Friendly Delphinus the Dolphin wins a place among the stars
The Sun’s southwards motion carries it across the sky’s equator at 08:50 BST on the 23rd, marking our autumnal equinox when days and nights are about equal around the Earth. It also means that our nights are lengthening at their fastest pace of the year.
The Summer Triangle (Vega, Deneb and Altair) remains in prime position high in the south at nightfall with the Milky Way flowing through it as it arches across the east from south to north. The brighter stars of Ursa Major, the Great Bear, form the familiar pattern of the Plough which stands in the north-west at nightfall as it begins to swing below the Pole Star, Polaris, in the north.
By our map times, the relatively empty expanse of the Square of Pegasus is climbing in the south-east while below it is the lengthy but dim constellation of Pisces, neatly book-ended by the Sun’s most distant planets Neptune and Uranus. Over the following few hours, though, this same region is invaded by the glorious form of Orion and his entourage of sparkling winter constellations
Last month I mentioned that Vega’s constellation, Lyra, was named for a lyre, but how that musical instrument came to be up there is associated with a myth that also involves Delphinus the Dolphin, a small but distinctive constellation that lies just to the left of the Summer Triangle.
That myth concerns Arion, a (real) poet and musician of ancient Greece. It has him returning by sea following lucrative performances in Sicily, only to be robbed and cast overboard before being rescued by a dolphin and delivered safely to shore. In gratitude, Apollo subsequently elevated the dolphin and Arion’s instrument, a lyre, to their places among the stars.
The four stars that represent the Dolphin’s head form a diamond or kite while telescope reveal that one of these, Gamma (see chart), is a superb double consisting of two stars of contrasting colours that appear only 9 arcseconds apart – in fact, they are separated by 330 times the Earth-Sun distance and take 3,200 years to orbit each other.
How two of the other stars in the diamond, Rotanev and Sualocin, came to be named was a mystery after they first appeared in a star catalogue issued by the Palermo Observatory in 1814. Then it was realised that spelled backwards they became Nicolaus Venator, the Latin equivalent of Niccolò Cacciatore which just happened to be the name of the assistant astronomer at the observatory. While his ruse succeeded, it is worth remembering that the modern craze for “buying” star names has no official standing and the names are not recognised by astronomers worldwide.
The prominent planet Jupiter is already past its best as the sky darkens and sinks from almost 10° high in the south-south-west to set in the south-west by our map times. Still brighter than any star, it dims slightly from magnitude -2.2 to -2.0 this month as it edges eastwards in southern Ophiuchus. The first quarter Moon stands 6° right of Jupiter on the 5th when a telescope shows it to be 38 arcseconds wide at its range of 767 million km.
Telescopes and good binoculars show Jupiter’s four main moons, Io, Europa, Ganymede and Callisto. Europa, with its ice-covered surface and likely sub-surface ocean, is of particular interest and the main target of a just-confirmed NASA mission, the Europa Clipper, which may launch as early as 2023 with arrival in 2026. This would see it beat the European Space Agency’s Jupiter Icy Moons Explorer (or JUICE) probe which is scheduled to launch in 2022 but only arrive at Jupiter in 2029. JUICE will explore all the main moons apart from Io, the volcanic innermost moon which appears to have less water than any other object in the solar system.
Saturn, our only other easy naked eye planet, is at its best at nightfall, albeit barely 12° high in the south and just below the so-called Teaspoon of Sagittarius. Non-twinkling and fading slightly this month between magnitude 0.3 and 0.5, Saturn moves to set in the south-west two hours after the map times. Catch it right of the Moon on the 8th when it appears 17 arcseconds wide with rings spanning 39 arcseconds and tipped 25° to our view.
Sunrise/sunset times for Edinburgh change from 06:16/20:08 BST on the 1st to 07:13/18:52 on the 30th. The Moon is at first quarter near Jupiter on the 6th, full near Neptune on the 14th, at last quarter above Orion on the 22nd and new on the 28th. On the 29th, the Moon’s sliver stands 6° high in the west-south-west at sunset and 3° above the evening star Venus. We have only the slimmest of chances of spotting the pair from our latitudes but binoculars may help – just don’t use them until the Sun is safely below the horizon.
We do need binoculars, at least, to see either Neptune and Uranus which shine at magnitudes of 7.8 and 5.7 respectively. Neptune lies in eastern Aquarius where it tracks 0.8° westwards during the month to pass a mere 13 arcsecond south of the naked-eye star Phi Aquarii (magnitude 4.2) on the 6th. At that time, just four days before it reaches opposition, Neptune lies 4,328 million km away and appears as a tiny 2.3 arcsecond bluish disk. Being seven times brighter, Uranus would be easier to recognise were it not in a star-sparse region in south-western Aries.
Of the other planets, Mars is in conjunction on the Sun’s far side on the 2nd, as is Mercury two days later, while both remain hidden in the Sun’s glare.
Diary for 2019 September
Times are BST
2nd 12h Mars in conjunction with Sun
4th 03h Mercury in superior conjunction
6th 04h First quarter
6th 08h Moon 2.3° N of Jupiter
8th 15h Moon 0.04° S of Saturn
10th 08h Neptune at opposition at distance of 4,328 million km
14th 06h Full moon
18th 07h Saturn stationary (motion reverses from W to E)
20th 18h Moon 2.7° N of Aldebaran
22nd 04h Last quarter
23rd 08:50 Autumnal equinox
24th 23h Moon 0.7° N of Praesepe
26th 10h Moon 3° N of Regulus
28th 02h Moon 4° N of Mars
28th 19h New moon
29th 23h Moon 6° N of Mercury
This is a slightly revised version, with added diary, of Alan’s article published in The Scotsman on August 31st 2019, with thanks to the newspaper for permission to republish here.
Giant planets hang low in evenings as Perseid meteors fly
Recent weeks have seen the Earth pass between the Sun and its two largest planets, the gas giants Jupiter and Saturn. Now they hang low in our evening sky, with Jupiter brighter than any star but less than 12° high in the south-south-west at nightfall as it sinks to set in the south-west one hour after our star map times. Saturn, one tenth as bright, trails 30° behind Jupiter and crosses our meridian a few minutes before the map times.
