Blog Archives

Scotland’s Sky in November, 2018

InSight probe to land on bright evening planet Mars

The maps show the sky at 21:00 GMT on the 1st, 20:00 on the 16th and 19:00 on the 30th. An arrow depicts the motion of Mars. (Click on map to enlarge)

The maps show the sky at 21:00 GMT on the 1st, 20:00 on the 16th and 19:00 on the 30th. An arrow depicts the motion of Mars. (Click on map to enlarge)

The Summer Triangle, still high in the south at nightfall, shifts to the west by our map times as our glorious winter constellations climb in the east. Taurus with the Pleiades and its leading star Aldebaran (close to the Moon on the 23rd) stands well clear of the horizon while Orion is rising below and dominates our southern sky after midnight.

In the month that should see NASA’s InSight lander touch down on its surface, the planet Mars continues as a prominent object in the south at nightfall. Venus springs into view as a spectacular morning star but we must wait to see whether the Leonids meteor shower, which has produced some storm-force displays in the past, gives us any more than the expected few meteors this year.

InSight is due to land on the 26th on a broad plain called Elysium Planitia that straddles Mars’ equator. There it will place an ultra-sensitive seismometer directly onto the surface and cover it with a dome-like shell to shield it from the noise caused by wind and heat changes. This should be able of detect marsquakes and meteor impacts that occur all around Mars. Other InSight experiments will hammer a spike up to five metres into the ground to measure Mars’ heat flow, and further investigate the planet’s interior structure by using radio signals to track tiny wobbles in its rotation.

Until recently, Mars has remained low down as it performed a loop against the stars in the south-western corner of Capricornus. That loop, resulting entirely from our changing vantage point as the Earth overtook Mars and came within 58 million km on 31 July, took Mars more than 26° south of the sky’s equator and 3° further south than the Sun stands at our winter solstice.

Now, though, Mars is climbing east-north-eastwards on a track that will take it further north than the Sun ever gets by the time it disappears into Scotland’s night-long twilight next summer. One by-product of this motion is that Mars’ setting time is remarkably constant over the coming months, being (for Edinburgh) within 13 minutes of 23:42 GMT from now until next May.

This month sees Mars leave Capricornus for Aquarius and shrink as seen through a telescope from 12 to 9 arcseconds as it recedes from 118 million to 151 million km. Its path, indicated on our southern chart, carries it 0.5° (one Moon’s breadth) north of the multiple star Deneb Algedi, the goat’s tail, on the 5th. It almost halves in brightness, from magnitude -0.6 to 0.0, but its peak altitude above Edinburgh’s southern horizon early in the night improves from 16° to 25°, though by our map times it is sinking lower towards the south-west.

Mars is not our sole evening planet since Saturn shines at magnitude 0.6 low down in the south-west at nightfall. It is only a degree below-right of the young Moon on the 11th and sets more than 90 minutes before our map times. The two most distant planets, Neptune and Uranus, are also evening objects and may be glimpsed through binoculars at magnitudes 7.9 and 5.7 in Aquarius and Aries respectively.

Edinburgh’s sunrise/sunset times vary from 07:19/16:32 on the 1st to 08:17/15:45 on the 30th. The Moon is new on the 7th, at first quarter and below-right of Mars on the 15th, full on the 23rd and at last quarter on the 30th.

Jupiter is hidden in the solar glare as it approaches conjunction on the Sun’s far side on the 26th. Mercury stands furthest east of the Sun (23°) on the 6th but is also invisible from our northern latitudes.

Venus, though, emerges rapidly from the Sun’s near side into our morning twilight where it stands to the left of the star Spica in Virgo. Shining brilliantly at magnitude -4.1, the planet rises in the east-south-east only 29 minutes before the Sun on the 1st. By the 6th, though, it rises 80 minutes before sunrise and stands 8° below and right of the impressively earthlit waning Moon. Venus itself is 58 arcseconds wide and 4% illuminated on that morning, its slender crescent being visible through binoculars. By the 30th, Venus rises four hours before the Sun, climbs to stand 23° high in the south-south-east at sunrise and appears as a 41 arcseconds and 25% sunlit crescent.

It is just as well that my previous note led on the usually neglected Draconids meteor shower because observers, at least those under clear skies, were thrilled to see it provide perhaps the best meteor show of 2018. For just a few hours around midnight on 8-9th October, the sky became alive with slow meteors at rates of up to 100 meteors per hour or more.

