Summer Triangle stars as autumn evenings begin
We may be edging towards autumn, but the Summer Triangle, the asterism formed by the bright stars Vega, Altair and Deneb, looms high in the south as night falls and shifts into the high south-west by our star map times later in the evening. Vega, almost overhead as the night begins, is the brightest of the three and lies in the small box-shaped constellation of Lyra the Lyre.
The next brightest, Altair in Aquila the Eagle, stands lower in the middle of our southern sky and, at 16.7 light years (ly), is one of the nearest bright stars to the Sun – eight light years closer than Vega. Flanking Altair, like the two sides of a balance, are the fainter stars Alshain (below Altair) and Tarazed (above) whose names come from “shahin-i tarazu”, the Arabic phrase for a balance.
Deneb, 25° from Vega, lies very high in the south-east at nightfall and overhead at our map times. It marks the tail of Cygnus the Swan which is flying overhead with wings outstretched and its long neck reaching south-westwards to Albireo, traditionally the swan’s beak. Although it is the dimmest corner-star of the Triangle, Deneb is one of the most luminous stars in our galaxy. Current estimates suggest that it shines some 200,000 time more brightly than our Sun from a distance of perhaps 2,600 ly, but its power and distance are hard to measure and the subject of some controversy.
Also controversial is the nature of Albireo. Even small telescopes show it as a beautiful double star in which a brighter golden star contrasts with a dimmer blue one. The mystery concerns whether the pair make up a real binary, with the two stars locked in orbit together by gravity, or whether this is just the chance alignment of two stars at different distances. Now measurement by the European Space Agency’s Gaia spacecraft appear to confirm the chance alignment theory.
The Milky Way, the band of countless distant stars in the plane of our galaxy, flows through the Summer Triangle and close to Deneb as it arches across our evening sky. Scan it through binoculars to glimpse a scattering of other double stars and star clusters.
One interesting stellar group is the so-called Coathanger which lies 8°, a little more than a normal binocular field-of-view, south of Albireo. It is also easy to locate one third of the way from Altair to Vega. Its line of stars, with a hook of stars beneath, gives it the appearance of an upside-down coat hanger. For decades this was regarded as a true star cluster, whose stars formed together, and its alternative designations as Brocchi’s Cluster and Collinder 399 reflect this. In 1998, though, results from the Hipparcos satellite, Gaia’s predecessor, proved that the Coathanger’s stars are at very different distances so that it, like Albireo, is simply a fortuitous chance alignment.
The Sun sinks 11.5° southwards during September to cross the sky’s equator at 02:54 BST on the 23rd. This marks our autumnal equinox and, by one definition, the beginning of autumn in the northern hemisphere. Sunrise/sunset times for Edinburgh change from 06:17/20:07 BST on the 1st at 07:13/18:51 on the 30th. The Moon is at last quarter on the 3rd, new on the 9th, at first quarter on the 17th and full on the 25th.
Venus is brilliant at magnitude -4.4 and 45° from the Sun on the 1st but it is only 4° above Edinburgh’s west-south-western horizon at sunset and sets 35 minutes later as its evening apparition as seen from Scotland comes to an end.
The other inner planet, Mercury, is prominent but low in the east-north-east before dawn until about the 14th. Glimpse it at magnitude -1.1 when it lies 1° above-left of Regulus in Leo on the 6th and 9° below-left of the impressively earthlit waning Moon on the 8th.
Jupiter is conspicuous but very low in the south-west at nightfall, sinking to set in the west-south-west one hour before our map times. Look for it below-right of the Moon on the 13th.
Saturn and Mars are in the far south of our evening sky. Saturn, the fainter of the two at magnitude 0.4 to 0.5, stands above the Teapot of Sagittarius and is just below and right of the Moon on the 17th when a telescope shows that its rings span 38 arcseconds around its 17 arcseconds disk. It sets in the south-west some 70 minutes after our map times.