With the exception of Mercury, these are our only naked eye planets. Both Venus and Mars are hidden on the Sun’s far side where Venus reaches its superior conjunction on the 14th. Mars stands at the far-point in its orbit of the Sun on the 26th and, by my reckoning, is further from the Earth on the 28th (400 million km) than it has been for 32 years.
The Summer Triangle of bright stars, Deneb, Vega and Altair, fills the high southern sky at our map times as the Plough stands in the north-west and “W” of Cassiopeia climbs high in the north-east. Below Cassiopeia is Perseus and the Perseids radiant, the point from which meteors of the annual Perseids shower appear to diverge as they disintegrate in the upper atmosphere at 59 km per second.
The meteoroids, debris from Comet Swift-Tuttle, encounter the Earth between about 17 July and 24 August but arrive in their greatest numbers around the shower’s maximum, expected at about 08:00 BST on the 13th. Sadly, the bright moonlight around that date means that we may see only a fraction of the 80-plus meteors that an observer might count under ideal moonless conditions. It is just as well that Perseids include a high proportion of bright meteors prone to leaving glowing trains in their wake. Our best night is likely to be the 12th-13th as the radiant climbs to stand around 70° high in the east as the morning twilight takes hold.
The Sun drops almost 10° lower in our midday sky during August as the sunrise/sunset times for Edinburgh change from 05:16/21:21 BST on the 1st to 06:14/20:10 BST on the 31st. New moon on the 1st is followed by first quarter on the 7th, full moon on the 15th, last quarter on the 23rd and new moon again on the 30th.
In a month that sees Jupiter dim slightly from magnitude -2.4 to -2.2 and its distance increase from 691 million to 756 million km, its westerly motion in southern Ophiuchus slows to a halt and reverses at a so-called stationary point on the 11th. Its cloud-banded disk, around 41 arcseconds wide, remains a fascinating telescopic sight, particularly given the recent disruption to its Great Red Spot.
Saturn recedes from 1,362 million to 1,409 million km and dims from magnitude 0.2 to 0.3 as it creeps westwards below the Teaspoon, a companion asterism to the Teapot of Sagittarius. Through a telescope, Saturn’s disk appears 18 arcseconds wide while the rings span 41 arcseconds and have their north face tipped at 25° towards the Earth.
Catch the Moon close to Jupiter on the 9th and to the left of Saturn as the Perseids peak on the 12th-13th.
Mercury stands between 2.5° and 5° high in the east-north-east one hour before Edinburgh’s sunrise from the 5th and 22nd. It becomes easier to spot later in this period as it brightens from magnitude 1.0 to -1.2, though we need a clear horizon and probably binoculars to spot it. It is furthest from the Sun, 19°, on the 10th.
The only constellation named for a musical instrument, Lyra the Lyre, stands high on the meridian as darkness falls. Its leading star, the white star Vega, is more than twice as massive as the Sun and 40 times more luminous, making it the second brightest star in our summer night sky (after Arcturus) at its distance of 25 light years (ly). Infrared studies show that Vega is surrounded by disks of dust, but whether this hints at planets coalescing or asteroids smashing together is a matter of controversy – perhaps a mixture of the two.
Some 162 ly away and three Moon-breadths above-left of Vega is the interesting multiple star Epsilon, the Double Double. Binoculars show two almost-equal stars, but telescopes reveal that each of these is itself double. One of the four has its own dim companion and the whole system is locked together gravitationally, though the orbital motions are so slow that little change in their relative positions is noticeable over a lifetime.
The more dynamic system, Beta Lyrae (see map), lies almost 1,000 ly away and has two main component stars that almost touch as they whip around each other in only 12.9 days. Tides distort both stars and, as they eclipse each other, Beta’s total brightness varies continuously between magnitudes of 3.2 and 4.4 – sometimes it can rival its neighbour Gamma while at others it can be less than half as bright.
At a distance of 2,570 ly and 40% of the way from Beta to Gamma is the dim Ring Nebula or M57. At magnitude 8.8 and appearing through a telescope like a small smoke ring around one arcminute across, it surrounds a much fainter white dwarf star which is what remains of a Sun-like star that puffed away its atmosphere towards the end of its life. The Dumbbell Nebula, M27, lies further to the southeast in Vulpecula, some 3° north of the arrowhead of Sagitta the Arrow. At 1,230 ly, its origin is identical to that of the Ring though it is larger and brighter and readily visible through binoculars.
Diary for 2019 August
Times are BST
1st 04h New moon
7th 19h First quarter
10th 00h Mercury furthest W of Sun (19°)
10th 00h Moon 2.5° N of Jupiter
11th 17h Jupiter stationary (motion reverses from W to E)
12th 11h Moon 0.04° S of Saturn
13th 08h Peak of Perseids meteor shower
14th 07h Venus in superior conjunction
15th 13h Full moon
17th 11h Mercury 0.9° S of Praesepe
23rd 16h Last quarter
24th 11h Moon 2.4° N of Aldebaran
26th 02h Mars farthest from Sun (249m km)
28th 13h Moon 0.6° N of Praesepe
30th 12h New moon
This is a slightly revised version, with added diary, of Alan’s article published in The Scotsman on July 31st 2019, with thanks to the newspaper for permission to republish here.
A partial eclipse of the Moon next to Saturn on the 16th
July brings a slow return to darker and longer nights as the Sun’s trek southwards continues in the wake of our summer solstice. Until the 12th, the Sun dips no more than 12° below Edinburgh’s northern horizon so that twilight persists through every night. By the month’s end, though, we enjoy almost four hours of effective darkness and, with the Moon out of the way, the fainter stars are once again on show.
The 50th anniversary of Apollo 11’s pioneering lunar landing occurs on the 20th while, for stargazers and the weather permitting, the highlight may be the partial eclipse of the Moon which comes four days earlier on the evening of the 16th, though the Moon rises, and the Sun sets, while the eclipse is already underway.