Leonid meteors arrive this month between the 15th and 20th, with the shower expected to hit its usually-brief peak at around 01:00 on the 18th. Although they flash in all parts of the sky, they diverge from a radiant point in the so-called Sickle of Leo which rises in the north-east before midnight and climbs high into the south before dawn. No Leonids appear before the radiant rises, but even with the radiant high in a dark sky we may see fewer than 20 per hour – all of them very swift and many of the brighter ones leaving glowing trains in their wake.

Leonid meteoroids come from Comet Tempel-Tuttle which orbits the Sun every 33 years and was last in our vicinity in 1998. There has not been a Leonids meteor storm since 2002 and we may be a decade or more away from the next one, or are we?

Diary for 2018 November

2nd           05h Moon 2.1° N of Regulus

6th            16h Mercury furthest E of Sun (23°)

7th            16h New moon

11th         16h Moon 1.5° N of Saturn

15th         15h First quarter

16th         04h Moon 1.0° S of Mars

18th         01h Peak of Leonids meteor shower

23rd         06h Full moon

23rd         22h Moon 1.7° N of Aldebaran

26th         07h Jupiter in conjunction with Sun

26th         20h InSight probe to land on Mars

27th         09h Mercury in inferior conjunction on Sun’s near side

27th         21h Moon 0.4° S of Praesepe

30th         00h Last quarter

Alan Pickup

This is a slightly revised version, with added diary, of Alan’s article published in The Scotsman on October 31st 2018, with thanks to the newspaper for permission to republish here.
Advertisements

Scotland’s Sky in October, 2018

Draconid meteors glide away from the Dragon’s head

The maps show the sky at 23:00 BST on the 1st, 22:00 BST (21:00 GMT) on the 16th and at 20:00 GMT on the 31st. An arrow depicts the motion of Mars. Summer time ends at 02:00 BST on the 28th when clocks are set back one hour to 01:00 GMT. (Click on map to enlarge)

The maps show the sky at 23:00 BST on the 1st, 22:00 BST (21:00 GMT) on the 16th and at 20:00 GMT on the 31st. An arrow depicts the motion of Mars. Summer time ends at 02:00 BST on the 28th when clocks are set back one hour to 01:00 GMT. (Click on map to enlarge)

Mars dominates our southern evening sky but most of the other bright planets are poorly placed this month. Even so, our October nights are full of interest, from the Summer Triangle in the evening to the star-fest around Orion before dawn.

Although Mars dims from magnitude -1.3 to -0.6, its reddish light remains prominent as it moves from low in the south-south-east at nightfall to the south-south-west at our map times and onwards to set in the south-west a little before 01:00 BST (midnight GMT). As its distance grows from 89 million to 118 million km, and its diameter shrinks from 16 to 12 arcseconds, the planet speeds through Capricornus to climb 6° northwards and that much higher in our sky. Catch it to the left of the Moon on the 17th and below-right of the Moon on the 18th.

The Sun tracks 11° southwards as Edinburgh’s sunrise/sunset times change from 07:15/18:48 BST (06:15/17:48 GMT) on the 1st to 07:17/16:35 GMT on the 31st. The Moon is at last quarter on the 2nd, new on the 9th, at first quarter on the 16th, full (the Hunter’s Moon) on the 24th and back at last quarter on the 31st.

Our charts show the Plough in the north as it moves below Polaris, the Pole Star. Mizar, in the Plough’s handle, forms a famous double star with the fainter Alcor – the pair being separated by about one third the diameter of the Moon. Once held as a (not very rigorous) test of eyesight, they were dubbed “The Horse and Rider”.

Both lie 83 light years (ly) from us although we can’t be certain that they are tied together by gravity. In any case, we are not talking about just two stars, for Alcor has a faint companion and most telescopes show Mizar to be a binary star – the first to be discovered telescopically in the 17th century. Spectroscopes reveal that each of Mizar’s components is itself binary, so Mizar and Alcor, if they are truly associated, together form a sextuplet star system.

Mizar is the same brightness, magnitude 2.2, as Eltanin which lies 14° to the right of Vega and very high in the west at nightfall, falling into the north-west overnight. It is the brightest star in Draco and a member of a quadrilateral that marks the head of the Dragon whose body and tail twist to end between the Plough and Polaris. It lies 154 ly away but is approaching the Sun and will pass within 28 ly in another 1.5 million years to become the brightest star in Earth’s night sky.

