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Scotland’s Sky in December, 2019

The Bronze Age bull that leads Orion across our night sky

Sky maps looking north and south, showing the position of the main constellations at different times during the month.

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 two brightest planets hug our south-south-western horizon after sunset at present, but soon set themselves to leave Orion to dominate our December nights which include the longest ones of the year.

The Sun’s southwards motion halts at our winter solstice at 04:19 GMT on the 22nd. Sunrise/sunset times for Edinburgh vary from 08:19/15:44 on the 1st, to 08:42/15:40 on the 22nd at 08:44/15:47 on Hogmanay. Because the Earth is tipped on its axis and in an elliptical orbit about the Sun, the solstice coincides with neither our latest sunrise nor earliest sunset. Instead, Edinburgh’s latest sunrise at 08:44 is not until the 29th, while our earliest sunset at 15:38 comes on the 15th.

The Moon is at first quarter on the 4th, full on the 12th, at last quarter on the 19th and new on the 26th when it appears too small to hide the Sun completely. Instead, an annular or ring solar eclipse is visible from Saudi Arabia to Indonesia by way of southern India.

Venus blazes at magnitude -3.9 as it stands 5° high thirty minutes after sunset on the 1st. It lies 7° to the left of Jupiter, one seventh as bright at magnitude -1.8, but we lose sight of the latter within a few days as it heads towards the Sun’s far side on the 27th.

Venus, meanwhile, tracks eastwards to pass 2° below the much fainter planet Saturn (magnitude 0.6) on the 10th. By the 27th, Saturn is hard to spot in the twilight when it stands 3° right of the very slender young and earthlit Moon. The next evening has the Moon 5° below and right of Venus which, by then, is established as an impressive evening star that stands 12° high thirty minutes after sunset.

Vega, the brightest star in the Summer Triangle, stands high in the south-west at nightfall, but sinks into the north-west sky by our map times. Meanwhile, Taurus the Bull, with its leading star Aldebaran and the Pleiades star cluster, climbs from low in the east-north-east into the south-east. Below Taurus is the unmistakable form of Orion with the three stars of his Belt slanting up to Aldebaran. By midnight, Taurus stands high on the meridian, above and to the right of Orion whose Belt also points downwards to our brightest nighttime star, Sirius in Canis Major.

The Pleiades, a so-called open star cluster, is sometimes called the Seven Sisters, though I leave you to judge whether this is fair description of its naked-eye appearance. Certainly, binoculars and telescopes show impressive views of many more than seven stars. Photographs reveal them to be embedded in bluish wispy haze that astronomers once took to be the remains of the material from which the stars formed. Now we understand the haze to be a cloud of dust which the cluster has encountered as it orbits our Milky Way. The cluster lies 444 light years (ly) away and may be less than 100 million years old – much older and the young blue and luminous stars that illuminate the dust would not have survived.

Taurus has represented a bull in the mythologies of many ancient civilisations since the early Bronze Age, though typically only the horns, head and forequarters are imagined in the sky. Taurus’ face is marked by a V-shaped pattern of stars that comprise the Hyades, the nearest of all the known open star clusters in the sky. The measurement of its distance as 153 ly is a vital yardstick in the fixing of other stellar distances in our galaxy and beyond. The bright red giant star Aldebaran, sometimes taken to be the Bull’s bloodshot eye, is not, though, a member of the Hyades, being a foreground object at 65 ly.

Perhaps the foremost astrophysical object in Taurus is the Crab Nebula which lies 1.1°, or two Moon-diameters, north-west of the star Zeta, the tip of Taurus’ unfeasibly long southern horn. Also known as M1, it is the remains of a supernova witnessed by Chinese observers in 1054, being seen as a naked-eye object for around two years and even being visible in daylight. The expanding debris of the stellar explosion now appears as an eight-magnitude smudge in small telescopes and contains a pulsar, a neutron star some 30 km wide that spins thirty times a second and beams out flashes of radiation at every wavelength from gamma rays to radio waves.

Above and to the left of Orion lies Gemini the Twins whose main stars, Castor and Pollux, sit one above the other as they climb through our eastern sky. Slow meteors of the Geminids shower diverge from a radiant near Castor (see chart) between the 4th and 17th. The display is expected to peak on the 14th at rates that could exceed 100 meteors per hour for an observer under a clear dark sky. It is a pity that the Moon lies just a few degrees below Pollux at the maximum and sheds enough light to swamp many of the fainter Geminids this time around.