The event begins at 19:44 BST when the Moon begins to encroach of the edge of the outer shadow of the Earth, the penumbra. Between 21:02 and midnight, part of the Moon lies within the central dark umbra where, in the absence of any direct sunlight, it is illuminated in only a dim reddish glow by sunlight refracting around the edge of the Earth. The Moon rises above Edinburgh’s south-eastern horizon at 21:44 and greatest eclipse occurs at 22:31 when the Moon stands only 8° high in the twilight with its northern 65% covered by the umbra. The eclipse ends when disk exits the penumbra at 01:18.
Sunrise/sunset times for Edinburgh change from 04:31/22:01 on the 1st to 05:14/21:23 on the 31st. New moon on the 2nd brings a total eclipse of the Sun visible along a track that crosses the south-eastern Pacific to Chile and Argentina. Surrounding areas, but not Europe, enjoy a partial eclipse. The Moon reaches first quarter on the 9th, is full at the lunar eclipse on the 16th and at last quarter on the 25th.
Saturn, the fainter of the two bright planets low down in our southern night sky, lies 8° to the right of the Moon and a little higher on the evening of the lunar eclipse. One week earlier, on the 9th, it reaches opposition in the constellation Sagittarius when it lies directly opposite the Sun so that it rises at sunset and is highest in the south in the middle of the night, albeit at an elevation of only 12°. It is also closest to us, 1,351 million km, and at its brightest at magnitude 0.1.
A telescope shows Saturn’s disk to be 18 arcseconds wide while its rings extend across 42 arcseconds and have their northern face tipped Earthwards at 24 degrees. Saturn’s globe is blanketed with clouds of ammonia crystals whose shade varies in bands running parallel to its equator. They appear much less prominent than the equivalent bands on Jupiter and there are few noticeable spots.
Jupiter, itself, lies in southern Ophiuchus some 31° to the right of Saturn and far outshines its neighbour even though it dims slightly in July from magnitude -2.6 to -2.4 as its distance grows from 649 million to 691 million km. It lies just below the bright Moon on evening of the 13th when a telescope shows its disk to be 44 arcseconds wide.
The previous evening, the 12th, would be an opportune time to check out Jupiter’s Red Spot which, as I mentioned last time, appears to have been spooling some of its reddish material into its adjacent cloud belt. It is certainly measurably smaller than it was just a few weeks ago but there are also signs that the disturbance may have subsided.
The other naked eye planets are poorly placed. The brightest, magnitude -3.9 Venus, rises 50 minutes before the Sun and stands less than 5° high in the north-east at sunrise. Mercury stands 4° to the left of Mars in the west-north-west during the hour after sunset on the 1st but both are too dim, at magnitudes of 1.2 and 1.8 respectively, to be glimpsed in our bright twilight.
Mars is edging towards the Sun’s far side and reappears before dawn in October. Mercury reaches inferior conjunction between the Sun and the Earth on the 21st when it passes 5° south of the Sun on its way to our morning sky in August. When Mercury next reaches inferior conjunction, on 11 November, it passes almost centrally across the Sun’s disk in a transit that we hope to witness from Scotland.
Our charts show Jupiter and Saturn low down in our southern sky while the star Vega, only slightly brighter than Saturn, crosses the meridian high above them. Vega, of course, forms the Summer Triangle with Deneb in Cygnus, high in the east, and Altair, lower in Aquila. Our map projection squashes the Triangle somewhat – in fact, Deneb and Vega are closer together in the sky than is either star to Altair. An even larger triangle, this one almost equilateral, fills our south-western sky and has its corners at Vega, Jupiter and Arcturus, which lies due west as the leading star of Bootes.
Capella in Auriga stands at its lowest below Polaris in the north and often features in images of noctilucent clouds or NLCs. Little did I realise when I mentioned these silvery-blue “night-shining” clouds last time that 2019 was to unleash some of our best NLC displays in decades. Formed by ice condensing on high-altitude dust particles, some perhaps debris from meteors, they catch the sunlight when our usual lower-level clouds are in darkness during the middle hours of our nights. Researchers are investigating why this should be such a special year for NLCs but expect the bonanza to continue into August.
Diary for 2019 July
Times are BST
2nd 20h New moon and total solar eclipse
4th 23h Earth farthest from Sun (152,100,000 km)
6th 04h Moon 3° N of Regulus
7th 15h Mercury 4° S of Mars
9th 12h First quarter
9th 18h Saturn at opposition at distance of 1,351 million km
13th 21h Moon 2.3° N of Jupiter
16th 08h Moon 0.2° S of Saturn
16th 23h Full moon and partial lunar eclipse
20th 21h 50th anniversary of first manned lunar landing
21st 14h Mercury in inferior conjunction
24th 00h Mercury furthest E of Sun (25°)
25th 02h Last quarter
28th 02h Moon 2.3° N of Aldebaran
This is a slightly revised version, with added diary, of Alan’s article published in The Scotsman on June 29th 2019, with thanks to the newspaper for permission to republish here.
Is Jupiter’s Great Red Spot unfurling before our eyes?
The Sun reaches its most northerly point at 16:54 BST on the 21st, marking the summer solstice in our northern hemisphere. Between its setting in the north-west and its rising in the north-east, it follows only a shallow arc below Edinburgh’s horizon and stands, at most, 10.6° below Edinburgh’s due-north horizon at 01:14 BST. As a result, twilight persists throughout our June nights and we must stay up late to glimpse even the brighter stars and planets.
The sunrise/sunset times for Edinburgh change from 04:36/21:46 BST on the 1st, to 04:26/22:03 on the 21st and 04:30/22:02 on the 30th. The Moon is new on the 3rd, at first quarter on the 10th, full on the 17th and at last quarter on the 25th.
At times like these, some stargazers forsake their hobby for a couple of months while others switch to observing the Sun, or, perhaps, noctilucent clouds. This June, though, the giant planet Jupiter is well worth a look as it comes to opposition on the 10th. It is then closest to us (641 million km) and stands directly opposite the Sun, so that it rises in the south-east at sunset and passes (for Edinburgh) less than 12° high in the south in the middle of the night.