Meteors from the Draconids shower diverge from a radiant point that lies close to Draco’s head (see our north map) between the 7th and 10th. Don’t expect a major display – perhaps no more than 10 meteors per hour, though all of them are very slow as they glide away from the radiant. The shower’s peak is due in a moonless sky around midnight on the 8th-9th and is worth checking because some years surprise us with strong displays and the shower’s parent comet, Comet Giacobini-Zinner, was visible through binoculars when it swept within 59 million km last month.

A better-known comet, Halley, is responsible for the meteors of the Orionids shower which lasts from the 16th to the 30th and has a broad but not very intense peak of fast meteors between the 21st and 24th. The radiant point, between Orion and Gemini, rises in the east-north-east soon after our map times and passes high in the south before dawn. Sadly, the peak coincides with the full moon, so don’t expect much of a show.

From high in the south at nightfall, the Summer Triangle (Vega, Deneb and Altair) tumbles into our western sky by the map times. By then, the less impressive and rather empty Square of Pegasus is in the south and Taurus and the Pleiades star cluster are climbing in the east. Orion rises below Taurus over the next two hours and crosses the meridian as the night ends.

Neptune and Uranus, now well placed in the evening, may be located through binoculars using better charts than I can provide here. A web search, for example for “Neptune finder chart”, should help. Neptune shines at magnitude 7.8 and lies in Aquarius at a distance of 4,342 million km on the 1st. Uranus is 2,824 million km away in Aries, near its border with Pisces, when it stands opposite the Sun in the sky (opposition) on the 24th. Although the full Moon stands close to it on that day, its magnitude of 5.7 makes it just visible to the unaided eye under a good dark and moonless sky.

October should see the launch of the European Space Agency’s BepiColombo mission to Mercury, but the planet itself is too low in our evening twilight to be seen. Venus sweeps around the Sun’s near side at inferior conjunction on the 26th and remains hidden in the Sun’s glare.

Jupiter is bright (magnitude -1.8) but less than 8° high in the south-west at sunset as the month begins. One of our last chances of spotting it in our bright evening twilight comes on the 11th when it lies 4° below-left of the young earthlit Moon.

Saturn, magnitude 0.5 and edging eastwards in Sagittarius, stands less than 10° high above Edinburgh’s south-south-western horizon as the sky darkens and sets in the south-west some 45 minutes before our map times. Look for it to the left of the Moon on the 14th.

Diary for 2018 October

Times are BST until the 28th

2nd           11h Last Quarter

9th            00h Peak of Draconids meteor shower

9th            05h New moon

11th         22h Moon 4° N of Jupiter

15th         04h Moon 1.8° N of Saturn

16th         19h First quarter

18th         14h Moon 1.9° N of Mars

21st – 24th         Peak of Orionids meteor shower

24th         02h Uranus at opposition at distance of 2,824m km

24th         18h Full moon

26th         15h Venus in inferior conjunction on Sun’s near side

28th         02h BST = 01h GMT End of British Summer Time

31st         17h GMT Last quarter

Alan Pickup

This is a slightly revised version, with added diary, of Alan’s article published in The Scotsman on September 29th 2018, with thanks to the newspaper for permission to republish here.

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.

Scotland’s Sky in August, 2018

Perseid meteor shower peaks in planet-rich sky

The maps show the sky at midnight BST on the 1st, 23:00 on the 16th and 22:00 on the 31st. An arrow depicts the motion of Mars. (Click on map to enlarge)

The maps show the sky at midnight BST on the 1st, 23:00 on the 16th and 22:00 on the 31st. An arrow depicts the motion of Mars. (Click on map to enlarge)

The persistent twilight that has swamped Scotland’s night sky since May is subsiding in time for us to appreciate four bright evening planets and arguably the best meteor shower of the year.

The Perseid shower returns every year between 23 July and 20 August as the Earth cuts through the stream of meteoroids that orbit the Sun along the path of Comet Swift-Tuttle. As they rush into the Earth’s atmosphere at 59 km per second, they disintegrate in a swift streak of light with the brighter ones often laying down a glowing train that may take a couple of seconds or more to dissipate.

The shower is due to peak in the early hours of the 13th at around 02:00 BST with rates in excess of 80 meteors per hour for an observer under ideal conditions – under a moonless dark sky with the shower’s radiant point, the place from which the meteors appear to diverge, directly overhead. We should lower our expectations, however, for although moonlight is not a problem this year, most of us contend with light pollution and the radiant does not stand overhead.