Mars stands more than 25° east (left) of Saturn, tracks 7° eastwards and northwards in Capricornus and stands near the Moon on the 19th and 20th. It is easily the brightest object (bar the Moon) in the sky at our map times though it more than halves in brightness from magnitude -2.1 to -1.3. As its distance increases from 67 million to 89 million km, its ochre disk shrinks from 21 to 16 arcseconds. The dust storm that blanketed the planet since June has now died down.
Finally, we have a chance to spot the Comet Giacobini-Zinner as it tracks south-eastwards past the bright star Capella in Auriga, low in the north-east at our map times but high in the east before dawn. The comet takes only 6.6 years to orbit the Sun and should appear in binoculars as a small oval greenish smudge only 0.9° to the right of Capella on the evening of the 2nd when it is 60 million km away. Moving at almost 2° per day, it passes less than 7° north-east of Elnath in Taurus (see chart) on the morning of the 11th, just a day after it reaches perihelion, its closest (152 million km) to the Sun.
Diary for 2018 September
Times are BST
2nd 10h Venus 1.4° S of Spica
3rd 03h Moon 1.2° N of Aldebaran
3rd 04h Last quarter
6th 11h Saturn stationary (motion reverses from W to E)
7th 04h Moon 1.1° S of Praesepe in Cancer
7th 19h Neptune at opposition
8th 23h Moon 0.9° N of Mercury
9th 19h New moon
10th 08h Comet Giacobini-Zinner closest to Sun (152 million km)
14th 03h Moon 4° N of Jupiter
16th 14h Mars closest to Sun (206,661,000 km)
17th 00h First quarter
17th 17h Moon 2.1° N of Saturn
20th 08h Moon 5° N of Mars
21st 03h Mercury in superior conjunction
23rd 02:54 Autumnal equinox
25th 04h Full moon
30th 09h Moon 1.4° N of Aldebaran
This is a slightly revised version, with added diary, of Alan’s article published in The Scotsman on August 31st 2018, with thanks to the newspaper for permission to republish here.
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.
Total eclipse of the harvest supermoon on the 28th
There is no shortage of stellar interest in our September evening sky but anyone willing and able to observe later in the night may see the largest full moon of the year turn dull and red as it is totally eclipsed on the morning of the 28th. There is also a nice grouping of planets in the east before dawn.
Our chart depicts the sky in our late evening at present and shows the star Deneb in Cygnus almost at the zenith as the Summer Triangle it forms with Vega and Altair begins to topple westwards. The Square of Pegasus is climbing in the south-east in a rather sparsely populated region of the sky.
The Pleiades in Taurus glimmer low in the east-north-east as they begin their climb to the high meridian by dawn. They stand above the Moon on the Friday night of the 5th/6th as the Moon draws closer to Aldebaran, the leading star of Taurus, eventually to occult the star during the morning twilight. As seen from Edinburgh the star winks out as it disappears behind the sunlit eastern edge of the Moon at 05:51 BST on Saturday, 6th.
Our sole bright evening planet, Saturn, hovers only 10° above Edinburgh’s south-western horizon as darkness falls and sinks a little lower each evening as it creeps eastwards against the stars of eastern Libra. Telescopically, its disk appears 16 arcseconds broad while the rings are 37 arcseconds across with their north face inclined at 24°, but it is past its best as a target and, since it sets 30 minutes before our map times, we need to be quick to catch it. Look for it to the left of the young Moon on the 18th.
The Sun slips southwards over the celestial equator at 09:21 BST on the 23rd, marking the autumnal equinox in our northern hemisphere. Meanwhile, sunrise/sunset times for Edinburgh change from 06:16/20:08 BST on the 1st to 07:13/18:52 to the 30th as the duration of nautical twilight at dawn and dusk shrinks from 89 to 80 minutes.