Conspicuous at magnitude -2.6 as it creeps westwards against the stars of southern Ophiuchus, Jupiter outshines every other object in our night sky except for the Moon which lies close to it on the night of the 16th-17th. A small telescope or good steadily-held binoculars reveal its four main moons, the Galilean moons, as they orbit from east to west of the planet in periods that range from 1.8 days for Io to 16.7 days for Callisto. Jupiter has more moons, 79 at the latest count, than any other planet, with Saturn’s tally of 62 coming second. Jupiter’s 75 lesser moons, though, are too small and dim to be spotted using any but the largest telescopes.
Jupiter’s globe is shrouded in clouds, mainly of ammonia crystals but tinted red and brown by other compounds which may include hydrocarbons. Telescopes show bands of darker cloud and a plethora of streaks and spots that transit smartly across the disk as the planet rotates in its sub-ten-hour day.
The most famous feature, the Great Red Spot, is an anticyclonic storm that may be more than 300 years old and was once larger than three Earths. It has shrunk significantly over the last century but observations over the past two weeks suggest something startling may be afoot and even that the spot may be disintegrating. It appears that 10,000-km-long streamers of reddish gas, perhaps methane-rich, are peeling away from the spot into the adjacent cloud band, the South Equatorial Belt, that circles the planet. Likened by some to the spot unfurling, nothing on this scale has been seen before so it is just as well that NASA’s Juno probe has a ring-side view as it orbits Jupiter.
Some 30° to the east of Jupiter, below the so-called Teaspoon of Sagittarius, is our Sun’s other gas giant planet, Saturn. Rising in the south-east about one hour before our map times, it brightens slightly from magnitude 0.3 to 0.1 to rival the two brightest stars on our south map – Vega in Lyra which stands very high in the east-south-east and Arcturus in Bootes in the middle of our south-western sky. When Saturn lies just left of the Moon on the night of the 18th, it lies 1,361 million km away and a telescope shows its disk and rings to span 18 and 41 arcseconds respectively.
The constellations of Ophiuchus and Hercules sprawl across the meridian at the map times, though our twilight means that this is not the best month for spotting M13, the Great Globular Cluster in Hercules (see map). Discovered by Edmond Halley of comet fame in 1714, this ball of hundreds of thousands of stars is some 160 light years across, 22,200 light years away and is thought to have formed 11.65 billion years ago. Under the best conditions, binoculars show it as a fuzzy circular patch around two-thirds as wide as the Moon.
Although Venus is brilliant at magnitude -3.9, it rises in the north-east around 40 minutes before the Sun and is unlikely to be noticed in Scotland’s dawn twilight. Mars is now as dim as magnitude 1.8 and becoming much harder to spot low down in our north-western evening twilight. Tracking eastwards in Gemini to pass below Castor and Pollux, it sets for Edinburgh at 00:05 BST on the 5th when it is 4° to the right of the slender young earthlit Moon. Mercury, much easier at magnitude -0.7, lies 11° below-right of Mars at that time and is 4° above the horizon one hour after sunset between the 5th and 19th. Mercury passes 0.2° above Mars on the 18th and stands furthest east of the Sun (25°) on the 24th.
Scotland’s noctilucent cloud season is just beginning and we can look forward to occasional displays of these “night-shining” clouds until August. Often with a bluish-white sheen, they may appear as wisps, streaks and whirls and merge into banks with cirrus-like herring-bone patterns. The clouds are formed when ice crystallises on dust particles in a narrow range of altitudes near 82 km. Here they are high enough to catch the sun’s light when our more typical lower-level terrestrial clouds are in darkness, from, say, one hour after sunset until one hour before sunrise. They rarely reach more than 20° above the horizon and favour directions towards the north-west at nightfall shifting to the north-east before dawn.
Diary for 2019 June
Times are BST
3rd 11h New moon
4th 17h Moon 4° S of Mercury
5th 16h Moon 1.6° S of Mars
7th 09h Moon 0.5° N of Praesepe
8th 21h Moon 3° N of Regulus
10th 07h First quarter
10th 16h Jupiter at opposition at distance of 641 million km
16th 20h Moon 2.0° N of Jupiter
17th 10h Full moon
18th 16h Mercury 0.2° N of Mars
19th 05h Moon 0.4° S of Saturn
21st 16:54 Summer solstice
24th 00h Mercury furthest E of Sun (25°)
25th 11h Last quarter
30th 17h Moon 2.3° N of Aldebaran
This is a slightly revised version, with added diary, of Alan’s article published in The Scotsman on May 31st 2019, with thanks to the newspaper for permission to republish here.
Giant world Jupiter becoming obvious in May’s twilit nights
With its lengthening days and increasingly twilit nights, May is hardly a vintage month for stargazing from Scotland’s latitudes. Official (nautical) darkness for Edinburgh lasts for more than five hours around midnight as the month begins but dwindles to nothing by the start of June and does not return until 12 July
Edinburgh’s sunrise/sunset times change from 05:30/20:51 on the 1st to 04:37/21:45 on the 31st, while the Moon is new on the 4th, at first quarter on the 12th, full on the 18th and at last quarter on the 26th.
Our charts show Leo diving westwards as the Summer Triangle formed by Vega, Altair and Deneb is climbing in the east. After the Moon, our most obvious nighttime object is the planet Jupiter which rises in the south-east 30 minutes before our map times and reaches less than 12° high in the south before dawn. In fact, look for the Moon above-right of Jupiter on the night of the 19th and closer to the planet’s left on the 20th.
The giant world is now edging westwards against the stars of southern Ophiuchus and brightens from magnitude -2.4 to -2.6 as its distance falls from 678 million to 644 million km. The Jovian globe spans 45 arcseconds in mid-May and telescopes show that it is crossed by bands of cloud that lie parallel to its equator. The four principal moons of Jupiter are also easy targets, though sometimes one or more hide from view as they pass in front of, or behind, the disk or are eclipsed in Jupiter’s shadow.