Even so, observable rates of 20-40 per hour make for an impressive display and, unlike for the rival Geminid shower in December, we don’t have to freeze for the privilege. Indeed, some people enjoy group meteor parties, with would-be observers reclining to observe different parts of the sky and calling out “meteor!” each time they spot one. Target the night of 12th-13th for any party, though rates may still be respectable between the 9th and 15th.

The shower takes its name from the fact that its radiant point lies in the northern part of the constellation Perseus, see the north map, and climbs from about 30° high in the north-north-east as darkness falls to very high in the east before dawn. Note that Perseids fly in all parts of the sky – it is just their paths that point back to the radiant.

Records of the shower date back to China in AD 36 and it is sometimes called the Tears of St Lawrence after the saint who was martyred on 10 August AD 258, though it seems this title only dates from the nineteenth century.

Sunrise/sunset times for Edinburgh change this month from 05:17/21:20 BST on the 1st to 06:15/20:10 on the 31st. The Moon is at last quarter on the 4th, new on the 11th, at first quarter on the 18th and full on the 26th.

A partial solar eclipse on the 11th is visible from the Arctic, Greenland, Scandinavia and north-eastern Asia. Observers in Scotland north of a line from North Uist to the Cromarty Firth see a thin sliver of the Sun hidden for just a few minutes at about 09:45 BST. Our best place to be is Shetland but even in Lerwick the eclipse lasts for only 43 minutes with less than 2% of the Sun’s disk hidden at 09:50. To prevent serious eye damage, never look directly at the Sun.

Vega in Lyra is the brightest star overhead at nightfall and marks the upper right corner of the Summer Triangle it forms with Deneb in Cygnus and Altair in Aquila. Now that the worst of the summer twilight is behind us, we have a chance to glimpse the Milky Way as it flows through the Triangle on its way from Sagittarius in the south to Auriga and the star Capella low in the north. Other stars of note include Arcturus in Bootes, the brightest star in our summer sky, which is sinking in the west at the map times as the Square of Pegasus climbs in the east.

Of the quartet of planets in our evening sky, two have already set by our map times. The first and brightest of these is Venus which stands only 9° high in the west at Edinburgh’s sunset on the 1st and sets itself 68 minutes later. By the 31st, these numbers change to 4° and 35 minutes, so despite its brilliance at magnitude -4.2 to -4.4, it is becoming increasingly difficult to spot as an evening star. It is furthest east of the Sun (46°) on the 17th.

Jupiter remains conspicuous about 10° high in the south-west as darkness falls and sets in the west-south-west just before the map times. Edging eastwards in Libra, it dims slightly from magnitude -2.1 to -1.9 and slips 0.6° north of the double star Zubenelgenubi on the 15th. A telescope shows it to be 36 arcseconds wide when it lies below-right of the Moon on the 17th.

The two planets low in the south at our map times are Mars, hanging like a prominent orange beacon only some 7° high in south-western Capricornus, and Saturn which is a shade higher above the Teapot of Sagittarius almost 30° to Mars’ right. Mars stood at opposition on 27 July and is at its closest to the Earth (57.6 million km) four days later. A planet-wide dust storm has hidden much of the surface detail on its small disk which shrinks during August from 24 to 21 arcseconds as its distance increases to 67 million km. Although Mars dims from magnitude -2.8 to -2.1, so it remains second only to Venus in brilliance. Catch the Moon near Saturn on the 20th and 21st and above Mars on the 24th.

Finally, we cannot overlook Mercury which is a morning star later in the period. Between the 22nd and 31st, it brightens from magnitude 0.8 to -0.7, rises more than 90 minutes before the Sun and stands around 7° high in the east-north-east forty minutes before sunrise. It is furthest west of the Sun (18°) on the 26th.

Diary for 2018 August

Times are BST

4th            19h Last quarter

9th            01h Mercury in inferior conjunction on Sun’s near side

11th          11h New moon and partial solar eclipse

13th         02h Peak of Perseids meteor shower

14th         15h Moon 6° N of Venus

17th         12h Moon 5° N of Jupiter

17th         19h Venus furthest E of Sun (46°)

18th         09h First quarter

21st         11h Moon 2.1° N of Saturn

23rd         18h Moon 7° N of Mars

26th         13h Full moon

26th         22h Mercury furthest W of Sun (18°)

28th         11h Mars stationary (motion reverses from W to E)

Alan Pickup

This is a slightly revised version, with added diary, of Alan’s article published in The Scotsman on July 31st 2018, with thanks to the newspaper for permission to republish here.

