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.
Saturn at its best as summer begins
The Sun reaches its most northerly place in the sky at the summer solstice on the 20th, regarded by many as the start of summer in our northern hemisphere. Contradictorily, though, the days around then are also classed as midsummer though the actual days of any midsummer celebrations vary from country to country. More sensibly, in my view, the Met Office defines summer to span the months of June to August which would place the middle of summer in mid-July and, consequently, means that summer begins on June 1.
The solstice occurs late on the 20th, at 23:34 BST, while sunrise/sunset times for Edinburgh change from 04:35/21:47 on the 1st, to 04:26/22:03 on the 20th and 04:31/22:02 on the 30th. Scotland’s nights remain twilight throughout, with little hope of spotting the fainter stars and, from the north of the country, only the brighter stars and planets may be seen.
One is the beautiful ringed world Saturn which stands opposite the Sun in the sky on June 3, only twelve days after Mars’ closest opposition since 2005. Both planets shine brightly in the south at our star map times as they track westwards across the sky. Unfortunately, Saturn climbs less than 14° above Edinburgh’s horizon and Mars is a degree or so lower still so telescopic views are hindered by their low altitudes.
Having stood at its closest (75 million km) on May 30, Mars fades from magnitude -2.0 to -1.5 as it recedes to 86 million km while telescopes show it contracting from almost 19 to 16 arcseconds in diameter, still large enough to show some detail on the disk. It tracks 5° westwards into the heart of Libra this month, its motion slowing to a halt on the 30th before resuming as an easterly progress that will persist for the next two years.
Saturn’s disk is similar in size, 18 arcseconds at opposition, but its rings are 42 arcseconds wide and have their north face tipped 26° towards us. Not since 2003 have the rings been so wide open to inspection. It dims slightly, from magnitude 0.0 to 0.2 as it creeps westwards in southern Ophiuchus about 7° above-left of Antares in Scorpius.
Third but not least in our planetary line-up, Jupiter is prominent at magnitude -2.0 in the south-west as the sky darkens at present, but sinks lower with each day and sets in the west a little more than one hour after our map times. Now moving eastwards below the main figure of Leo, it passes within 0.1° south of the magnitude 4.6 double star Chi Leonis on the 10th and dims a shade to magnitude -1.9 by the 30th.
Of the other naked-eye planets, Mercury stands 24° west of the Sun on the 4th and, while well placed for observers south of the equator, is swamped in our predawn twilight. Venus reaches superior conjunction on the Sun’s far side on the 6th and is not visible either.
The Moon is new on the 5th, close to Jupiter on the 11th, at first quarter on the 12th, above Spica in Virgo on the 14th, above-right of Mars on the 17th, close to Saturn on the 18th, full in the 20th and at last quarter on the 26th.
Our star charts show the stars of the Summer Triangle, Vega, Deneb and Altair, climbing in the east to south-east as the Plough stands high in the north-west. The curve of the Plough’s handle extends to the brightest star visible at our map times, Arcturus in Bootes. Look some 20° above and to the left of Arcturus for the pretty arc of stars that make up Corona Borealis, the Northern Crown, which, because it is incomplete, should perhaps be called the Northern Tiara.
Corona’s leading star has the dual names of Gemma, for an obvious reason, and Alphecca which derives from the Arabic for “the bright of the broken ring”. At magnitude 2.2, though, it was surpassed briefly and unexpectedly exactly 150 years ago, in 1866, by the appearance of a nova (“new star”) just beyond the crown’s south-eastern edge.
Now called T Coronae Borealis (or T CrB), this reached magnitude 2.0 but plunged below naked eye visibility after only eight days to became slightly variable in brightness as a telescopic object just fainter than the tenth magnitude. To much surprise it burst into prominence again in 1946 though this time it was already fading at magnitude 3.2 when it was first spotted.
T CrB thus earned its nickname as the Blaze Star and became the brightest known of ten such recurrent novae in the sky. Studies over the past year show it slightly brighter and bluer than usual and hint that a new outburst may occur at any time, so this is one to check regularly.
Another variable star, R CrB, is usually near the sixth magnitude and the brightest star within the crown. However, normally for a few weeks or months every few years, it fades to become a dim telescopic object when, so it is thought, clouds of soot form in its atmosphere and block its light. Strangely, it has yet to recover following a record-breaking fade in 2007 and was still near the 14th magnitude a few days ago.
Despite our summer twilight, Scotland is best placed to see noctilucent or “night-shining” clouds which may appear cirrus-like and often bluish low down between the north-west after sunset and the north-east before dawn. Formed by layers of ice-crystals near 82 km in height, these are Earth’s highest clouds and able to shine in the sunlight long after our normal clouds have dimmed to darkness.
This is a slightly-revised version of Alan’s article published in The Scotsman on June 1st 2016, with thanks to the newspaper for permission to republish here.