Saturn trails almost two hours behind Jupiter but is fainter at magnitude 0.5 to 0.3. It lies in Sagittarius, below the Teaspoon asterism, where it stands above the Moon but low down in the south-south-east before dawn on the 23rd. Always an impressive sight through a telescope, though not helped by its low altitude, its disk appears 18 arcseconds wide at mid-month, circled by rings that measure 40 by 16 arcseconds.
Mercury and Venus are too deep in the morning twilight to be seen at present, though Mercury slips around the Sun’s far side on the 21st. The morning twilight also hinders views of the Eta-Aquarids meteor shower which peaks around the 6th-7th and brings swift meteors that stream from a point which hovers low in our east-south-eastern sky for two hours before sunrise.
Mars sets a few minutes before our star map times and may be hard to spot low down in our west-north-western evening sky. It stands between the horns of Taurus on the 1st and shines at magnitude 1.6 to rival the star Elnath, which lies 5° above Mars and marks the tip of the Bull’s northern horn.
Mars’ pinkish-orange hue is best appreciated through binoculars as the planet dims further to magnitude 1.8 and speeds 20° eastwards during May, crossing into Gemini at mid-month and sweeping only 0.2° north of the star cluster M35 (use binoculars) on the 19th. It recedes from 335 million to 363 million km during May but, at a mere 4 arcseconds in diameter, is too small to be of telescopic interest. Catch Mars above the slim earthlit Moon on the 7th.
NASA’s InSight lander used its sensitive French-built seismometer to detect its first likely marsquake on 6 April. The faint vibrations are now being studied for clues as to Mars’ interior. Another instrument, a German heat probe designed to drill up to five metres into the surface, seems to have encountered a rock and is currently stalled well short of its target depth.
The Plough looms directly overhead at nightfall and stands high in the west by our map times. If we extend a curving line along its handle, we reach the prominent star Arcturus which, at magnitude -0.05, is the brightest of all the stars in the sky’s northern hemisphere and, after Sirius, the second brightest (nighttime) star visible from Scotland, although both Vega and Capella come close.
Classed officially as a red giant star, though more yellow-orange in hue, Arcturus is slightly more massive than our Sun and perhaps 50% older. As such, it has depleted the hydrogen used to power its core through nuclear fusion, progressed to fusing helium instead and inflated to 25 times the Sun’s radius and 170 times its luminosity. Eventually, after shedding its outer layers, it will settle down as a dim white dwarf star comparable in size to the Earth.
At present, though, we admire it as the leading star in the constellation of Bootes which has been likened to a pale imitation of Orion or even an ice-cream cone. Bootes takes its name from the Greek for herdsman or plowman, apparently in relation to the seven stars of the Plough which were also known as the “Seven Oxen” in early times.
Arcturus’ own name comes from the Greek for “guardian of the bear”, another reference to its role in following Ursa Major across the sky. In truth, it is something of a temporary guardian since it is rushing past our solar system at 122 km per second at a distance of 36.7 light years and will likely fade from naked-eye view within (only) half a million years as it tracks south-westwards in the direction of Virgo and the bright star Spica.
It is in the north of Virgo, and roughly coincident with the “D” of the label for Denebola on our south star map, that we find the galaxy M87, the owner of the supermassive black hole whose image was released a few weeks ago. M87 is 54 million light years away and visible as a smudge in small telescopes.
Diary for 2019 May
Times are BST
5th 00h New moon
6th 15h Peak of Eta-Aquarids meteor shower
6th 23h Moon 2.3° N of Aldebaran
8th 01h Moon 3° S of Mars
11th 03h Moon 0.3° N of Praesepe
12th 02h First quarter
12th 16h Moon 3° N of Regulus
18th 22h Full moon
19th 18h Mars 0.2° N of star cluster M35 in Gemini
20th 18h Moon 1.7° N of Jupiter
21st 14h Mercury in superior conjunction
22nd 23h Moon 0.5° S of Saturn
26th 18h Last quarter
This is a slightly revised version, with added diary, of Alan’s article published in The Scotsman on April 30th 2019, with thanks to the newspaper for permission to republish here.
Galaxy clusters of interest in April’s southern evening sky
Orion stands in the south-west at nightfall as the sparkling skies of winter give way to the less flamboyant constellations of spring, led by Leo and Virgo. By our map times, Orion has mostly set the west and the Milky Way arcs only some 30° above Edinburgh’s north-western horizon as it flows between Auriga and the “W” formation of Cassiopeia.
The Milky Way, of course, marks the plane of our disk-shaped galaxy, itself dubbed the Milky Way, around which our Sun orbits every 240 million years. If we look along it, we encounter numerous distant stars but countless more are forever hidden from sight behind intervening clouds of gas and dust – the raw material from which new stars and planets may eventually coalesce. If we gaze in directions away from the plane of the Milky Way, though, the star numbers fall away and there is negligible gas and dust to hide our view of galaxies far beyond our own.
It follows that we might expect our best view of the distant universe to be in directions at right angles to the plane, towards the galactic poles. Regions around the North Galactic Pole are ideally placed in our April evening sky and host some of the most interesting clusters of galaxies in the entire sky.
The pole itself lies in the less-than-startling Coma Berenices which is approaching the high meridian at our map times. As the only modern constellation named for a historic person, this celebrates Queen Berenice II of Egypt who is said to have sacrificed her long hair as an offering to Aphrodite. Her tresses are represented by a cascade of stars that spill southwards through the “M” of “COMA” on the chart. These make up a dispersed but nearby star cluster at a distance of about 280 light years – the second closest star cluster after the Hyades in Taurus.
Roughly coincident with the “C” of “COMA” is another cluster, but this one of more than 1,000 galaxies at a distance of some 320 million light years. The Coma Cluster’s brightest galaxies are only around the twelfth magnitude and, as such, a challenge for many amateur telescopes. It was by studying this cluster that the Swiss astronomer Fritz Zwicky uncovered evidence as long ago as 1933 for the existence of what we now call dark matter. He found its galaxies were simply moving too fast to be held together unless addition material was present to supply an extra gravitational pull. Now we suspect that up to 90% of the Coma Cluster consists of this still-mysterious dark matter.