Scotland’s Sky in July, 2018

Dust storm rages on Mars as it stands closest since 2003

The maps show the sky at 01:00 BST on the 1st, midnight on the 16th and 23:00 on the 31st. An arrow depicts the motion of Mars. (Click on map to enlarge)

The maps show the sky at 01:00 BST on the 1st, midnight on the 16th and 23:00 on the 31st. An arrow depicts the motion of Mars. (Click on map to enlarge)

Mars comes closer to the Earth in July than at any time since its once-in-60,000-years record approach in 2003. It is just our luck that a dust storm that began a month ago now engulfs the entire planet so that the surface markings may now be glimpsed only through a patchy reddish haze.

Both current Mars rovers, Opportunity and Curiosity, are also affected. This is the most intense storm to impact Opportunity since it landed in 2004 and the vehicle has shut down because it lost power as the dust hid the Sun and coated its solar panels. It is hoped that, after the storm subsides, friendly gusts of wind will waft the dust from the panels and Opportunity will revive. If not, this would mark the end of a remarkable mission which had been planned, initially, to last for only 90 days. Its sister rover, Spirit, succumbed in 2010 after becoming stuck in soft soil.

Meanwhile, the more advanced Curiosity rover has been operating since 2012. Being nuclear powered, it is less vulnerable to the dust but its cameras are recording a dull reddened landscape beneath dusty orange skies.

For watchers in Edinburgh, Mars rises in the south-east just before midnight at the beginning of July and is conspicuous at magnitude -2.2 but only 11° high in the south during morning twilight. Look for it 4° below the Moon on the 1st as Mars moves westwards in the constellation of Capricornus.

Mars reaches opposition on the 27th when it stands opposite the Sun, rises during our evening twilight and is highest in the south in the middle of the night. By then it blazes at magnitude -2.8, making it second only to Venus in brilliance, and stands 58 million km away. A telescope shows it to be 24 arcseconds wide, with its southern polar cap tilted 11° towards us. Because Mars is edging inwards in its relatively elongated orbit, it is actually around 100,000 km closer to us on the 31st.

As Mars rises at its opposition on the 27th it once again lies below Moon, but this time the Moon is deep in eclipse as it passes almost centrally through the Earth’s shadow. The total phase of the eclipse, the longest this century, lasts from 20:30 to 22:13 BST and it is in the middle of this period, at 21:22, that the Moon rises for Edinburgh. By 22:13, and weather permitting, it may be possible to see the Moon’s dull ochre disk 5° high in the south-east. From then until 23:19, the Moon emerges eastwards from the Earth’s dark umbral shadow, and at 00:29 it is free of the penumbra, the surrounding lighter shadow.

The Earth stands at its furthest from the Sun for 2018 (152,100,000 km) on the 6th. Edinburgh’s sunrise/sunset times change from 04:31/22:01 on the 1st to 05:15/21:22 on the 31st. The Moon is at last quarter on the 6th and new on the 13th when a partial solar eclipse is visible to the south of Australia. First quarter on the 19th is followed by full moon and the total lunar eclipse on the 27th.

Our chart shows the corner stars of the Summer Triangle, Vega in Lyra, Altair in Aquila and Deneb in Cygnus, high in the south to south-east as the fainter corner stars of the Square of Pegasus are climbing in the east. The Plough stands in the middle of our north-western sky and the “W” of Cassiopeia is similarly placed in the north-east.

Venus sets before our chart times but is brilliant in the west at nightfall. It brightens from magnitude -4.0 to -4.2 but is sinking lower from night to night as it tracks southwards relative to the Sun. It passes 1.1° north of the star Regulus in Leo on the 9th as the much fainter planet Mercury (magnitude 0.4) stands 16° below-right of Venus. The little innermost planet stands furthest east of the Sun (26°) on the 12th but is a challenge to glimpse in the twilight this time around.

Venus lies to the left of the young earthlit Moon on the 15th, below-right of the Moon on the 16th and, by month’s end, stands less than 10° high at sunset before setting itself some 70 minutes later.

Jupiter lingers as a conspicuous evening object in the south-south-west at nightfall, sinking to set in the west-south-west one hour after our map times. Moving very little against the stars of Libra, it dims slightly from magnitude -2.3 to -2.1 and shows a 39 arcseconds disk when it lies below-left of the Moon on the 20th.

Saturn reached opposition on June 27 and is at its best at our star map times, albeit low in the south at a maximum altitude of less than 12° for Edinburgh. At magnitude 0.0 to 0.2, it is creeping westwards above the Teapot of Sagittarius where it lies near the Moon on the 24th and 25th. Its disk and wide-open rings appear 18 and 41 arcseconds wide respectively.

