Cassini’s scheduled suicide at Saturn
The heroic Cassini mission to Saturn is set to reach its dramatic conclusion on 15 September. After a seven-year journey from Earth, the probe has been studying the planet, its glorious rings and its fascinating moons for the past thirteen years. Now, with its fuel running low, it is time for the NASA probe to plunge into the Saturnian atmosphere where, in the interest of so-called planetary protection, it will disintegrate and vaporise.
To leave it in orbit around the planet would run the risk of it colliding with the rings or one of the moons, with the outside possibility of contaminating them with microbes from the Earth. This was of little concern when Cassini’s mission was planned, and it carried and delivered the European-built Huygens probe which parachuted to the surface of Saturn’s largest moon, Titan. It touched down on a world in which rivers of liquid hydrocarbons, chiefly methane, flow into lakes in a landscape dominated by water-ice mountains.
Now, though, we realise that despite Saturn’s remoteness from the Sun, the possibility of alien life there cannot be discounted. Indeed, it seems clear that its small moon Enceladus has a subsurface watery ocean and there has been talk of sending a mission to search for organic compounds in the plumes of water erupting from geysers on its surface.
Recent orbits of Saturn have seen Cassini piercing the gap between Saturn and its rings, and even skimming the planet’s outer atmosphere. It will continue to collect data as it begins its final suicidal dive into Saturn’s atmosphere on the 15th, but its signal will be lost at around 13:00 BST as aerodynamic forces cause it to tumble and, eventually, break apart and burn up.
The Sun crosses southwards over the equator at 21:02 BST on the 22nd, the moment of our autumnal equinox. Sunrise/sunset times for Edinburgh change from 06:17/20:07 BST on the 1st to 07:14/18:50 on the 30th. The Moon is full on the 6th, at last quarter on the 13th, new on the 20th and at first quarter on the 28th.
Now that Scotland’s persistent summer twilight is behind us, our nights offer views of the Milky Way as it arches directly overhead from the south-west to the north-east at our chart times, carving through the Summer Triangle formed by Deneb, Altair and Vega which now lies just west of the high meridian.
To the east of the Triangle is the distinctive form of the celestial dolphin, Delphinus, where the celebrated English amateur astronomer George Alcock discovered a famous and unusual naked-eye nova fifty summers ago in 1967. I remember watching the stellar outburst as it took five months to reach its peak brightness at magnitude 3.5. Now assigned the variable-star tag HR Delphini, the star is still visible as a twelfth magnitude object through telescopes.
Another 13° east of Delphinus is the globular star cluster Messier 15, 4° north-west of Pegasus’s brightest star, Enif. A tightly packed globe of perhaps 100,000 stars, all very much older than our Sun, M15 lies around 34,000 light years away and looks like a fuzzy star through binoculars.
Saturn is the sole bright planet to appear on our star maps. Look for it as the brightest object low down in the south-south-west at nightfall and even lower in the south-west by our map times, only thirty minutes before it sets. Edging eastwards in Ophiuchus, it shines 4° below-left of the Moon on the 26th.
Jupiter is bright at magnitude -1.7 but hard to see very low in the west-south-west just after sunset. By mid-month it is likely to be lost in the twilight.
Our charts plot the two outer planets, the ice giant world Uranus in Pisces and its near-twin Neptune in Aquarius, though we probably need more detailed charts to identify them through binoculars or telescopes. At magnitude 5.7, Uranus is at the verge of naked-eye visibility, while Neptune reaches opposition on the 5th and is dimmer at magnitude 7.8.
The other planets are about to join Venus low down in our eastern sky at the end of the night. The brilliant morning star shines at magnitude -4.0 when it rises in the north-east at 03:04 for Edinburgh on 1 September, and climbs 25° high into the east by sunrise. Catch it through binoculars before the twilight intervenes on that day and look 1.2° to its left for the Praesepe or Beehive cluster of stars in Cancer. Leaving the cluster behind, Venus tracks east-south-eastwards into Leo to pass 0.5° (a Moon’s breadth) north of the star Regulus on the 20th.
Mercury emerges from the Sun’s glare to stand 18° west of the Sun and 11° below-left of Venus on the 12th. Between the 6th and 23rd it rises more than 80 minutes before sunrise and brightens eightfold from magnitude 1.1 to -1.1. On the 6th, in fact, Mercury lies 2.5° to the right of Regulus which, in turn, is 0.8° to the right of the fainter magnitude 1.8 planet Mars. As Regulus climbs above them, the two planets then converge to lie less than 0.5° apart on the 16th and 17th.
Early risers are in for a special treat when the waning earthlit Moon joins the party on the 17th. On that morning, Venus stands 10° below-left of the Moon and almost 4° above-right of Regulus, with the Mars-Mercury conjunction another 8° below and to the left. On the 18th, the line-up is even more compact as the Moon shifts to lie 0.7° below Regulus. By the 30th, Venus rises in the east-north-east at 04:41 and is 3° above-right of Mars.