The radiant of the month’s second shower, the Ursids, lies just below the first “R” in “URSA MINOR” on our north star map. The shower is active between the 17th and 26th with its peak of some 10 medium-speed meteors per hour under (thankfully) moonless skies on the 23rd.

The normally shy innermost planet Mercury is currently shining brightly at about magnitude -0.5 low down in the south-east for two hours before sunrise. However, it sinks lower each morning and is likely lost in the dawn twilight by midmonth. Higher and to its right, and in line with the bright star Spica in Virgo, is the fainter (magnitude 1.7) Mars which tracks 20° east-south-eastwards in Libra this month, and passes a mere 0.2° north of the double star Zubenelgenubi on the 12th. Catch the Red Planet to the right of the waning Moon before dawn on the 23rd.

Diary for 2019 December

4th           07h First quarter

8th           13h Interstellar Comet Borisov closest to Sun (300m km)

11th         05h Venus 1.8° S of Saturn

11th         12h Moon 3° N of Aldebaran

12th         05h Full moon

14th         14h Peak of Geminids meteor shower

15th         16h Moon 1.3° N of Praesepe

17th         05h Moon 4° N of Regulus

19th         05h Last quarter

22nd         04:19 Winter solstice

23rd         Peak of Ursids meteors shower

23rd         02h Moon 4° N of Mars

26th         05h New moon and annular solar eclipse

27th         12h Moon 1.2° S of Saturn

27th         18h Jupiter in conjunction with Sun

29th         02h Moon 1.0° S of Venus

Alan Pickup

This is an extended version, with added diary, of Alan’s article published in The Scotsman on November 30th 2019, with thanks to the newspaper for permission to republish here.
Please note, this is the last time the monthly sky update will appear on the Journal.  From now on, the articles will appear in the news section of the Astronomical Society of Edinburgh website.

Scotland’s Sky in November, 2019

Mercury crosses Sun as bright planets converge in evening sky

Sky maps looking north and south, showing the position of the main constellations at different times during the month.

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

With all the planets in view and a sky brimming with interest from dusk to dawn, November is a rewarding month for stargazers, particularly since temperatures have yet to plumb their wintry lows. Our astro-highlight of the month, if not the year, though, comes in daylight on the 11th when Mercury appears as a small inky dot crossing the Sun’s face.

Perhaps one puzzle is why such transits of Mercury are not more frequent. After all, Mercury orbits the Sun every 88 days and, as we see it, passes around the Sun’s near side at its so-called inferior conjunction every 116 days on average.

The reason we don’t enjoy around three transits each year is that the orbits of Mercury and the Earth are tipped at 7° in relation to each other. For a transit to occur, we need Mercury to reach inferior conjunction near the place where its orbit crosses the orbital plane of the Earth, and currently this can occur only during brief windows each May and November. This condition restricts us to around one transit of Mercury every seven years on average but there are wide variations. Indeed, our last transit occurred as recently as May 2016 while we need to wait until November 2032 for the next. We must hang around even longer, and travel beyond Europe, for the next transit of Venus in 2117.

This month’s transit begins at 12:35 on the 11th when the tiny disk of Mercury, only 10 arcseconds wide, begins to enter the eastern (left) edge of the Sun. The Sun stands 16° high in Edinburgh’s southern sky at that time but it falls to 5° high in the south-west by 15:20 when Mercury is at mid-transit, only one twenty-fifth of the Sun’s diameter above the centre of the solar disk. The Sun sets for Edinburgh at 16:13 so we miss the remainder of the transit which lasts until 18:04.

The usual warnings about solar observation apply so that, if you value your eyesight, you must never observe the Sun directly. Solar glasses that you might have used for an eclipse will be no help since Mercury is too small to see seen without magnification. Instead, use binoculars or, better, a telescope which has been equipped securely with an approved solar filter.

A few days after its transit, Mercury begins its best morning apparition of the year. Between the 23rd and 30th, it rises more than two hours before the Sun and shines brightly at magnitude -0.1 to -0.5 while 7° high in the south-east one hour before sunrise. Higher but fainter in the south-east before dawn is Mars (magnitude 1.7) which tracks south-eastwards in Virgo to pass 3° north of Spica on the 8th and end the period 11° above-right of Mercury. Catch it below the waning Moon on the 24th.