Lying south of Coma Berenices, and about 9° to the east (left) of Leo’s star Denebola, is the closer Virgo Cluster of galaxies. This sprawls across 8° of sky and holds about 1,500 galaxies at a distance of 54 million light years or so. Small telescopes show several, though we’d struggle to locate them without a better chart than I can supply here. In fact, The Virgo Cluster lies at the heart of a much larger family of galaxies and galaxy clusters dubbed the Virgo Supercluster which includes the so-called Local Group of galaxies in which the Milky Way is a major player. The Coma Cluster rules another supercluster.
Edinburgh’s sunrise/sunset times change from 06:44/19:50 BST on the 1st to 05:32/20:49 on the 30th as the Moon stands at new on the 5th, first quarter on the 12th, full on the 19th and last quarter on the 26th. As I mentioned last time, satellites may now be spotted at any time of night though the current spell of evening passes by the International Space Station ends on or about the 5th.
Mars stands some 30° high and alongside the Pleiades in our western sky as our nights begin at present. The planet, though, is tracking east-north-eastwards against the stars and passes north of Taurus’ main star, Aldebaran, to lie between the Bull’s horns later in the month. The young earthlit Moon is an impressive sight 9° below Mars on the 8th and stands above Aldebaran and to the left of Mars on the 9th.
Mars no longer glares like an orange beacon in our sky and is now only half as bright as Aldebaran. As its distance grows from 302 million to 335 million km in April, it dims a little more from magnitude 1.5 to 1.6. Even large telescopes reveal little detail on its small ochre disk, less than 5 arcseconds in diameter, and viewing conditions can only deteriorate as it sinks towards the north-western horizon where it sets in the middle of the night.
There are two brighter planets in our predawn sky, both of them low in the south to south-east as the Summer Triangle formed by Vega, Deneb and Altair climbs through the east.
Jupiter, conspicuous at magnitude -2.2 to -2.5, rises in the south-east less than three hours after our map times and stands 11° above Edinburgh’s southern horizon before dawn. Slow-moving in southern Ophiuchus, it reaches a stationary point on the 10th when its motion appears to reverse from easterly to westerly as it begins to be overtaken by the Earth. Saturn, rather fainter at magnitude 0.6 to 0.5 and at its own stationary point on the 30th, lies in Sagittarius some 25° to Jupiter’s left. Catch the Moon near Jupiter on the 23rd and Saturn on the 25th.
Although Venus is brilliant at magnitude -4.0, it rises in the east less than 38 minutes before sunrise and is unlikely to be noticed. Mercury is furthest west of the Sun (28°) on the 11th but is much fainter and lower still in the morning twilight.
Diary for 2019 April
Times are BST
2nd 05h Moon 2.7° S of Venus
3rd 00h Moon 4° S of Mercury
5th 10h New moon
9th 08h Moon 5° S of Mars
9th 17h Moon 2.1° N of Aldebaran
10th 18h Jupiter stationary (motion reverses from E to W)
11th 21h Mercury furthest W of Sun (28°)
12th 20h First quarter
13th 22h Moon 0.1° N of Praesepe
15th 10h Moon 2.8° N of Regulus
16th 23h Mars 7° N of Aldebaran
19th 12h Full moon
23rd 00h Uranus in conjunction with Sun
23rd 13h Moon 1.6° N of Jupiter
25th 15h Moon 0.4° S of Saturn
26th 23h Last quarter
30th 03h Saturn stationary (motion reverses from E to W)
This is a slightly revised version, with added diary, of Alan’s article published in The Scotsman on March 30th 2019, with thanks to the newspaper for permission to republish here.
Watch earth satellites transit our vernal equinox sky
The Sun climbs northwards at its fastest for the year in March and crosses the sky’s equator at 21:58 on the 20th, the time of our vernal or spring equinox. As the days lengthen rapidly, the stars in the evening sky appear to drift sharply westwards so that Orion, which is astride the meridian as the night begins on the 1st, stands 45° over in the south-west by nightfall on the 31st.
Another consequence of the Sun’s motion is that the Earth’s shadow, on the night side of the planet, is tilting increasingly southwards so that it no longer reaches so far above Scotland at midnight. Indeed, by the end of March the shadow is shallow enough that satellites passing a few hundred kilometres above our heads may be illuminated by the Sun at any time of night. This allows them to appear as moving points of light against the stars as they take a few minutes to cross the sky. Some are steady in brightness while others pulsate or flash as they tumble or spin in orbit.
Dozens of satellites are naked-eye-visible every night, while many times this number may be glimpsed through binoculars. Predictions of when and where to look, including plots of their tracks against the stars, may be obtained online for free, or example from heavens-above.com, or via smartphone apps. Of particular interest are the so-called Iridium satellites which can outshine every other object in the sky, bar the Sun and Moon, during brief flares when their orientation to the Sun and the observer is just right. Although online predictions also include these, Iridium flares are falling rapidly in frequency since the satellites responsible are being deorbited as they are replaced by 2nd generation (and non-flaring) craft.
The most obvious steadily-shining satellite is, of course, the International Space Station which can outshine Sirius as it transits up to 40° high from west to east across Edinburgh’s southern sky. As it orbits the Earth every 93 minutes at a height near 405 km, it is visible before dawn until about the 15th and begins a series of evening passes a week later.
Sunrise/sunset times for Edinburgh change from 07:05/17:46 GMT on the 1st to 05:47/18:48 GMT (06:47/19:48 BST) on the 31st which is the day that we set our clocks to British Summer Time.
The Moon is new on the 6th and spectacular over the following days as its brightly earthlit crescent stands higher each evening in the west-south-west. Catch the Moon 12° below Mars on the 10th and 6° below and left of the planet on the 11th. Mars itself stands around 30° high in the west-south-west at nightfall and is well to the north of west when it sets before midnight. This month it dims from magnitude 1.2 to 1.4 as it speeds more than 20° north-eastwards from Aries into Taurus to end the period only 3° below-left of the Pleiades.