Venus and Jupiter converge for twilight rendezvous
The perpetual twilight during Scotland’s all-too-brief June nights means that the month is rarely a vintage one for stargazing. Indeed, from the north of the country, all but the brighter stars and planets are swamped by the “gloaming” or, for those in the Northern Isles, the “simmer dim”.
This year, though, we have two powerful excuses for staying up late. Not only is the beautiful world Saturn at its stunning best, but the two brightest planets, Venus and Jupiter, are converging in our western evening sky on their way to a spectacular close conjunction low down in the twilight at the month’s end. Just expect a flurry of UFO reports.
The Sun is at its furthest north over the Tropic of Cancer at 17:38 BST on the 21st, the moment of our summer solstice when days are at their longest over Earth’s northern hemisphere. For Edinburgh, sunrise/sunset times vary from 04:35/21:46 BST on the 1st, to 04:26/22:03 on the 21st and 04:30/22:02 on the 30th. The Moon is full on the 2nd, at last quarter on the 9th, new on the 16th and at first quarter on the 24th.
Venus is brilliant in our western evening sky and sets in the west-north-west just prior to our star map times. It reaches its greatest angular distance of 45° east of the Sun on the 6th and grows even brighter from magnitude -4.3 to -4.4 – bright enough to be glimpsed in broad daylight. However, its motion against the stars is taking it southwards in the sky so that its altitude at Edinburgh’s sunset plunges from 27° on the 1st to 16° by the 30th.
As Venus approaches from 113 million to 77 million km, a telescope shows its disk swelling from 22 to 32 arcseconds across while the sunlit portion falls from 53% to 32%. The planet is said to reach dichotomy when it is 50% illuminated on the 6th, but it seems that optical effects involving its deep cloudy atmosphere mean that observers see the phase occurring several days earlier than predicted when Venus is an evening star.
Venus lies below and left of the Castor and Pollux in Gemini as the month begins, but it tracks east-south-eastwards across Cancer and into Leo to pass 0.9° north of the Praesepe star cluster (use binoculars) on the 13th.
As it does so, it closes on the night’s second brightest planet, Jupiter, which creeps much more slowly from Cancer into Leo. Jupiter stands 21° to the left of Venus on the 1st but is barely 0.4°, less than a Moon’s breadth, above-left of Venus by the evening of the 30th. The giant planet dims slightly from magnitude -1.9 to -1.8 during the period and shrinks from 34 to 32 arcseconds as it recedes from 851 million to 909 million km.
Don’t forget that Jupiter’s four main moons can be followed through a telescope or decent binoculars as they orbit from side to side of the planet. Our own Moon is a 19% sunlit crescent on the 20th when it stands 5° below Jupiter which, in turn, is 6° left of Venus. On the next evening the Moon lies 5° below-left of the star Regulus in Leo which is about to set in the west-north-west at our map times as the Plough stands high above.
Mars and Mercury are not observable this month. Mars reaches conjunction on the Sun’s far side on the 14th while Mercury reaches 22° west of the Sun on the 24th but is swamped by our morning twilight.
The constellations of Hercules and Ophiuchus, both sparsely populated with stars, loom in the south at the map times while the Summer Triangle formed by Vega, Altair and Deneb is high in the east to south-east. Were we under darker skies a few degrees further south, in southern Europe for example, then we might spy the Milky Way flowing through the Triangle as it arches across the eastern sky from Sagittarius and Scorpius in the south. The latter are rich in stars and star clusters, but they hug Scotland’s southern horizon where only the bright yellowish Saturn and the distinctly red supergiant star Antares, half as bright and more than 11° below-left of the planet, stand out.
Saturn stood at opposition on May 23 but is still observable throughout the night as it moves from the south-east at nightfall to the south-west before dawn, peaking only 16° above Edinburgh’s southern horizon thirty minutes before our map times. As the planet edges 2° westwards in eastern Libra, it tracks away from the fine double star Graffias in Scorpius, below and to its left. Look for it close to the Moon on the evenings of the 1st and 28th.
This month Saturn fades a little from magnitude 0.1 to 0.3 but remains a striking telescopic sight. Its iconic ring system is tilted 24° towards us and spans 42 arcseconds around the rotation-flattened 18 arcseconds globe.
On the opposite side of our sky to Saturn is the star Capella in Auriga which transits low across the north from the north-west at dusk to the north-north-east before dawn. It is in this region of our sky that we sometimes see noctilucent clouds. Appearing like wisps, ripples and sheets of silvery-blue cirrus, these form as ice condenses around particles near altitudes of 82 km where they glow in the sunlight after our more familiar weather or troposphere clouds are in darkness. Best seen from latitudes between 50° and 60° N, ideal for Scotland, they appear for just a few weeks around the solstice, from about mid-May to early-August.