Our noctilucent, or “night-shining”, cloud season is now in full swing with sightings of several displays of these high-altitude blue-white clouds since late-May and further ones expected until August.

Often with a wispy cirrus-like appearance, noctilucent clouds are composed of ice-crystals at heights near 82 km and glimmer above our northern horizon where they catch the sunlight long after our more usual lower-level clouds are in darkness. Their nature is still something of a mystery but it may not be coincidental that the first definite record of them dates only as far back as 1885, just two years after the cataclysmic eruption of the Krakatoa volcano in Indonesia.

Diary for 2018 July

Times are BST

1st            03h Moon 5° N of Mars

6th            09h Last quarter

6th            18h Earth farthest from Sun (152,100,000 km)

9th            21h Venus 1.1° N of Regulus

11th          05h Jupiter stationary (motion reverses from W to E)

12th         06h Mercury furthest E of Sun (26°)

13th         04h New moon and partial solar eclipse S of Australia

14th         23h Moon 2.2° N of Mercury

16th         04h Moon 1.6° N of Venus

19th         21h First quarter

21st         01h Moon 4° N of Jupiter

25th         07h Moon 2.0° N of Saturn

27th         06h Mars at opposition at distance of 58 million km

27th         21h Full moon and total lunar eclipse

27th         22h Moon 7° N of Mars

29th         Main peak of Delta Aquarids meteor shower

31st         09h Mars closest to Earth (57,590,000 km)

Alan Pickup

This is a slightly revised version, with added diary, of Alan’s article published in The Scotsman on June 30th 2018, with thanks to the newspaper for permission to republish here.

Scotland’s Sky in December, 2017

Geminid meteors sparkle during long December nights

The maps show the sky at 21:00 GMT on the 1st, 20:00 on the 16th and 19:00 on the 31st. (Click on map to enlarge)

The maps show the sky at 21:00 GMT on the 1st, 20:00 on the 16th and 19:00 on the 31st. (Click on map to enlarge)

December brings us spectacular night skies and arguably the richest meteor shower of the year, the Geminids. We still have the Summer Triangle of bright stars, Vega in Lyra, Deneb in Cygnus and Altair in Aquila, high in the south-west at nightfall while the unmistakable figure of Orion dominates the midnight hours, surrounded by his cohort of familiar winter constellations. By the predawn, the Plough sails overhead and the night’s only conspicuous planets shine to the south of east.

Our longest nights, of course, occur around the winter solstice when the Sun reaches its most southerly point in its annual trek around the sky. This occurs at 16:28 GMT on the 21st when Edinburgh’s night, measured from sunset to sunrise, lasts for 17 hours and 3 minutes, which no less than 10 hours and 39 minutes longer than at June’s summer solstice.

Sunrise/sunset times for Edinburgh during December vary from 08:19/15:44 on the 1st to 08:42/15:40 on the 21st and 08:44/15:48 on the 31st. The Moon is full on the 3rd, at last quarter on the 10th, new on the 18th and at first quarter on the 26th,

By our map times, the Summer Triangle has toppled low into the west and is being followed by the less impressive Square of Pegasus. The Square’s top-left star, Alpheratz, belongs to Andromeda whose other main stars, Mirach and Almach, line up to its left. A spur of fainter stars above Mirach leads us to the Andromeda Galaxy, whose oval glow reaches us from 2.5 million light years away.

Orion is in the east-south-east, his Belt pointing up Aldebaran and the Pleiades in Taurus and down to where the brightest nighttime star, Sirius in Canis Major, rises less than one hour later.

The Moon lies to the right of Aldebaran and below the Pleiades on the night of 2nd-3rd, to the left of Aldebaran a day later and comes around again to occult the star in the early hours of the 31st. We need a telescope to see Aldebaran wink out at the Moon’s limb at 01:01 and reappear at 01:57 as seen from Edinburgh.

It is from a radiant point near Castor in Gemini, north-east of Orion, that meteors from the Geminids shower diverge between the 8th and 17th although, of course, the meteors fly in all parts of the sky. With negligible moonlight this year, and given decent weather, we are in for a stunning display of sparkling long-trailed meteors whose paths point back to the radiant. Rates for an observer under an ideal dark sky could peak at more than 100 per hour at the shower’s peak on the night of the 13th-14th, though most of us may glimpse only a fraction of these.