This is a slightly-revised version of Alan’s article published in The Scotsman on August 31st 2017, with thanks to the newspaper for permission to republish here.
Countdown to the Great American Eclipse
With two eclipses and a major meteor display, August is 2017’s most interesting month for sky-watchers. Admittedly, Scotland is on the fringe of visibility for both eclipses while the annual Perseids meteor shower suffers moonlight interference.
The undoubted highlight is the so-called Great American Eclipse on the 21st. This eclipse of the Sun is total along a path, no more than 115km wide, that sweeps across the USA from Oregon at 18:17 BST (10:17 PDT) to South Carolina at 19:48 BST (14:48 EDT) – the first such coast-to-coast eclipse for 99 years.
Totality is visible only from within this path as the Moon hides completely the dazzling solar surface, allowing ruddy flame-like prominences to be glimpsed at the solar limb and the pearly corona, the Sun’s outer atmosphere, to be admired at it reaches out into space. At its longest, though, totality lasts for only 2 minutes and 40 seconds so many of those people fiddling with their gadgets to take selfies and the like may be in danger of missing the spectacle altogether.
The surrounding area from which a partial eclipse is visible even extends as far as Scotland. From Edinburgh, this lasts from 19:38 to 20:18 BST but, at most, only the lower 2% of the Sun is hidden at 19:58 as it hangs a mere 4° high in the west. Need I add that the danger of eye damage means that we must never look directly at the Sun – instead project the Sun through a pinhole, binoculars or a small ‘scope, or use an appropriate filter or “eclipse glasses”.
A partial lunar eclipse occurs over the Indian Ocean on the 7th as the southern quarter of the Moon passes through the edge of the Earth’s central dark umbral shadow between 18:23 and 20:18 BST. By the time the Moon rises for Edinburgh at 20:57, it is on its way to leaving the lighter penumbral shadow and I doubt whether we will see any dimming, It exits the penumbra at 21:51.
Our charts show the two halves of the sky around midnight at present. In the north-west is the familiar shape of the Plough while the bright stars Deneb in Cygnus and Vega in Lyra lie to the south-east and south-west of the zenith respectively. These, together with Altair in Aquila in the middle of our southern sky, make up the Summer Triangle. The Milky Way flows through the Triangle as it arches overhead from the south-west to the north-east where Capella in Auriga rivals Vega in brightness.
Of course, many of us have to contend with light pollution which swamps all trace of the Milky Way and we are not helped by moonlight which peaks when the Moon is full on the 7th and only subsides as last quarter approaches on the 15th. New moon comes on the 21st and first quarter on the 29th. The Sun, meantime, slips another 8° southwards during the month as sunrise/sunset times for Edinburgh change from 05:17/21:20 BST on the 1st to 06:15/20:09 on the 31st.
Meteors of the annual Perseids shower, the tears of St Lawrence, are already arriving in low numbers. They stream away from a radiant point in the northern Perseus which stands in the north-east at our map times, between Capella and the W-pattern of Cassiopeia. We spot Perseids in all parts of the sky, though, and not just around Perseus.
Meteor numbers are expected to swell to a peak on the evening of the 12th when upwards of 80 per hour might be counted under ideal conditions. Even though moonlight will depress the numbers seen this time, we can expect the brighter ones still to impress as they disintegrate in the upper atmosphere at 59 km per second, many leaving glowing trains in their wake. The meteoroids concerned come from Comet Swift-Tuttle which last approached the Sun in 1992.
Although Neptune is dimly visible through binoculars at magnitude 7.8 some 2° east of the star Lambda Aquarii, the only naked-eye planet at our map times is Saturn. The latter shines at magnitude 0.3 to 0.4 low down in the south-west as it sinks to set less than two hours later. It is a little higher towards the south at nightfall, though, where it lies below-left of the Moon on the 2nd when a telescope shows its disk to be 18 arcseconds wide and its stunning wide-open rings to span 40 arcseconds. Saturn is near the Moon again on the 29th.
Jupiter is bright (magnitude -1.9 to -1.7) but very low in our western evening sky, its altitude one hour after sunset sinking from 6° on the 1st to only 1° by the month’s end as it disappears into the twilight. Catch it just below and right of the young Moon on the 25th.
Venus is brilliant at magnitude -4.0 in the east before dawn. Rising in the north-east a little after 02:00 BST at present, and an hour later by the 31st, it climbs to stand 25° high at sunrise. Viewed through a telescope, its disk shrinks from 15 to 12 arcseconds in diameter as it recedes from 172 million to 200 million km and its gibbous phase changes from 74% to 83% sunlit.
As Venus tracks eastwards through Gemini, it passes below-right of the star cluster M35 (use binoculars) on the 2nd and 3rd, stands above-left of the waning earthlit Moon on the 19th and around 10° below Castor and Pollux as it enters Cancer a few days later. On the 31st it stands 2° to the right of another cluster, M44, which is also known as Praesepe or the Beehive.