The Moon is at last quarter on the 5th and new on the 13th when a partial solar eclipse is visible from southern Africa and Antarctica. Following first quarter on the 21st, the full moon on the 28th is a much more intriguing event because not only is it totally eclipsed, but it coincides with the Moon’s closest approach to the Earth (at perigee) in the whole of 2015. As a result, the Moon appears 7% wider than it does on average and, while the enlargement is not startlingly obvious to the casual observer, it has led to near-perigee full moons being called supermoons.
As the full moon closest to the equinox, this is also the harvest moon, a title that comes from the fact that, for several nights in a row, the bright Moon hangs low in our eastern sky as the night begins and extends the period by which the harvest may be gathered. The illusion that makes the Moon appear larger than usual when it stands low in the sky is also pronounced around this time and can only be enhanced by the supermoon circumstance.
The eclipse on the morning of the 28th, though, begins at 01:12 BST with the Moon well up in our southern sky, in the constellation Pisces and below the Square of Pegasus. The Moon meets the outer edge of the Earth’s penumbral shadow at that time, but little darkening may be noticed for another 30 minutes or more.
The dark shadow of the umbra beings to invade the lunar disk at 02:07, while from 03:11 until 04:23 the Moon is totally eclipsed as it crosses the southern part of the umbra. Here the Moon is only illuminated, usually with a reddish hue, by light scattered around the edge of the Earth. Varying atmospheric conditions, clouds and volcanic dust, on our home world mean that the appearance and brightness of the eclipsed Moon is always of interest. The Moon leaves the umbra behind at 05:27 and stands only 7° above Edinburgh’s western horizon when it exits the last of the penumbra at 06:23.
Venus, already a spectacular morning star, rises at Edinburgh’s east-north-eastern horizon 100 minutes before the Sun on the 1st and more than four hours before sunrise at the month’s end. Improving from magnitude -4.4 to -4.5, it shows a slender but dazzling crescent through binoculars though it shrinks in diameter from 51 to 43 arcseconds as it recedes from 48 million to 76 million km.
The giant planet Jupiter lags some 20° below-left of Venus and emerges from the Sun’s far side by mid-month to shine at magnitude -1.7, just brighter than Sirius which is low in the south-east at the time.
Our third morning planet, Mars, is much fainter at magnitude 1.8 and stands 9° above-left of Venus on the 1st. Also still on the far side of its orbit, it slips down and to the left in the direction of Jupiter and passes 0.8° north of Leo’s leading star Regulus on the 24th. The blue-white of Regulus outshines the orange tinted Mars and the contrasting pair make an interesting sight roughly half-way between Venus and Jupiter for a few mornings around that day.
The little innermost planet, Mercury, stands furthest east of the Sun (27°) on the 4th but is much too low in our western evening twilight to be seen this month. After sweeping around the Sun’s near side on the 30th, it is due to make its best appearance of the year as a morning star in October.
This is a slightly-revised version of Alan’s article published in The Scotsman on September 1st 2015, with thanks to the newspaper for permission to republish here.
Mars greets a rival and two new orbiters
The Summer Triangle, formed by the bright stars Vega, Deneb and Altair, still has pride of place high in our southern sky at nightfall. Mars and Saturn are visible on our September evenings, too, but we must look low in the south-west to catch them. Both are well past their best and less interesting telescopic targets than Jupiter which is now resplendent in the east before dawn.
Having swept 3°, or six Moon-widths, to the south of Saturn on 27 August, Mars has a trio of further notable encounters later in September. Two new spacecraft, NASA’s MAVEN and India’s MOM or Mangalyaan, are on course to enter orbit around Mars on the 21st and 24th respectively while the planet is due to pass 3° north of the enormous red supergiant star Antares in Scorpius on the 27th. The name Antares comes from the Ancient Greek for “rival to Mars” and, while they may indeed be similar in brightness by the month’s end, it will be fascinating to see how their colours compare.