Mercury has been enjoying its best spell of evening visibility this year, but is now fading rapidly and may be lost from view by the 7th. Binoculars show it shining at magnitude 0.1 on the 1st as it stands 10° directly above the sunset position forty minutes after sunset.
The Moon and planets never stray far from the ecliptic, the line around the sky that traces the apparent path of the Sun during our Earth’s orbit. The ecliptic slants steeply across our south-west at nightfall towards the Sun’s most northerly point which it reaches to the north of Orion at our summer solstice in June.
Given a clear dark evening, this is the best time of year to spy a broad cone of light stretching along the ecliptic from the last of the fading twilight. Dubbed the zodiacal light, this glow comes from sunlight scattering from interplanetary dust particles and was the subject on which Brian May, the lead guitarist of Queen, gained his doctorate.
As the Moon continues around the sky, it reaches first quarter on the 14th and passes just north of the star Regulus in Leo on the night of the 18/19th. Regulus, 45° high on Edinburgh’s meridian at our map times, lies less than a Moon’s breadth above the ecliptic and marks the handle of the Sickle of Leo.
Algieba in the Sickle is a splendid binary whose contrasting orange and yellow component stars lie 4.7 arcseconds apart and may be separated telescopically as they orbit each other every 510 years or so. The larger of the pair has at least one companion which may be a planet much larger than Jupiter or, perhaps, a brown dwarf star.
Between full moon on the 21st and last quarter on the 28th, the Moon passes very close to the conspicuous planet Jupiter on the 27th. The giant planet rises in the south-east in the small hours and is unmistakable at magnitude -2.0 to -2.2 low in the south before dawn where it is creeping eastwards against the stars of southern Ophiuchus.
The red supergiant star Antares in Scorpius lies some 13° to the right of Jupiter while Saturn, fainter at magnitude 0.6, is twice this distance to Jupiter’s left and lower in the twilight. Look for Saturn to the Moon’s left on the 1st and just above the Moon on the 29th.
Venus is brilliant (magnitude -4.1) but becoming hard to spot very low down in our morning twilight. More than 10° to the left of Saturn as the month begins and rushing further away, it rises in the south-east 81 minutes before sunrise tomorrow and only 39 minutes before on the 31st.
Diary for 2019 March
1st 18h Moon 0.3° N of Saturn
2nd 21h Moon 1.2° S of Venus
6th 16h New moon
7th 01h Neptune in conjunction with Sun
11th 12h Moon 6° S of Mars
13th 11h Moon 1.9° N of Aldebaran
14th 10h First quarter
15th 02h Mercury in inferior conjunction
17th 13h Moon 0.1° S of Praesepe
19th 00h Moon 2.6° N of Regulus
20th 21:58 Vernal equinox
21st 02h Full moon
27th 02h Moon 1.9° N of Jupiter
28th 04h Last quarter
29th 05h Moon 0.1° S of Saturn
30th 10h Mars 3° S of Pleiades
31st 01h GMT = 02h BST Start of British Summer Time
This is a slightly revised version, with added diary, of Alan’s article published in The Scotsman on February 28th 2019, with thanks to the newspaper for permission to republish here.
Orion and Winter Hexagon in prime-time view
Even though the two brightest planets, Venus and Jupiter, hover low in the south-east before dawn, the shortest month brings what many consider to be our best evening sky of the year. After all, the unrivalled constellation of Orion is in prime position in the south, passing due south for Edinburgh one hour before our star map times. Surrounding it, and ideally placed at a convenient time for casual starwatchers, are some of the brightest stars and interesting groups in the whole sky.
I mentioned some of the sights in and around Orion last time, including the bright stars Procyon, Betelgeuse and Sirius which are prominent in the south at the map times and together form the Winter Triangle.
Like the Summer Triangle, this winter counterpart is defined as an asterism which is a pattern of stars that do not form one of the 88 constellations recognised by the International Astronomical Union. Both triangles are made up of stars in different constellations, but we also have asterisms that lie entirely within a single constellation, as, for example, the Sickle of Leo which curls above Regulus in the east-south-east at our map times, and the Plough which comprises the brighter stars of the Ursa Major, the Great Bear, climbing in the north-east.
Yet another asterism, perhaps the biggest in its class, includes the leading stars of six constellations and re-uses two members of the Winter Triangle. The Winter Hexagon takes in Sirius, Procyon, Pollux in Gemini and Capella in Auriga which lies almost overhead as Orion crosses the meridian. From Capella, the Hexagon continues downwards via Aldebaran in Taurus and Rigel at Orion’s knee back to Sirius.
Edinburgh’s sunrise/sunset times change from 08:08/16:45 on the 1st to 07:07/17:44 on the 28th. The Moon is new on the 4th and at first quarter on the 12th when it stands 12° below the Pleiades in our evening sky. The 13th sees it gliding into the Hyades, the V-shaped star cluster that lies beyond Aldebaran. Both the Pleiades and the Hyades are open clusters whose stars all formed at the same time. Another fainter cluster, Praesepe or the Beehive in Cancer, is visible through binoculars to the left of the Moon late on the 17th. Full moon is on the 19th with last quarter on the 26th.
A number of other open star clusters lie in the northern part of the Hexagon, two of them plotted on our chart. At the feet of Gemini and almost due north of Betelgeuse is M35, visible as a smudge to the unaided eye but easy though binoculars and telescopes which begin to reveal its brighter stars. It lies 3,870 ly (light years) away, as compared with 440 ly for the Pleiades and 153 ly for the Hyades. Further north in Auriga is the fainter M37 (4,500 ly) which binoculars show 7° north-east of Elnath, the star at the tip of the upper horn of Taurus. M36 (4,340 ly) and M38 (3,480 ly) lie from 4° and 6° north-west of M37.
Mars dims a little from magnitude 0.9 to 1.2 but remains the brightest object near the middle of our south-south-western evening sky, sinking westwards to set before midnight. Mars is 241 million km distant when it stands above the Moon on the 10th, with its reddish 5.8 arcseconds disk now too small to show detail through a telescope. As it tracks east-north-eastwards against the stars, it moves from Pisces to Aries and passes 1° above-right of the binocular-brightness planet Uranus (magnitude 5.8) on the 13th.