The Sun’s southwards progress leads to sunrise/sunset time for Edinburgh changing from 07:19/16:33 GMT on the 1st to 08:17/15:45 on the 30th. The Moon is at first quarter on the 4th, full on the 12th, at last quarter on the 19th and new on the 26th.

Three bright planets vie for attention in our early evening sky but the brightest, Venus, is currently also the first to drop below the horizon as the twilight fades. Blazing at magnitude -3.9, it stands less than 4° high in the south-west at Edinburgh’s sunset on the 1st and sets itself only 38 minutes later.

Second in brightness comes Jupiter, magnitude -1.9, which lies some 24° to the left of Venus on the 1st and sets two hours after sunset. Then we have magnitude 0.6 Saturn which lies another 22° to Jupiter’s left so that it is about 10° high in the south-south-west as darkness falls tonight and sets about 50 minutes before our map times.

Venus tracks quickly eastwards to pass 1.4° south of Jupiter on the 24th when it stands 6° high at sunset as it embarks on an evening spectacular that lasts until May. The young Moon lies 7° below-right of Saturn on the 1st, makes a stunning sight between Jupiter and Venus on the 28th and is nearing again Saturn on the 29th.

Vega, the leader of the Summer Triangle, blazes just south-west of overhead at nightfall at present but is sinking near the middle of our western sky by our map times. Well up in the south by then is the Square of Pegasus whose top-left star, Alpheratz, leads the three main stars of Andromeda, lined up to its left. A spur of two fainter stars above the middle of these, Mirach, points the way to the oval glow of the Milky Way’s largest neighbouring galaxy, the famous Andromeda Galaxy, M31.

Below the Square is the dim expanse of Pisces that lies between the distant binocular-brightness planets Neptune and Uranus, in Aquarius and Aries respectively.

Orion, the centerpiece of our winter’s sky, is rising in the east at our map times and takes six hours, until the small hours of the morning, to reach its highpoint in the south. Preceding Orion is Taurus and the Pleiades while on his heals comes Sirius in Canis Major which twinkles its way across our southern sky before dawn.

The morning hours, particularly on the 19th, are also optimum for glimpsing members of the Leonids meteor shower. Arriving between the 6th and 30th, but with a sharp peak expected late on the 18th, these swift meteors diverge from Leo’s Sickle which rises in the north-east before midnight and climbs to stand in the south before dawn. Fewer than 15 meteors per hour may be sighted this year, far below the storm-force levels witnessed around the turn of the century.

Diary for 2019 November

2nd           07h Moon 0.6° S of Saturn

4th           10h First quarter

8th           15h Mars 3° N of Spica

11th         15h Mercury transits Sun at inferior conjunction

12th         14h Full moon

14th         04h Moon 3.0° N of Aldebaran

18th         11h Moon 1.2° N of Praesepe

18th         23h Peak of Leonids meteor shower

19th         21h Last quarter

20th         00h Moon 4° N of Regulus

24th         09h Moon 4° N of Mars

24th         14h Venus 1.4° S of Jupiter

25th         03h Moon 1.9° N of Mercury

26th         15h New moon

28th         10h Mercury furthest W of Sun (20°)

28th         11h Moon 0.7° N of Jupiter

28th         19h Moon 1.9° N of Venus

29th         21h Moon 0.9° S of Saturn

Alan Pickup

This is an extended version, with added diary, of Alan’s article published in The Scotsman on October 31st 2019, with thanks to the newspaper for permission to republish here.

Scotland’s Sky in December, 2016

Geminids suffer in the supermoonlight

The maps show the sky at 21:00 GMT on the 1st, 20:00 on the 16th and 19:00 on the 31st. An arrow depicts the motion of Mars after the first week of the month. (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. An arrow depicts the motion of Mars after the first week of the month. (Click on map to enlarge)

The Sun reaches its farthest south at our winter solstice at 10:44 GMT on the 21st, as Mars and the brilliant Venus stand higher in our evening sky than at any other time this year. This is not a coincidence, for both planets are tracking eastwards and, more importantly, northwards in the sky as they keep close to the ecliptic, the Sun’s path over the coming weeks and months. Meantime, Jupiter is prominent during the pre-dawn hours while Orion is unmistakable for most of the night and strides proudly across the meridian at midnight in mid-December.

As the sky darkens this evening, Pegasus with its iconic, but rather empty, Square is nearing the meridian and the Summer Triangle (Vega, Deneb and Altair) stands high in the south-west.

