Saturn at its best as noctilucent clouds gleam
The first day of June marks the start of our meteorological summer, though some would argue that summer begins on 21 June when (at 05:25 BST) the Sun reaches its most northerly point at the summer solstice.
Sunrise/sunset times for Edinburgh vary surprisingly little from 04:35/21:47 BST on the 1st, to 04:26/22:03 at the solstice and 04:31/22:02 on the 30th. The Moon is at first quarter on the 1st, full on the 9th, at last quarter on the 17th and new on the 24th.
The Sun is already so far north that our nights remain bathed in twilight and it will be mid-July before Edinburgh sees its next (officially) dark and moonless sky. This is a pity, for the twilight swamps the fainter stars and, from northern Scotland, only the brightest stars and planets are in view.
If we travel south, though, the nights grow longer and darker, and the spectacular Milky Way star fields in Sagittarius and Scorpius climb higher in the south. From London at the solstice, for example, official darkness, with the Sun more than 12° below the horizon, lasts for three hours, while both Barcelona and Rome rejoice in more than six hours.
It is in this same area of sky, low in the south in the middle of the night, that we find the glorious ringed planet Saturn. This stands just below the full moon on the 9th and is at opposition, directly opposite the Sun, on the 15th when it is 1,353 million km away and shines at magnitude 0.0, comparable with the stars Arcturus in Bootes and Vega in Lyra. The latter shines high in the east-north-east at our map times and, together with Altair in Aquila and Deneb in Cygnus, forms the Summer Triangle which is a familiar feature of our nights until late-autumn.
Viewed telescopically, Saturn’s globe appears 18 arcseconds wide at opposition while its rings have their north face tipped 27° towards us and span 41 arcseconds. Sadly, Saturn’s low altitude, no more than 12° for Edinburgh, means that we miss the sharpest views although it should still be possible to spy the inky arc of the Cassini division which separates the outermost of the obvious rings, the A ring, from its neighbouring and brighter B ring.
Other gaps in the rings may be hard to spot from our latitudes – we can only envy the view for observers in the southern hemisphere who have Saturn near the zenith in the middle of their winter’s night. For us, Saturn is less than a Moon’s breadth further south over our next two summers, while the ring-tilt begins to decrease again.
On the other hand, we can sympathize with those southern observers for most of them never see noctilucent clouds, a phenomenon for which we in Scotland are ideally placed. Formed by ice condensing on dust motes, their intricate cirrus-like patterns float at about 82 km, high enough to shine with an electric-blue or pearly hue as they reflect the sunlight after any run-of-the-mill clouds are in darkness. Because of the geometry involving the Sun’s position below our horizon, they are often best seen low in the north-north-west an hour to two after sunset, shifting towards the north-north-east before dawn – along roughly the path taken by the bright star Capella in Auriga during the night.
Jupiter dims slightly from magnitude -2.2 to -2.0 but (after the Moon) remains the most conspicuous object in the sky for most of the night. Indeed, the Moon lies close to the planet on the 3rd – 4th and again on the 30th. As the sky darkens at present, it stands some 30° high and just to the west of the meridian, though by the month’s end it is only half as high and well over in the SW. Our star maps plot it in the west-south-west as it sinks closer to the western horizon where it sets two hours later.
The giant planet is slow-moving in Virgo, about 11° above-right of the star Spica and 3° below-left of the double star Porrima. As its distance grows from 724 million to 789 million km, its disk shrinks from 41 to 37 arcseconds in diameter but remains a favourite target for observers.
The early science results from NASA’s Juno mission to Jupiter were released on 25 May. They reveal the atmosphere to be even more turbulent than was thought, with the polar regions peppered by 1,000 km-wide cyclones that are apparently jostling together chaotically. This is in stark contrast to the meteorology at lower latitudes, where organized parallel bands of cloud dominate in our telescopic views. In addition, the planet’s magnetic field is stronger and more lumpy than was expected. Juno last skimmed 3,500 km above the Jovian clouds on 19 May and is continuing to make close passes every 53 days.
Both Mars and Mercury are hidden in the Sun’s glare this month, the latter reaching superior conjunction on the Sun’s far side on the 21st.
Venus, brilliant at magnitude -4.3 to -4.1, is low above our eastern horizon before dawn. It stands at its furthest west of the Sun in the sky, 46°, on 3 June but it rises only 78 minutes before the Sun and stands 10° high at sunrise as seen from Edinburgh. By the 30th, it climbs to 16° high at sunrise, having risen more than two hours earlier. Between these days, it shrinks in diameter from 24 to 18 arcseconds and changes in phase from 49% to 62% illuminated. It lies left of the waning crescent Moon on the 20th and above the Moon on the following morning.
This is a slightly-revised version of Alan’s article published in The Scotsman on May 31st 2017, with thanks to the newspaper for permission to republish here.
