Three planets outshine the stars during June’s twilit nights

The maps show the sky at 01:00 BST on the 1st, midnight on the 16th and 23:00 on the 30th. An arrow depicts the motion of Mars. (Click on map to enlarge)

The maps show the sky at 01:00 BST on the 1st, midnight on the 16th and 23:00 on the 30th. An arrow depicts the motion of Mars. (Click on map to enlarge)

Unless our passion is for observing the Sun, Scotland’s brief twilit nights can make June a frustrating month for stargazers. This month, though, three planets outshine all the stars, while a fourth, the handsome ringed world Saturn, is at its best for the year.

We are approaching the summer solstice, due this year at 11:07 BST on the 21st when the Sun is overhead at the Tropic of Cancer. On that day, it passes 57.5° high in the south for Edinburgh at 13:14 BST, the time of local noon.

The middle of the following night sees the Sun 10.6° below the northern horizon for Edinburgh and a mere 6.4° below for Lerwick in Shetland which is why, over northern Scotland in particular, only the brighter stars and planets may be glimpsed.

Edinburgh’s sunrise and sunset times change from 04:35/21:47 BST on the 1st, to 04:26/22:03 on the 21st and 04:31/22:02 on the 30th. The Moon is at last quarter on the 6th, new on the 13th, at first quarter on the 20th and full on the 28th.

Our chart is timed for around the middle of the night at present and depicts three of those planets as they line up low across our southern sky. Even brighter, though, is the brilliant Venus which blazes at magnitude -4.0 low in the west-north-west after sunset and sinks to set in the north-west at 00:36 BST on the 1st and just before midnight by the 31st.

Although it is still drawing away from the Sun, Venus sinks lower each evening as it tracks further south in the sky, moving from below Castor and Pollux in Gemini to the western fringe of Leo by the 30th. Look for it 6° to the right of the young Moon on the 16th with the Praesepe star cluster in Cancer just above and to the Moon’s right. On that evening, the planet is 174 million km distant and appears through a telescope as a 75% illuminated disk of diameter 14 arcseconds.

Mercury joins Venus in the evening twilight later in the month but is a real challenge to spot through binoculars from our latitudes. For us, the little innermost planet shines at about magnitude 0.0 but stands only 2° high in the north-west one hour after sunset from the 20th onwards.

Foremost among the planets on our star chart is Jupiter which is prominent at magnitude -2.5 in Libra as it moves from low in the south-south-east at nightfall to the south-south-west by the map times. Having stood directly opposite the sun at opposition on 9 May, it dims slightly to magnitude -2.3 and shrinks to 41 arcseconds across by June’s end. Binoculars reveal its four main moons to either side and the interesting double star Zubenelgenubi less than a degree to its south over the coming nights. Catch it just below the Moon on the 23rd.

This month it is Saturn’s turn to reach opposition when it stands 1,354 million km away on the 27th when it also happens to lie close to the Moon. It passes less than 12 degrees high in the south as seen from Edinburgh in the middle of the night as Vega, the leading star in the Summer Triangle, passes just to the south of overhead.

Improving from magnitude 0.2 to 0.0 to equal Vega in brightness, Saturn is creeping slowly westwards just above the Teapot of Sagittarius though this asterism barely clears our southern horizon. Viewed telescopically, Saturn’s disk is 18 arcseconds broad at opposition while its rings span 41 arcseconds and have their north face tipped 26° towards us.

The night’s final planet, Mars, is rising above Edinburgh’s south-eastern horizon at our map times and climbs to lie 10° high in the south-south-east before dawn. Its orange hue is already conspicuous at magnitude -1.2 and it more than doubles in brightness to magnitude -2.1 by the 30th. Moving eastwards against the stars of Capricornus, it reaches a so-called stationary point on the 28th when its motion reverses to westerly.

Mars approaches from 92 million to only 67 million km during June while its orange-red disk swells from 15 to 21 arcseconds in diameter, becoming large enough for most decent telescopes to reveal something of its surface detail and that its icy south polar cap is tipped at 15° to our view. Mars lies near the Moon on the morning of the 3rd and to the left of the Moon on the 30th.

I mentioned solar observing at the beginning of this note since our long summer days give ample opportunities for viewing the Sun’s surface, or so we hope. Of course, I should repeat the serious warning that we must never look directly at the Sun through any binoculars or telescopes – to do so invites critical damage to the eyes, if not blindness. Instead it is possible to project the Sun’s image onto a card held away from the eyepiece. Alternatively, obtain an inexpensive but certified “solar filter” and follow the instructions carefully on how to employ this.

Of particular interest are sunspots, dark regions on the solar surface that last for anything from a day to several weeks and mark magnetic storms. Their numbers fluctuate in a cycle of roughly 11 years and, following a peak in activity in 2014, are low at present as we near a so-called sunspot minimum which might be due in 2020. However, sunspot numbers have plummeted in recent months and more than half the days in 2018 have been spotless so far, so it is suggested that the official minimum could occur rather earlier than expected.

Diary for 2018 June

Times are BST

1st            02h Moon 1.6° N of Saturn

3rd            13h Moon 3° N of Mars

6th            03h Mercury in superior conjunction on Sun’s far side

6th            20h Last quarter

13th          21h New moon

14th          14h Moon 5° S of Mercury

16th         14h Moon 2.3° S of Venus

16th         21h Moon 1.2° S of Praesepe in Cancer

20th         06h Venus 0.8° N of Praesepe

20th         12h First quarter

21st         11:07 Summer solstice

23rd         20h Moon 4° N of Jupiter

27th         14h Saturn at opposition at distance of 1,354 million km

28th         05h Moon 1.8° N of Saturn

28th         06h Full moon

28th         15h Mars stationary (motion reverses from E to W)

Alan Pickup

This is a slightly revised version, with added diary, of Alan’s article published in The Scotsman on May 31st 2018, with thanks to the newspaper for permission to republish here.