Meanwhile, Mars, or rather the spacecraft in orbit around it, are due for a more challenging encounter when the icy nucleus of comet C/2013 A1 Siding Spring sweeps within some 130,000 km of the planet on 19 October. The operators of NASA’s Mars Reconnaissance Orbiter and Mars Odyssey, and of Europe’s Mars Express, are arranging to shield their craft from the worst of the dust storm that is likely to be accompanying the comet, and similar precautions may be needed for MAVEN and MOM.
In other space news, Europe’s Rosetta craft is now studying five potential landing sites for its Philae lander on the nucleus of Comet Churyumov-Gerasimenko. The landing is not due until November, but it is planned to choose a primary and a backup site this month as Rosetta closes to with 30 km of the nucleus.
By our star map times, the Summer Triangle lies just west of our meridian as it gives way to the stars of autumn led by the topsy-turvy winged horse Pegasus whose nose is marked by the star Enif. Use binoculars to look 4° north-west of Enif for the star cluster M15 which appears as a fuzzy blob less than half as wide as the Moon. In fact, it is one of the finest globular clusters in the sky and contains more than 100,000 stars at a distance in excess of 30,000 light years.
The Sun tracks 11.5° southwards in the sky during September and crosses the equator at 03:29 BST on the 23rd, the time of this year’s autumnal equinox. Sunrise/sunset times for Edinburgh change from 06:17/20:07 on the 1st to 07:13/18:51 on the 30th as the duration of nautical twilight at dawn and dusk falls from 89 to 80 minutes.
The Moon is at first quarter on the 2nd, full on the 9th, at last quarter on the 16th and new on the 24th. As the full moon nearest to the equinox, the one on the 9th is also our Harvest Moon and, since it comes less than a day after the Moon is closest to the Earth, it is yet another supermoon.
Saturn stands about 11° high in the south-west and only 0.3° above the northern tip of the crescent Moon as the evening twilight fades on 31 August, with Mars another 4° below and to their left.
On 27 September, the young Moon returns to lie 6° to the right of Saturn which, by then, is 4° lower in the sky and becoming hard to spot in the twilight. Both planets begin the period at magnitude 0.6, but Mars dims slightly to magnitude 0.8 by the 29th when it stands 5° below the Moon and 3° above Antares. It is also 20° to the left of Saturn and drops below Edinburgh’s horizon at 20:51 BST. Viewed through a telescope, Mars is only 6 arcseconds in diameter at midmonth, while Saturn is 16 arcseconds wide within rings that span 36 arcseconds and have their north face tilted 22° towards us.
After Mars and Saturn set, the sky is devoid of bright planets until Jupiter rises more than five hours later. True, Neptune and Uranus are binocular objects at magnitudes of 7.8 and 5.7 in Aquarius and Pisces respectively, but we need better charts to identify them.
There is no mistaking Jupiter, though. The conspicuous giant planet rises at Edinburgh’s east-north-eastern horizon at 03:29 on the 1st and by 02:07 on the 30th. climbing well clear of the eastern to south-eastern horizon by dawn. As it brightens slightly from magnitude -1.8 to -1.9, it also tracks 6° eastwards, below and away from the Praesepe or Beehive star cluster in Cancer. Look for the waning earthlit Moon 6° below and right of Jupiter before dawn on the 20th. Viewed through a telescope on that morning, the cloud-banded Jovian disk is 33 arcseconds across.
Venus is also a morning object and, although it remains brilliant at magnitude -3.9, it is sinking deeper into the twilight as it approaches conjunction on the Sun’s far side in October. On the 1st, it rises 87 minutes before the Sun and stands 14° below and left of Jupiter as it climbs 12° above our eastern horizon by sunrise. Jupiter soon leaves it behind, though, so by the 30th it rises 32 minutes before the Sun and is only 6° high at sunrise. Viewed telescopically, its almost full disk is only 10 arcseconds across.
The other inner planet, Mercury, moves to lie 26° east of the Sun on the 21st, but hugs the western horizon at sunset and is not observable from our latitudes.