 March 2023, with thanks to the newspaper for permission to republish here.