This is a slightly-revised version of Alan’s article published in The Scotsman on May 29th 2015, with thanks to the newspaper for permission to republish here.
The mysterious noctilucent clouds of summer
If we are prepared to do battle with June’s night-long twilight, and provided the weather improves at last, there is plenty of interest in our June sky. Saturn is the pick of the planets while the bright star Vega in Lyra leads the onslaught as the constellations of summer invade from the east at our star map times. We also need to be alert for noctilucent clouds as they make their seasonal appearance low in our northern sky.
The Sun is furthest north at 11:51 BST on the 21st, the instant of our summer solstice. On that day, the Sun dips only 10.6° below Edinburgh’s northern horizon in the middle of the night, so that our sky remains bathed in twilight throughout the night while from further north in Scotland the sky is brighter still. This obviously impedes our ability to see the dimmer stars and “faint fuzzies” such as galaxies and nebulae. On the other hand, it means that satellites remain sunlit whenever they pass overhead. Indeed, the International Space Station is conspicuous two or three times each night until 10 June as it transits from west to east across Scotland’s southern sky – visit heavens-above.com for predictions customised for your location.
The Sun’s shallow sweep below our northern horizon overnight also allows us occasional views of noctilucent or “night-shining” clouds. Composed of tiny ice crystals in a thin layer at a height near 82 km, they catch the sunlight long after our usual low-level clouds are in darkness and can appear like chaotic banks of electric-blue cirrus, sometimes in a herringbone pattern. Their preferred direction follows the Sun around the horizon, so they are more commonly seen low in the north-west after nightfall and towards the north-east before dawn. They occur from mid-May to mid-August but why they are more frequent than they were a century ago remains a mystery. Could the rise be due to global warming, increased industrial pollution or even particles from rocket launches?
Sunrise/sunset times for Edinburgh 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 first quarter on the 5th, full on the 13th, at last quarter on the 19th and new on the 27th.
At magnitude -1.9, our brightest evening planet continues to be Jupiter, but we must look lower into the west to catch it below Pollux in Gemini as the twilight fades. Shining at magnitude -1.9, it stands 9° above-right of the Moon on the 1st. Jupiter sinks to set in the north-west almost three hours after the Sun as June begins but by the 30th it is only 6° high at sunset and may already be lost from view.
Mercury lies 18° below and to the right of Jupiter on the 1st but is one twentieth as bright at magnitude 1.4 and fading rapidly as it moves to pass through inferior conjunction between the Sun and Earth on the 19th.
The bright star Arcturus in Bootes stands high on the meridian at nightfall but has moved to the middle of our south-western sky by the map times. This leaves our high southern sky devoid of bright stars until we come to Vega in Lyra high in the east-south-east. Directly below Vega is Altair in Aquila while Deneb in Cygnus, almost due east, completes the Summer Triangle. The arc from Vega to Arcturus cuts through Hercules and Corona Borealis, the pretty semi-circular Northern Crown whose main star has the dual names of Alphecca or, perhaps more appropriately, Gemma.
Mars fades from magnitude -0.5 to 0.0 as it tracks eastwards in Virgo towards Spica. It also recedes from 119 million to 148 million km during the month as its small disk contracts from 12 to 9 arcseconds if viewed through a telescope. Look for its reddish light about 26° high in the south-west at nightfall and catch it above the Moon on the 7th. Our maps show it sinking towards the west where it sets two hours later.
Saturn, magnitude 0.2 to 0.4, stands almost 20° high in the south at nightfall at present and continues to creep westwards in Libra almost 4° above-left of the double star Zubenelgenubi. After standing close to Spica on the 8th, the Moon lies near Saturn on the 10th when the planet appears 18 arcseconds wide, its disk set within rings that span 41 arcseconds and have their north face inclined 21° towards us. Don’t miss an opportunity to observe it this month for it will soon be following Mars lower into the south-west at nightfall, and it stands even further south in our summer sky during every year until 2022.
Continuing as a brilliant morning star of magnitude -4.0 to -3.9, Venus rises above Edinburgh’s east-north-eastern horizon 61 minutes before the Sun tomorrow and in the north-east 102 minutes before sunrise on the 30th. Before dawn on the 24th, it lies 5° left of the slender waning Moon and 6° below the Pleiades in Taurus.
Last month, I reported the prediction that the Earth would slice through streams of particles from Comet 209P/LINEAR on the morning on 24 May and that the resulting meteor shower might be spectacular. In fact, it appears that the encounter occurred as forecast, but that the resulting display was disappointing with only a few bright meteors, even for observers in the Americas for whom the timing of the outburst was ideal. Radar studies suggest that the vast majority of meteoroids were unusually small and their meteors too dim to be seen by the unaided eye.