This is a slightly-revised version of Alan’s article published in The Scotsman on July 31st 2017, with thanks to the newspaper for permission to republish here.
Pole stars of the future in the Summer Triangle
The Sun’s southerly motion since the solstice on 21 June has yet to gain speed and not until 12 July does it lie sufficiently far south for Edinburgh to enjoy any so-called nautical darkness, with the Sun more than 12° below the northern horizon in the middle of the night. Even then, moonlight is troublesome for a few more days to delay our first views of a dark summer night sky.
If there is one star-pattern that dominates our skies over the summer, it is the Summer Triangle. Formed by the bright stars Vega in the constellation Lyra, Deneb in Cygnus and Altair in Aquila, it occupies the upper part of our south star map, though its outline is not depicted. In fact, the projection used means that the Triangle’s proportions are squashed, because Vega and Deneb are significantly closer together in the sky than either are to Altair.
The leader and brightest of the Triangle’s stars is Vega which moves from high in the east at nightfall to stand even higher in the south at our map times. Blazing at magnitude 0.0 from a distance of 25 light years, it is a white star, twice as massive as our Sun but very much younger. Excess heat revealed by infrared astronomy indicates that Vega is encircled by a disk of dust which may be evidence that a planetary system is forming around it.
Set your time machine for about AD 13,700 and you will be able to glimpse Vega close to where we currently find Polaris, our current Pole Star. This is because the Earth’s axis is slowly toppling in space, taking 26,000 years to complete a 47° circle in the sky and carrying the axis to within 4° of Vega. Polaris happens to lie within 0.8° of the axis at present so that, as the Earth rotates once each day, it stays almost fixed in our sky and the other stars appear to circle counterclockwise around it
Altair is the second brightest of the Triangle’s stars and one of the closest bright stars at “only” 16.7 light years. Shining at magnitude 0.8, half as bright as Vega, it is 80% more massive than our Sun but, remarkably, spins on its axis in about nine hours as compared with the more leisurely 25 days taken by the Sun. As a result, it is noticeably oblate, measuring 20% wider across its equator than it does pole-to-pole.
Deneb’s magnitude of 1.2 makes it the dimmest of the Triangle’s corner stars but it is also one of the most luminous stars in our Milky Way Galaxy. Because its distance may be around 2,600 light years, it very difficult to measure the minuscule shift in its position when viewed from opposite sides of the Earth’s orbit around the Sun – the parallax technique that gives us accurate distances to Vega and Altair. Indeed, estimates of Deneb’s distance differ by well over 1,000 light years.
White-hot and shining at some 200,000 Sun-power, Deneb is large enough to engulf the Earth were it to swap places with the Sun. It is also burning its nuclear fuel at such a rate that it seems destined to disintegrate in a supernova within a few million years, although it should survive to be another of our future pole stars as it comes as close as 5° to the pole in AD 9,800.
The Sun eventually tracks 5° southwards during July as Edinburgh’s sunrise/sunset times change from 04:32/22:01 BST on the 1st to 05:15/21:22 on the 31st. The Moon is at first quarter on the 1st, full on the 9th, at last quarter on the 16th, new on the 23rd and returns to first quarter on the 30th.
At magnitude -2.0, Jupiter remains our brightest evening planet though it stands lower in the south-west to west as it sinks to set in the west just before our star map times. Above and to the right of the star Spica in Virgo, it lies to the right of the Moon in the south-west as the sky darkens on the 1st and is just below the Moon and much lower in the west-south-west on the 28th. The cloud-banded Jovian disk appear 39 arcseconds wide at mid-month if viewed telescopically, while binoculars allow glimpses of its four main moons.
Saturn is less conspicuous at magnitude 0.1 to 0.3 but continues as the brightest object low in our southern night sky. Creeping westwards against the stars of southern Ophiuchus, it crosses Edinburgh’s meridian at an altitude of 12° one hour before our map times and may be spotted 3° below-left of the Moon on the 6th. Binoculars show it as more than a round dot, while small telescopes reveal the beauty of ring system which is tilted wide open to our view and spans 41 arcseconds in mid-July.
Venus is brilliant at magnitude -4.1 in the east before dawn. After rising in the north-east at about 02:15 BST throughout the month. it climbs to stand 17° high at sunrise as the month begins and higher still by its end. Seen through a telescope, it is 16 arcseconds across and 70% illuminated when it lies to the left of waning (15% sunlit) and earthlit Moon on the 20th. Against the background stars of Taurus, the planet moves from 8° below-right of the Pleiades tomorrow to pass 3° above-left of Aldebaran on the 14th.
Of the other bright planets, Mars is out of sight as it reaches conjunction on the Sun’s far side on the 27th, while Mercury stands furthest east of the Sun (27°) on the 30th but is unlikely to be seen near our west-north-western horizon in the bright evening twilight.