By our map times, Altair is setting in the west and Orion stands in the south-east, the three stars of Belt pointing down to where Sirius, our brightest night-time star, will soon rise. Sirius, the red supergiant Betelgeuse at Orion’s shoulder and Procyon in Canis Minor, almost due east of Betelgeuse, form a near-equilateral triangle which has come to be known as the Winter Triangle.

Above Orion is Taurus, home to the Pleiades star cluster and the bright orange giant star Aldebaran, the latter located less than halfway between us and the V-shaped Hyades cluster.

Look for the almost-full Moon below the Pleiades and to the right of Aldebaran and the Hyades on the evening of the 12th and watch it barrel through the cluster during the night, occulting (hiding) several of the cluster’s stars on the way. As they dip low into the west on the following morning, the Moon occults Aldebaran itself, the star slipping behind the Moon’s northern edge between 05:26 and 05:41 as seen from Edinburgh. Even though this is the brightest star to be occulted this year, the Moon’s brilliance means we may well need a telescope to view the event.

Sunrise/sunset times for Edinburgh vary from 08:20/15:44 on the 1st to 08:42/15:40 on the 21st and 08:44/15:48 on the 31st. The Moon is at first quarter on the 7th and full on the 14th when, once again, it is near its perigee, its closest point to the Earth. Despite the fact that the Moon appears a barely perceptible 7% wider than it does on average, we can look forward to yet another dose of over-hyped supermoon hysteria in the media. The Moon’s last quarter comes on the 21st and it is new on the 29th.

Sadly, the Moon does its best to swamp the annual Geminids meteor shower which lasts from the 8th to the 17th and is expected to peak at about 20:00 on the 13th. Its meteors are medium-slow and, thankfully, there are enough bright ones that several should be noticeable despite the moonlight. Without the moonlight, and under perfect conditions, this might have been our best display of 2016, with 100 or more meteors per hour.

Geminids are visible in all parts of the sky, but perspective makes them appear to diverge from a radiant point near the star Castor in Gemini, marked near the eastern edge of our north map. This radiant climbs from our north-eastern horizon at nightfall to pass high in the south at 02:00.

Venus stands 10° above Edinburgh’s southern horizon at sunset on the 1st and shines spectacularly at magnitude -4.2 as it sinks to set in the south-west almost three hours later. The young earthlit Moon stands 10° above-right of Venus on the 2nd, 5° above the planet on the 3rd and, one lunation later, 20° below-right of the Moon on Hogmanay. By then, Venus is twice as high at sunset and (just) brighter still at magnitude -4.3. A telescope shows its dazzling gibbous disk which swells from 17 to 22 arcseconds in diameter as the sunlit portion shrinks from 68% to 57%.

As Venus speeds from Sagittarius to Capricornus, so Mars keeps above and to its left as it moves from Capricornus into Aquarius and into the region of sky above our south-western horizon at the map times. Mars is only a fraction as bright, though, and fades from magnitude 0.6 to 0.9. It also appears much smaller, only 6 arcseconds, so that telescopes now struggle to reveal any surface features. Spot Mars to the left of the Moon on the 4th and below-right of the Moon on the 5th.

Mercury is farthest east of the Sun, 21°, on the 11th but hugs our south-western horizon at nightfall and is unlikely to be seen. It reaches inferior conjunction between the Sun and Earth on the 28th by which time Saturn, which passes beyond the Sun on the 10th, might just be glimpsed low above the south-eastern horizon before dawn. On the 27th, it shines at magnitude 0.5 and lies 7° below-left of the slender waning Moon.

Jupiter is conspicuous at magnitude -1.8 to -1.9 and the real star of our morning sky. Rising in the east for Edinburgh at 03:04 on the 1st and 01:31 on the 31st, it climbs well up into our southern sky before dawn where it stands above Virgo’s leading star Spica and draws closer during the month.

Jupiter, Spica and the Moon form a neat triangle before dawn on the 23rd, when Jupiter is 850 million km away and appears 35 arcseconds wide through a telescope. Any decent telescope shows its parallel cloud belts, while binoculars reveal its four main moons which swap places from side to side of the disk as they orbit the planet in periods of between 1.8 and 17 days.

Alan Pickup

This is a slightly-revised version of Alan’s article published in The Scotsman on December 1st 2016, with thanks to the newspaper for permission to republish here.