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Scotland’s Sky in September, 2018

Summer Triangle stars as autumn evenings begin

The maps show the sky at 23:00 BST on the 1st, 22:00 on the 16th and 21:00 on the 30th. An arrow depicts the motion of Mars. (Click on map to enlarge)

The maps show the sky at 23:00 BST on the 1st, 22:00 on the 16th and 21:00 on the 30th. An arrow depicts the motion of Mars. (Click on map to enlarge)

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

Alan Pickup

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.
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Scotland’s Sky in August, 2015

Perseids meteor shower peaks under moonless skies

The maps show the sky at 00:00 BST on the 1st, 23:00 on the 16th and 22:00 on 31st. (Click on map to englarge)

The maps show the sky at 00:00 BST on the 1st, 23:00 on the 16th and 22:00 on 31st. (Click on map to englarge)

The revelations by New Horizons at Pluto were certainly the highlight for July, showing that even small ice-bound worlds far from the Sun can have an active and fascinating geology. No doubt we are in for further surprises as the data from the encounter are downloaded over the narrow-bandwidth link to the probe over the coming months.

August sees our attention return to Comet 67P/Churyumov–Gerasimenko which is due to experience its peak activity as it sweeps through perihelion, its closest to the Sun, on the 15th. We should enjoy a grandstand view courtesy of Europe’s Rosetta probe in orbit around the comet’s icy nucleus, but it is far from certain that Philae will be able to relay further measurements from the surface. The comet’s perihelion occurs 26 million km outside the Earth’s orbit so none of the icy debris being driven from its nucleus is destined to reach the Earth.

The Earth does, though, intersect the orbit of Comet 109P/Swift-Tuttle with the result that its debris or meteoroids plunge into the upper atmosphere to produce the annual Perseids meteor shower. Its meteors diverge from a radiant point in Perseus which lies in the north-east at our star map times and climbs to stand just east of the zenith before dawn. Note that the shower’s meteors appear in all parts of the sky, with many of them bright and leaving persistent trains in their wake as they disintegrate at 59 km per second.

According to the British Astronomical Association (BAA), the premiere organisation for amateur astronomers in Britain, the shower is active from July 23 until August 20 and, for an observer under ideal conditions, reaches a peak of 80 or more meteors per hour at about 07:00 on the 13th. This is obviously after our daybreak, but rates should be high throughout the night of the 12th-13th and particularly before dawn, and respectable on the preceding and following nights too. With the Moon new on the 14th and causing no interference, the BAA puts the Perseids’ prospects this year as very favourable, an accolade it shares with the Geminids shower in December.

The Sun dips 10° southwards during August as sunrise/sunset times for Edinburgh change from 05:16/21:21 BST on the 1st to 06:14/20:11 on the 31st. The duration of nautical twilight at dawn and dusk shrinks from 121 to 89 minutes. The Moon is at first quarter on the 7th, new on the 14th, at first quarter on the 22nd and full again on the 29th.

After the twilit nights during the weeks around the solstice, August should bring (if the weather ever improves!) a chance to reacquaint ourselves with the best of what the summer skies can offer. The Summer Triangle formed by Vega in Lyra, Deneb in Cygnus and Altair in Aquila stands high in the south at our star map times, somewhat squashed by the map projection used. After the Moon leaves the scene, look for the Milky Way as it flows diagonally through the Triangle, its mid-line passing between Altair and Vega and close to Deneb as it arches over the sky from the south-south-west towards Cassiopeia, Perseus and Auriga in the north-east.

The main stars of Cygnus the Swan are sometimes called the Northern Cross, particularly when the cross appears to stand upright in our north-western sky later in the year. The Swan’s neck stretches south-westwards from Sadr to Albireo, the beak, which is one of the finest double stars in the sky. A challenge for binoculars, almost any telescope shows Albireo as a contracting pair of golden and bluish stars.

The brightest star on the line between Sadr and Albireo is usually the magnitude 3.9 Eta. However, just 2.5° south-west of Eta is the star Chi Cygni which pulsates in brightness every 407 days or so and belongs to the class of red giant variable stars that includes Mira in Cetus. Chi is a dim telescopic object at its faintest, but it can become easily visible to the naked eye at its brightest. Last year, though, it only reached magnitude 6.5, barely visible to the naked eye. Now approaching maximum brightness again and as bright as magnitude 4.2 in late July, it may surpass Eta early in August, so is worth a look.

Venus and Jupiter have dominated our evening sky over recent months but are now lost in the Sun’s glare to leave Saturn as our only bright planet as the night begins. Although it dims slightly from magnitude 0.4 to 0.6, it remains the brightest object low down in the south-west as the twilight fades. Indeed, it stands only 5° or so above Edinburgh’s horizon at the end of nautical twilight and sets thirty minutes after our map times, so is now poorly placed for telescopic study. Catch it 2° below-right of the first quarter Moon on the 22nd when Saturn’s rings are tipped at 24° and span 38 arcseconds around its 17 arcseconds disk.

Jupiter reaches conjunction on the Sun’s far side on the 26th while Venus sweeps around the Sun’s near side on the 15th and reappears before dawn a few days later. Brilliant at magnitude -4.2, its height above Edinburgh’s eastern at sunrise doubles from 6° on the 25th to 12° by the 31st.

Also emerging in our morning twilight is the much dimmer planet Mars, magnitude 1.8. On the 20th and 21st it rises in the north-east two hours before the Sun and lies against the Praesepe or Beehive star cluster in Cancer. Before dawn on the 31st, Mars stands 9° above-left of Venus.

Alan Pickup

This is a slightly-revised version of Alan’s article published in The Scotsman on July 31st 2015, with thanks to the newspaper for permission to republish here.