This is a slightly-revised version of Alan’s article published in The Scotsman on June 30th 2017, with thanks to the newspaper for permission to republish here.
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.
Geminids suffer in the supermoonlight
The Sun reaches its farthest south at our winter solstice at 10:44 GMT on the 21st, as Mars and the brilliant Venus stand higher in our evening sky than at any other time this year. This is not a coincidence, for both planets are tracking eastwards and, more importantly, northwards in the sky as they keep close to the ecliptic, the Sun’s path over the coming weeks and months. Meantime, Jupiter is prominent during the pre-dawn hours while Orion is unmistakable for most of the night and strides proudly across the meridian at midnight in mid-December.
As the sky darkens this evening, Pegasus with its iconic, but rather empty, Square is nearing the meridian and the Summer Triangle (Vega, Deneb and Altair) stands high in the south-west.
By our map times, Altair is setting in the west and Orion stands in the south-east, the three stars of Belt pointing down to where Sirius, our brightest night-time star, will soon rise. Sirius, the red supergiant Betelgeuse at Orion’s shoulder and Procyon in Canis Minor, almost due east of Betelgeuse, form a near-equilateral triangle which has come to be known as the Winter Triangle.
Above Orion is Taurus, home to the Pleiades star cluster and the bright orange giant star Aldebaran, the latter located less than halfway between us and the V-shaped Hyades cluster.
Look for the almost-full Moon below the Pleiades and to the right of Aldebaran and the Hyades on the evening of the 12th and watch it barrel through the cluster during the night, occulting (hiding) several of the cluster’s stars on the way. As they dip low into the west on the following morning, the Moon occults Aldebaran itself, the star slipping behind the Moon’s northern edge between 05:26 and 05:41 as seen from Edinburgh. Even though this is the brightest star to be occulted this year, the Moon’s brilliance means we may well need a telescope to view the event.
Sunrise/sunset times for Edinburgh vary from 08:20/15:44 on the 1st to 08:42/15:40 on the 21st and 08:44/15:48 on the 31st. The Moon is at first quarter on the 7th and full on the 14th when, once again, it is near its perigee, its closest point to the Earth. Despite the fact that the Moon appears a barely perceptible 7% wider than it does on average, we can look forward to yet another dose of over-hyped supermoon hysteria in the media. The Moon’s last quarter comes on the 21st and it is new on the 29th.
Sadly, the Moon does its best to swamp the annual Geminids meteor shower which lasts from the 8th to the 17th and is expected to peak at about 20:00 on the 13th. Its meteors are medium-slow and, thankfully, there are enough bright ones that several should be noticeable despite the moonlight. Without the moonlight, and under perfect conditions, this might have been our best display of 2016, with 100 or more meteors per hour.
Geminids are visible in all parts of the sky, but perspective makes them appear to diverge from a radiant point near the star Castor in Gemini, marked near the eastern edge of our north map. This radiant climbs from our north-eastern horizon at nightfall to pass high in the south at 02:00.
Venus stands 10° above Edinburgh’s southern horizon at sunset on the 1st and shines spectacularly at magnitude -4.2 as it sinks to set in the south-west almost three hours later. The young earthlit Moon stands 10° above-right of Venus on the 2nd, 5° above the planet on the 3rd and, one lunation later, 20° below-right of the Moon on Hogmanay. By then, Venus is twice as high at sunset and (just) brighter still at magnitude -4.3. A telescope shows its dazzling gibbous disk which swells from 17 to 22 arcseconds in diameter as the sunlit portion shrinks from 68% to 57%.
As Venus speeds from Sagittarius to Capricornus, so Mars keeps above and to its left as it moves from Capricornus into Aquarius and into the region of sky above our south-western horizon at the map times. Mars is only a fraction as bright, though, and fades from magnitude 0.6 to 0.9. It also appears much smaller, only 6 arcseconds, so that telescopes now struggle to reveal any surface features. Spot Mars to the left of the Moon on the 4th and below-right of the Moon on the 5th.
Mercury is farthest east of the Sun, 21°, on the 11th but hugs our south-western horizon at nightfall and is unlikely to be seen. It reaches inferior conjunction between the Sun and Earth on the 28th by which time Saturn, which passes beyond the Sun on the 10th, might just be glimpsed low above the south-eastern horizon before dawn. On the 27th, it shines at magnitude 0.5 and lies 7° below-left of the slender waning Moon.
Jupiter is conspicuous at magnitude -1.8 to -1.9 and the real star of our morning sky. Rising in the east for Edinburgh at 03:04 on the 1st and 01:31 on the 31st, it climbs well up into our southern sky before dawn where it stands above Virgo’s leading star Spica and draws closer during the month.
Jupiter, Spica and the Moon form a neat triangle before dawn on the 23rd, when Jupiter is 850 million km away and appears 35 arcseconds wide through a telescope. Any decent telescope shows its parallel cloud belts, while binoculars reveal its four main moons which swap places from side to side of the disk as they orbit the planet in periods of between 1.8 and 17 days.
