The Bronze Age bull that leads Orion across our night sky
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
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.
Mercury crosses Sun as bright planets converge in evening sky
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