Smallest planet Mercury at its best for the year
It is surprising just how quickly our evening sky changes during April. If we look to the south-west as the twilight fades tonight, the foremost winter constellation of Orion stands well clear of the horizon, with Sirius sparkling to his left and Taurus and the Pleiades to the right, almost due west. By the month’s end, though, Orion has all-but-set as nautical twilight ends in the evening and only Betelgeuse at Orion’s shoulder remains (barely) in view, as shown by our monthly star chart. Taurus is setting, too, and Sirius has already gone.
Jupiter remains a striking object which is ideally placed for study in our evening sky as it slips westwards in the southern part of Leo, some 14° below and to the left of Regulus. Having stood at opposition, directly opposite the Sun, on March 8, the giant planet is unmistakable in the south-east at nightfall at present and passes 41° high in the S only thirty minutes before our map times.
Those maps have the Plough overhead, dragged out and distorted by the map projection used, while Virgo and the star Spica are nearing the meridian to the left of Jupiter. Spilling northwards from Virgo into the constellation of Coma Berenices is a region sometimes called the Realm of the Galaxies. This includes the Virgo cluster of well over 1,000 galaxies whose core lies some 54 million light years away and is roughly coincident with the “D” of “Denebola” on the chart.
More than a dozen of these Virgo galaxies are visible through medium-sizes telescopes and have entries in Charles Messier’s 18th century list of comet-like smudges in the sky. While it is impractical to plot their locations on our chart, a Web search and dark moonless skies over the next fortnight should allow some to be spotted.
The Sun climbs more than 10° northwards during April as sunrise/sunset times for Edinburgh change from 06:42/19:52 BST on the 1st to 05:30/20:51 on the 30th. New moon on the 7th is followed by first quarter on the 14th, full moon on the 22nd and last quarter on the 30th.
This month, Jupiter recedes from 676 million to 723 million km, dims only slightly from magnitude -2.4 to -2.3 and shrinks in diameter from 44 to 41 arcseconds. Binoculars show its four main moons and also show Jupiter passing only 7 arcminutes (one quarter of a Moon-breadth) above-left of the magnitude 4.6 star Chi Leonis on the 8th.
Jupiter loses its status as our sole evening planet this month. One of its usurpers is Mars which, as we’ll see, rises before midnight later in the period. The other is Mercury which emerges from the Sun’s glare over the coming week at the start of its best evening apparition of the year.
By the evening of the 8th, Mercury shines brightly at magnitude -0.9 and stands almost 8° high in the west-north-west 40 minutes after sunset. The very young crescent Moon, only 3% illuminated and brightly earthlit, lies 6° to its left but we will need a clear horizon, and perhaps even binoculars, to pick them out of the twilight. On the next evening, Mercury is 17° below-right of the still-spectacular earthlit Moon while, on the 10th, the Moon stands against the V-shaped Hyades star cluster in Taurus and sets as it draws close to the bright star Aldebaran.
As the Moon continues onwards, eventually to shine alongside Regulus on the 16th and Jupiter on the 17th, Mercury climbs to stand furthest east of the Sun (20°) on the 18th. By then, the innermost and smallest planet is 11° high 40 minutes after sunset and sets itself more than 90 minutes later still. It is fainter, though, at magnitude 0.2 and it dims further to magnitude 1.5 by the 25th when it is 2° lower and 7° below-right of the Pleiades. As Mercury dives towards the Sun’s near side, it is heading for a spectacular transit across the Sun’s face on May 9.
Mars is brightening rapidly as it draws closer to us on its way to opposition on May 22. It rises in the south-east about one hour after our chart times and from the 21st onwards rises before midnight as seen from Edinburgh. The best time to see it, though, is just before dawn when it reaches its highest point, admittedly only low down in the south. At magnitude -0.5 it is already the brightest object (after the Moon) down there, and by the month’s end it is more than twice as bright at magnitude -1.4.
Mars is currently in Scorpius, 6° north-north-west of the red supergiant star Antares, but it tracks 1.5° eastwards to a stationary point in Ophiuchus on the 17th before doubling back into Scorpius. Meantime, it approaches from 118 million to 87 million km and its disk swells from 12 to 16 arcseconds wide. Telescopes should reveal some markings on it desert surface, and perhaps its northern polar cap, but its low altitude is likely to make for poor observing conditions.
Not far to Mars’ left, and another victim of its low altitude, is the glorious ringed world Saturn. The second brightest object low in the south before dawn, it improves from magnitude 0.4 to 0.2 as it edges 1° westwards in Ophiuchus. Saturn lies almost 1,400 million km away in mid-April when its rotation-flattened disk is 18 arcseconds across and the rings span 40 arcseconds, their north face tipped 26° earthwards.
The Moon stands 4° above Mars before dawn on the 25th and a similar distance above-left of Saturn on the next morning.