This is a slightly-revised version of Alan’s article published in The Scotsman on December 1st 2016, with thanks to the newspaper for permission to republish here.
Nights begin with Venus and end at Jupiter
The end of British Summer Time means that we now enjoy six hours of official darkness before midnight, though I appreciate that this may not be welcomed by everyone. The starry sky as darkness falls, however, sees only a small shift since a month ago, with the Summer Triangle, formed by the bright stars Vega, Deneb and Altair, now just west of the meridian and toppling into the middle of the western sky by our star map times.
Those maps show the Square of Pegasus high in the south. The star at its top-left, Alpheratz, actually belongs to Andromeda whose other main stars, Mirach and Almach, are nearly equal in brightness and stand level to its left. A spur of two stars above Mirach leads to the oval glow of the Andromeda Galaxy, M31, which is larger than our Milky Way and, at 2.5 million light years, is the most distant object visible to the unaided eye. It is also approaching us at 225 km per second and due to collide with the Milky Way in some 4 billion years’ time.
Binoculars show M31 easily and you will also need them to glimpse more than a handful of stars inside the boundaries of the Square of Pegasus, even under the darkest of skies. In fact, there are only four such stars brighter than the fifth magnitude and another nine to the sixth magnitude, close to the naked eye limit under good conditions. How many can you count?
Mars is the easiest of three bright planets to spot in tonight’s evening sky. As seen from Edinburgh, it stands 11° high in the south as the twilight fades, shining with its customary reddish hue at a magnitude of 0.4, and appearing about half as bright as the star Altair in Aquila, 32° directly above it.
Now moving east-north-eastwards (to the left), Mars is 5° below-right of the Moon on the 6th and crosses from Sagittarius into Capricornus two days later. Soon after this, it enters the region covered by our southern star map, its motion being shown by the arrow. By the 30th, Mars has dimmed slightly to magnitude 0.6 but is almost 6° higher in the south at nightfall, moving to set in the west-south-west at 21:00. It is a disappointingly small telescopic sight, though, its disk shrinking from only 7.5 to 6.5 arcseconds in diameter as it recedes from 188 million to 215 million km.
We need a clear south-western horizon to spy Venus and Saturn, both low down in our early evening twilight. Venus, by far the brighter at magnitude -4.0, is less than 4° high in the south-west thirty minutes after sunset, while Saturn is 4° above and to its right, very much fainter at magnitude 0.6 and only visible through binoculars. The young earthlit Moon may help to locate them – it stands 3° above-right of Saturn on the 2nd and 8° above-left of Venus on the 3rd.
Mercury is out of sight in the evening twilight and Saturn will soon join it as it tracks towards the Sun’s far side. However, Venus’ altitude thirty minutes after sunset improves to 9° by the 30th when it sets for Edinburgh at 18:30 and is a little brighter at magnitude -4.1. Viewed telescopically, Venus shows a dazzling gibbous disk that swells from 14 to 17 arcseconds as its distance falls from 178 million to 149 million km.
Sunrise/sunset times for Edinburgh change from 07:20/16:31 on the 1st to 08:18/15:44 on the 30th. The Moon reaches first quarter on the 7th, full on the 14th, last quarter on the 21 and new on the 28th.
The full moon on the 14th occurs only three hours after the Moon reaches its perigee, the closest point to the Earth in its monthly orbit. As such, this is classed as a supermoon because the full moon appears slightly (7%) wider than it does on average. By my reckoning, this particular lunar perigee, at a distance of 356,509 km, is the closest since 1948 when it also coincided with a supermoon.
Of the other planets, Neptune and Uranus continue as binocular-brightness objects in Aquarius and Pisces respectively in our southern evening sky, while Jupiter, second only to Venus in brightness, is now obvious in the pre-dawn.
Jupiter rises at Edinburgh’s eastern horizon at 04:28 on the 1st and stands more than 15° high in the south-east as morning twilight floods the sky. It outshines every star as it improves from magnitude -1.7 to -1.8 by the 30th when it rises at 03:07 and is almost twice as high in the south-south-east before dawn.
Currently close to the famous double star Porrima in Virgo, Jupiter is 13° above-right of Virgo’s leader Spica and draws 5° closer during the period. Catch it less than 3° to the right of the waning earthlit Moon on the 25th. Jupiter’s distance falls from 944 million to 898 million km during November while its cloud-banded disk is some 32 arcseconds across.
The annual Leonids meteor shower has produced some stunning storms of super-swift meteors in the past, but probably not this year. Active from the 15th to 20th, it is expected to peak at 04:00 on the 17th but with no more than 20 meteors per hour under a dark sky. In fact, the bright moonlight is likely to swamp all but the brightest of these this year. Leonids diverge from a radiant point that lies within the Sickle of Leo which climbs from low in the east-north-east at midnight to pass high in the south before dawn.