Although most meteors originate as cometary debris, the Geminids appear to be rocky splinters from the 5 km-wide asteroid, Phaethon, which dives within 21 million km of the Sun every 523 days. In what is its closest approach to the Earth since its discovery in 1983, Phaethon sweeps only 10.3 million km from the Earth on the 16th when a telescope might show it as a tenth magnitude speck speeding past Alpheratz.

December’s second shower, the Ursids, derives from Comet Tuttle and is active between the 17th and 25th, peaking on the 23rd. Typically it yields fewer than ten meteors per hour so I rarely mention it here – I believe my last time was 37 years ago – but very occasionally it rivals the Geminids in intensity, if only for a few hours. The radiant point lies near the star Kochab in Ursa Minor and is plotted on our northern chart.

The unprecedented interstellar asteroid, discovered using a telescope in Hawaii and featured here hast time, has now been called 1I/’Oumuamua. This indicates that it is our first known interstellar visitor and employs the Hawaiian word ’Oumuamua to reflect its supposed status as a scout from the distant past. Further observations imply that it is remarkably elongated, being at least five times longer than it is wide.

Venus shines brilliantly at magnitude -3.9 very low in the south-east as the night ends, but is soon lost from view as it dives towards the Sun’s far side. It leaves Jupiter as our most prominent (magnitude -1.7 to -1.8) morning object. The giant world rises at Edinburgh’s east-south-eastern horizon at 05:31 on the 1st and 04:07 on the 31st, climbing southwards in the sky to stand some 15° high before dawn. Tracking eastwards in Libra, it passes 0.7° north of the celebrated double star Zubenelgenubi on the 21st.

Mars, fainter at magnitude 1.7 to 1.5, lies 16° above-right of Jupiter on the 1st when it is also about half as bright as Virgo’s star Spica, 3° below and to its right. As Mars tracks east-south-eastwards from Virgo to Libra it almost keeps pace with the Sun, so that it rises at around 03:50 throughout the month. By the 31st, it stands 3° from Jupiter, with Zubenelgenubi below and to Mars’ left in the same binocular field of view. The waning Moon forms a nice triangle with Mars and Spica on the 13th and with Mars and Jupiter on the 14th.

Saturn sets in our bright evening twilight as it heads towards conjunction beyond the Sun on the 21st. Mercury slips around the Sun’s near side on the 13th to become best placed as a morning star between Christmas and New Year. Between the 21st and 31st it brightens between magnitude 0.8 and -0.3, rises 100 or more minutes before Edinburgh’s sunrise and stands around 8° high in the south-east thirty minutes before sunrise.

Alan Pickup

This is a slightly revised version of Alan’s article published in The Scotsman on November 30th 2017, with thanks to the newspaper for permission to republish here.

Scotland’s Sky in October, 2017

Saturn at full tilt as Comet Halley’s meteors fly

The maps show the sky at 23:00 BST on the 1st, 22:00 BST (21:00 GMT) on the 16th and at 20:00 GMT on the 31st. Summer time ends at 02:00 BST on the 29th when clocks are set back one hour to 01:00 GMT. (Click on map to enlarge)

The maps show the sky at 23:00 BST on the 1st, 22:00 BST (21:00 GMT) on the 16th and at 20:00 GMT on the 31st. Summer time ends at 02:00 BST on the 29th when clocks are set back one hour to 01:00 GMT. (Click on map to enlarge)

Our charts capture the sky in transition between the stars of summer, led by the Summer Triangle of Deneb, Vega and Altair in the west, and the sparkling winter groups heralded by Taurus and the Pleiades star cluster climbing in the east. Indeed, if we look out before dawn, as Venus blazes in the east, we see a southern sky centred on Orion that mirrors that of our spectacular February evenings. October also brings our second opportunity this year to spot debris from Comet Halley.

As the ashes of the Cassini spacecraft settle into Saturn, the planet reaches a milestone in its 29-years orbit of the Sun when its northern hemisphere and rings are tilted towards us at their maximum angle of 27.0° this month. In practice, our view of the rings’ splendour is compromised at present by its low altitude.

Although it shines at magnitude 0.5 and is the brightest object in its part of the sky, Saturn hovers very low in the south-west at nightfall and sets around 80 minutes before our map times. The rings span 36 arcseconds at mid-month while its noticeably rotation-flattened disk measures 16 arcseconds across the equator and 14 arcseconds pole-to-pole. Catch it below and to the right of the young crescent Moon on the 24th.