This is a slightly-revised version of Alan’s article published in The Scotsman on November 1st 2016, with thanks to the newspaper for permission to republish here.
Mars bright in evenings as ExoMars probe arrives
As we plunge into the final quarter of the year, our lengthening nights offer a procession of stellar views that stretch from the Summer Triangle in the evening to the stunning star-scapes around Orion during the morning hours. The brighter planets, though, are on show only low down around dusk and dawn.
The middle of the Summer Triangle stands some 60° high and due south as darkness falls tonight. Its brightest corner star, Vega in the constellation Lyra, lies just south-west of overhead, while Deneb in Cygnus is even higher in the south-east and Altair in Aquila lies below them on the meridian.
With no hindering moonlight over the next few evenings, now is a good time to spy the Milky Way as it arches almost overhead after dusk, climbing from Sagittarius on the south-south-western horizon and flowing through the heart of the Triangle on its way to Deneb and the “W” of Cassiopeia high in the north-east. Of course, unless we can find a dark site, away from the pollution of street lighting and the like, we may have trouble seeing the Milky Way or indeed any but the brighter stars on our chart.
Edinburgh’s sunrise/sunset times change this month from 07:16/18:47 BST (06:16/17:47 GMT) on the 1st to 07:18/16:34 GMT on the 31st after we set clocks back one hour with the end of BST on the morning of the 30th. The Moon is new on the 1st, at first quarter on the 9th, full on the 16th (the hunter’s moon), at last quarter on the 22nd and new again on the 30th.
Venus stands nearly 5° high in the south-west at sunset and sets itself only 43 minutes later on the 1st. By the 31st it is barely a degree higher in the south-south-west at sunset but remains visible for 73 minutes so is easier to spot if we enjoy an unobscured outlook. It blazes at magnitude -3.9 and stands 4° below-right of the slender earthlit Moon on the 3rd when its gibbous disk appears 12 arcseconds wide and 85% sunlit if viewed telescopically.
In the month that the first European-Russian ExoMars spacecraft reaches Mars, the planet is the brightest object low in the south-south-west as the twilight disappears. ExoMars consists of a Trace Gas Orbiter to study rare gases, and particularly methane, in Mars’ atmosphere and it also has the experimental Schiaparelli lander.
Mars fades slightly from magnitude 0.1 (almost equal to Vega) to 0.4 this month as it tracks 21° eastwards above the so-called Teapot of Sagittarius, clipping the top star of the Teapot’s lid (Kaus Borealis) on the 7th. The planet recedes from 160 million to 187 million km during October while its gibbous disk shrinks to 7.5 arcseconds in diameter which, with its low altitude, makes telescopic study all the more challenging. It lies below the Moon on the 8th.
A little fainter than Mars, and a little lower to Mars’ right in this evening’s sky, is the ringed planet Saturn. This shines at magnitude 0.6 in southern Ophiuchus and appears 16 arcseconds across, with its glorious rings spanning 36 arcseconds. Saturn lies to the left of the earthlit Moon on the 5th and dips lower with each evening until it is passed by Venus late in the month – catch Saturn 3° above Venus on the 29th.
By our map times, both Saturn and Mars have set and the Summer Triangle has toppled over into the west. High in the south is the Square of Pegasus, a line along its right-hand side pointing down to the southern bright star Fomalhaut in Piscis Austrinus the Southern Fish. Just to the right of this line, and 2° south-west (below-right) of the star Lambda Aquarii (magnitude 3.7), is the farthest of the Sun’s planets, Neptune. At magnitude 7.8 and a distance of 4,350 million km on the 1st, we need binoculars and a better chart to identify it, and probably a large telescope to glimpse its bluish disk only 2.3 arcseconds wide.
To the east of Aquarius lies the constellation of the two fish, Pisces, and the second most distant planet, Uranus, which stands directly opposite the Sun at opposition on the 15th at a distance of 2,835 million km. At magnitude 5.7 it is near the limit of naked-eye visibility under the darkest of skies, but is an easier binocular or telescope target with its diameter of 3.7 arcseconds.
Orion rises in the east less than two hours after our map times and strides across the meridian before dawn. To its north and east lies Gemini and between the two is the radiant point for the annual Orionids meteor shower. This is visible during our morning hours throughout the second half of the month and peaks at rates around 25 meteors per hour between the 21st and 24th. Its meteors are swift, with many leaving glowing trains in their wake, and represent the dusty debris laid down by Halley’s Comet.
The night ends with Mercury which is conspicuous at magnitude -0.7 and rises in the east 109 minutes before the Sun on the 1st, climbing to stand 9° high forty minutes before sunrise. By the 11th, as its favourable morning show draws to a close, it rises 76 minutes before sunrise. On that morning, the even brighter Jupiter lies only 0.7° below-right of Mercury as the giant planet climbs away from the Sun’s far side. By the 28th, Jupiter rises at about 05:40 BST and is an impressive sight 1.5° below the earthlit waning Moon.
This is a slightly-revised version of Alan’s article published in The Scotsman on October 1st 2016, with thanks to the newspaper for permission to republish here. Journal Editor’s apologies for the lateness of the article appearing here.
Harvest moon eclipsed on the 16th
Two eclipses and a couple of notable space exploration milestones make September an interesting month for astronomers. I’ll postpone until the close of this note, though, my thoughts on the exciting news that Proxima Centauri, the closest star to our Sun, has a planet which is probably rocky, slightly larger than the Earth and in the star’s so-called habitable zone where liquid water might exist.
The first eclipse, an annular or “ring” eclipse of the Sun, occurs on the 1st with the Moon too distant to hide the Sun completely. Instead, a dazzling ring of sunlight remains visible along a narrow path that stretches across Central Southern Africa into the Indian Ocean. Surrounding areas enjoy a partial solar eclipse but nothing is seen as far north as Europe
Of greater interest for us is a penumbral eclipse of the Moon on the 16th during which the Moon passes through the southern outer part of the Earth’s shadow, the penumbra. The event lasts from 17:55 to 21:54 BST although, as seen from Edinburgh, the Moon only rises in the east at 19:29. Maximum eclipse occurs 25 minutes later, at 19:54, when all but the southern 9% of the Moon is within the penumbra. Little darkening of the disk may be noticeable, except near the northern edge which is closest to the Earth’s umbra where all direct sunlight is extinguished.
Since this full moon is the one closest to the autumnal equinox, due at 15:21 BST on the 22nd, it is also called the harvest moon. The tradition is that the bright moon stands at a similar altitude in the eastern sky over several evenings at this time, so permitting the harvesting hours to be extended.
The Sun tracks 11.5° southwards during September to cross the celestial equator at the equinox when day and night have approximately equal lengths around the Earth. Sunrise/sunset times for Edinburgh change from 06:18/20:06 BST on the 1st to 07:14/18:50 on the 30th. The moon is new on the 1st, at first quarter on the 9th, full on the 16th, at last quarter on the 23rd and new again on 1 October.
Jupiter is now lost from view as it nears conjunction on the Sun’s far side on the 26th. It leaves Venus as an evening star, but even though Venus is brilliant at magnitude -3.9 it stands less than 5° above Edinburgh’s horizon at sunset and sets itself within the next 45 minutes. Catch it, if you can, in the west as September begins, shifting to the south-west by the month’s end.
Mars, Saturn and the star Antares in Scorpius form a triangle low in the south-west as darkness falls at present, with Saturn above Antares and Mars a few degrees to their left. Saturn is magnitude 0.5 while Mars is brighter and noticeably reddish, though it fades from magnitude -0.3 to 0.1 as it speeds 18° eastwards and further away. By month’s end, its motion brings it onto our chart and close to the so-called Teapot of Sagittarius, just setting in the south-west.
Look for the Moon close to Saturn on the 9th and above Mars on the 10th when, if viewed telescopically, the two planets appear 16 and 10 arcseconds wide respectively, with Saturn’s wide-open rings spanning 37 arcseconds.
Mercury begins its best morning appearance of the year late in the month. From the 24th onwards, it rises in the east more than 95 minutes before the Sun and reaches more than 8° high forty minutes before sunrise. It is furthest west of the Sun (18°) on the 28th and is magnitude -0.5 when it lies alongside the slender earthlit Moon on the 29th.
Just a day later, on the 30th, Europe’s Rosetta spacecraft is destined to end its mission when it collides with Comet Churyumov-Gerasimenko, the rubber-duck shaped body it has been orbiting and investigating since August 2014. The collision will be gentle but radio contact and data-collection is likely to be lost as the craft settles on the comet’s surface.
Earlier in the month, during a month-long launch window beginning on the 8th, NASA’s OSIRIS-REx spacecraft is due to embark on its seven-years mission to collect and return samples from the surface of Bennu, a small asteroid which has been given an outside chance of having a catastrophic impact with the Earth late in the next century.
Proxima Centauri lies at a distance of only 4.25 light years but is much too dim to be seen without a telescope, A small red dwarf star, it is less than 15% as massive and wide as our Sun and has less than 0.2% of the Sun’s energy output. Also called Alpha Centauri C, it was discovered in 1915 by the Edinburgh-born astronomer Robert Innes and lies 15° to the east of the Southern Cross in a part of the sky we never see from Britain. It is thought to form a triple star system with Alpha Centauri A and B, a tight binary of more Sun-like stars that lie 2° away in the sky.
The newly discovered world has been dubbed Proxima b but it is something of a stretch to call it Earth-like. It orbits its star in a year of 11.2 Earth-days at a distance of less than 8 million km where it is blasted by X-rays from dramatic flares that we see erupting on Proxima’s surface – far from ideal for life. It is also probably tidally locked – keeping its same face towards the star – and we do not even know (yet!) that it has water, never mind life.