The Sun moves 11° further south of the equator this month as sunrise/sunset times for Edinburgh change from 07:16/18:48 BST (06:16/17:48 GMT) on the 1st to 07:18/16:34 GMT on the 31st, after we set our clocks back on the 29th.

Jupiter is now lost in our evening twilight as it nears the Sun’s far side on the 26th. Saturn is not alone as an evening planet, though, for both Neptune and Uranus are well placed. They are plotted on our southern chart in Aquarius and Pisces respectively but we can obtain more detailed and helpful diagrams of their position via a Web search for a Neptune or Uranus “finder chart” – simply asking for a “chart” is more likely to lead you to astrological nonsense.

Neptune, dimly visible through binoculars at magnitude 7.8, lies only 0.6° south-east (below-left) of the star Lambda Aquarii at present and tracks slowly westwards to sit a similar distance south of Lambda by the 31st. It lies 4,346 million km away on the 1st and its bluish disk is a mere 2.3 arcseconds wide.

Uranus reaches opposition on the 19th when it stands directly opposite the Sun and 2,830 million km from Earth. At magnitude 5.7 it is just visible to the unaided eye in a good dark sky, and easy through binoculars. Currently 1.3° north-west of the star Omicron Piscium and also edging westwards, it shows a bluish-green 3.7 arcseconds disk if viewed telescopically.

North of Aquarius and Pisces are Pegasus and Andromeda, the former being famous for its relatively barren Square while the fuzzy smudge of the Andromeda Galaxy, M31, lies 2.5 million light years away and is easy to glimpse through binoculars if not always with the naked eye.

Mercury slips through superior conjunction on the Sun’s far side on the 8th and is out of sight. Venus remains resplendent at magnitude -3.9 in the east before dawn though it does rise later and stand lower each morning. On the 1st, it rises for Edinburgh at 04:44 BST (03:44 GMT) and climbs to stand 20° high at sunrise. By the month’s end, it rises at 05:30 GMT and is 13° high at sunrise. Against the background stars, it speeds from Leo to lie 5° above Virgo’s star Spica by the 31st.

Mars is another morning object, though almost 200 times dimmer at magnitude 1.8 as it moves from 2.6° below-left of Venus on the 1st to 16° above-right of Venus on the 31st. The pair pass within a Moon’s breadth of each other on the 5th and 6th when Venus appears 11 arcseconds in diameter and 91% sunlit and Mars (like Uranus) is a mere 3.7 arcseconds wide.

Comet Halley was last closest to the Sun in 1986 and will not return again until 2061. Twice each year, though, the Earth cuts through Halley’s orbit around the Sun and encounters some of the dusty debris it has released into its path over past millennia. The resulting pair of meteor showers are the Eta Aquarids in early-May and the Orionids later this month. Although the former is a fine shower for watchers in the southern hemisphere, it yields only the occasional meteor in Scotland’s morning twilight.

The Orionids are best seen in the morning sky, too, and produce fewer than half the meteors of our main annual displays. This time the very young Moon offers no interference during the shower’s broad peak between the 21st and 23rd. In fact, Orionids appear throughout the latter half of October as they diverge from a radiant point in the region to the north and east of the bright red supergiant star Betelgeuse in Orion’s shoulder and close to the feet of Gemini. Note that they streak in all parts of the sky, not just around the radiant.

Orionids begin to appear when the radiant rises in the east-north-east at our map times, building in number until it passes around 50° high in the south before dawn. Under ideal conditions, with the radiant overhead in a black sky, as many as 25 fast meteors might be counted in one hour with many leave glowing trains in their wake. Rates were considerably higher than this between 2006 and 2009, so there is the potential for another pleasant surprise.

Alan Pickup

This is a slightly revised version of Alan’s article published in The Scotsman on September 30th 2017, with thanks to the newspaper for permission to republish here.

Scotland’s Sky in September, 2017

Cassini’s scheduled suicide at Saturn

The maps show the sky at 23:00 BST on the 1st, 22:00 on the 16th and 21.00 on the 30th. (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. (Click on map to enlarge)

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.

Alan Pickup

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.

Scotland’s Sky in August, 2017

Countdown to the Great American Eclipse

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

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

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.

Alan Pickup

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.

Scotland’s Sky in July, 2017

Pole stars of the future in the Summer Triangle

The maps show the sky at 01:00 BST on the 1st, midnight on the 16th and 23:00 on the 31st. (Click on map to enlarge)

The maps show the sky at 01:00 BST on the 1st, midnight on the 16th and 23:00 on the 31st. (Click on map to enlarge)

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.

Alan Pickup

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.