This is a slightly-revised version of Alan’s article published in The Scotsman on September 1st 2016, with thanks to the newspaper for permission to republish here.
Perseids rain as Mars approaches his rival
Every year at this time the Earth sweeps through the stream of meteoroids released by Comet 109P/Swift-Tuttle which passed just inside the Earth’s orbit in 1992 and is not due to return until 2126. And every year at this time, some of those meteoroids plunge into our upper atmosphere at 59 km per second, producing a rich display of bright meteors, many leaving glowing trains in their wake. According to some claims, this year’s meteor spectacle could be even better than usual.
The meteors appear in all parts of the sky but, since they are moving in parallel, perspective causes their paths to point away from a so-called radiant point in the constellation Perseus. It has already been active for a week, but it is expected to peak at about 13:00 BST on the 12th when, typically, an observer beneath the radiant and with a perfect dark sky might count 80 or more Perseids per hour. Of course, this year’s peak occurs in daylight for Scotland, but we should still enjoy high rates on our nights of 11/12th and 12/13th.
The radiant, plotted on our north star map, stands in the north-east at nightfall and climbs to lie just east of overhead before dawn. As the radiant climbs, so we face more directly into the Perseids stream and meteor rates climb in sympathy. This means that our morning hours are favoured and we have the extra advantage that the Moon sets in the middle of the night on the critical nights, though moonlight will hinder evening watches. Another bonus is that the nights are much less cold than they are for the year’s other two major showers which occur in the depths of winter.
The suggestions that the Perseids might be particularly active in 2016, with perhaps twice as many meteors as usual, derive from the fact that Jupiter approaches the Perseids stream every 12 years and its gravity might be diverting a segment of the stream closer to the Earth on each encounter. Indeed, there does seem to be a 12-years periodicity in enhanced Perseids displays with the last one in 2004, so we may be due for another special show this month.
Sunrise/sunset times for Edinburgh change from 05:17/21:19 BST on the 1st to 06:16/20:09 on the 31st. The Moon is new on the 2nd, at first quarter on the 10th, full on the 18th and at last quarter on the 25th.
Our chart depicts the Summer Triangle, formed by Deneb, Vega and Altair, high on the meridian as the Plough sinks in the north-west and the “W” of Cassiopeia climbs in the north-east, above the Perseids radiant. The large but rather empty Square of Pegasus balances on a corner in the east-south-east while the Teapot of Sagittarius is toppling westwards on our southern horizon. To its right, and very low in the south-west, is Saturn, the only bright planet visible at our map times.
Saturn hardly moves this month, being stationary against the stars on the 13th when it reverses from westerly to easterly in motion. It lies in Ophiuchus, 6° north of the red supergiant star Antares in Scorpius. Antares is around magnitude 1.0 while Saturn is almost twice as bright at 0.4. Saturn stands above Antares low in the south-south-west as tonight’s twilight fades but are outshone by the Red Planet, Mars, which lies 10° to their right and is three times brighter than Saturn at magnitude -0.8.
Mars, though, is moving eastwards (to the left) at almost a Moon’s-breadth each day and passes between Antares and Saturn, and 1.8° above Antares, on the 24th. Even though Mars dims to magnitude -0.4 by then, it remains much brighter than Antares even though the star’s name comes from the Ancient Greek for “equal to Mars”. Both appear reddish, of course, but for very different reasons – Antares has a bloated “cool” gaseous surface that glows red at about 3,100°C while Mars has a rocky surface which is rich in iron oxide, better known as rust.
The Moon stands above-right of Mars and to the left of Saturn on the 11th when Mars appears only 12 arcseconds wide if viewed through a telescope. Saturn is 17 arcseconds across while its rings span 39 arcseconds and have their north face tipped 26° towards us. By the 31st, Mars has faded further to magnitude -0.3 and lies 4° above-left of Antares.
Observers at our northern latitudes must work hard to spot any other bright planet this month although anyone in the southern hemisphere can enjoy a spectacular trio of them low in the west at nightfall. Seen from Scotland, though, the brilliant (magnitude -3.9) evening star Venus stands barely 5° above our western horizon at sunset and sets itself less than 40 minutes later. We need a pristine western outlook to see it, and quite possibly binoculars to glimpse it against the twilight.
Fainter (magnitude -1.7) is Jupiter which stands currently 27° to the left of Venus and 5° higher so that it sets more than 70 minutes after the Sun. Between them, and considerably fainter, is Mercury which stands furthest from the Sun (27°) on the 16th and, perhaps surprisingly, is enjoying its best evening apparition of the year as seen from the southern hemisphere.
Jupiter sinks lower with each evening and meets Venus on the 27th when Venus passes less than 5 arcminutes north of Jupiter. This is the closest planetary conjunction of the year and would be spectacular were the two not so twilight-bound. As it is, binoculars might show Jupiter 9 arcminutes below and left of Venus on that evening.
This is a slightly-revised version of Alan’s article published in The Scotsman on August 1st 2016, 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.