The usually elusive planet Mercury begins its best evening apparition of 2019 in the middle of the month as it begins to emerge from our west-south-western twilight. Best glimpsed through binoculars, it stands between 8° and 10° high forty minutes after sunset from the 21st and sets itself more than one hour later still. It is magnitude -0.3 on the 27th when it lies furthest from the Sun in the sky, 18°, and its small 7 arcseconds disk appears 45% illuminated.
Venus, brilliant at magnitude 4.3, rises for Edinburgh at 05:11 on the 1st and stands 8° high by 06:30 as twilight begins to invade the sky. That morning also finds it 6° above and right of the waning earthlit Moon. A telescope shows Venus to be 19 arcseconds in diameter and 62% sunlit.
Jupiter is conspicuous 9° to the right of, and slightly above, Venus on the 1st though it is one ninth as bright at magnitude -1.9. Larger and more interesting through a telescope, its 34 arcseconds disk is crossed by bands of cloud running parallel to its equator while its four main moons may be glimpsed through binoculars. Edging eastwards (to the left) in southern Ophiuchus, it is 9° east of the celebrated and distinctly red supergiant star Antares in Scorpius, a star so big that it would engulf the Earth and Mars if it switched places with our Sun.
Our third predawn planet, Saturn rises at 06:38 on the 1st and is more of a challenge being fainter (magnitude 0.6) in the twilight. One hour before Edinburgh’s sunrise on the 2nd, it lies only 2° above the horizon and less than 10 arcminutes above-right of the Moon’s edge. Watchers in south-eastern England see it slightly higher and may glimpse it emerge from behind the Moon at about 06:31.
Venus speeds eastwards through Sagittarius to pass 1.1° north of Saturn on the 18th and shine at magnitude -4.1 even lower in the morning twilight by the month’s end. By then, the Moon has come full circle to stand above-right of Jupiter on the 27th and to Jupiter’s left on the 28th.
Diary for 2019 February
2nd 07h Moon 0.6° N of Saturn
4th 21h New moon
10th 16h Moon 6° S of Mars
12th 22h First quarter
13th 20h Mars 1.1° N of Uranus
14th 04h Moon 1.7° N of Aldebaran
18th 03h Moon 0.3° S of Praesepe
18th 14h Venus 1.1° N of Saturn
19th 13h Moon 2.5° N of Regulus
19th 16h Full moon
26th 11h Last quarter
27th 01h Mercury furthest E of Sun (18°)
27th 14h Moon 2.3° N of Jupiter
This is a slightly revised version, with added diary, of Alan’s article published in The Scotsman on January 31st 2019, with thanks to the newspaper for permission to republish here.
Rise early for a total lunar eclipse on the 21st
Any month that has the glorious constellation of Orion in our southern evening sky is a good one for night sky aficionados. Add one of the best meteor showers of the year, a total eclipse of the Moon, a meeting between the two brightest planets and a brace of space exploration firsts and we should have a month to remember
Orion rises in the east as darkness falls and climbs well into view in the south-east by our star map times. Its two leading stars are the blue-white supergiant Rigel at Orion’s knee and the contrasting red supergiant Betelgeuse at his opposite shoulder – both are much more massive and larger than our Sun and around 100,000 times more luminous.
Below the middle of the three stars of Orion’s Belt hangs his Sword where the famous and fuzzy Orion Nebula may be spied by the naked eye on a good night and is usually easy to see through binoculars. One of the most-studied objects in the entire sky, it lies 1,350 light years away and consists of a glowing region of gas and dust in which new stars and planets are coalescing under gravity.
The Belt slant up towards Taurus with the bright orange giant Aldebaran and the Pleiades cluster as the latter stands 58° high on Edinburgh’s meridian. Carry the line of the Belt downwards to Orion’s main dog, Canis Major, with Sirius, the brightest star in the night sky. His other dog, Canis Minor, lies to the east of Orion and is led by Procyon which forms an almost-equilateral triangle with Sirius and Betelgeuse – our so-called Winter Triangle.
The Moon stands about 15° above Procyon when it is eclipsed during the morning hours of the 21st. The event begins at 02:36 when the Moon lies high in our south-western sky, to the left of Castor and Pollux in Gemini, and its left edge starts to enter the lighter outer shadow of the Earth, the penumbra.
Little darkening may be noticeable until a few minutes before it encounters the darker umbra at 03:34. Between 04:41 and 05:46 the Moon is in total eclipse within the northern half of the umbra and may glow with a reddish hue as it is lit by sunlight refracting through the Earth’s atmosphere. The Moon finally leaves the umbra at 06:51 and the penumbra at 07:48, by which time the Moon is only 5° high above our west-north-western horizon in the morning twilight.
This eclipse occurs with the Moon near its perigee or closest point to the Earth so it appears slightly larger in the sky than usual and may be dubbed a supermoon. Because the Moon becomes reddish during totality, there is a recent fad for calling it a Blood Moon, a term which has even less of an astronomical pedigree than supermoon. Combine the two to get the frankly ridiculous description of this as a Super Blood Moon.
Sunrise/sunset times for Edinburgh change from 08:44/15:49 on the 1st to 08:10/16:43 on the 31st. New moon early on the 6th, UK time, brings a partial solar eclipse for areas around the northern Pacific. First quarter on the 14th is followed by full moon and the lunar eclipse on the 21st and last quarter on the 27th.
The Quadrantids meteor shower is active until the 12th but is expected to peak sharply at about 03:00 on the 4th. Its meteors, the brighter ones leaving trains in their wake, diverge from a radiant point that lies low in the north during the evening but follows the Plough high into our eastern sky before dawn. With no moonlight to hinder observations this year, as many as 80 or more meteors per hour might be counted under ideal conditions.
Mars continues as our only bright evening planet though it fades from magnitude 0.5 to 0.9 as it recedes. Tracking through Pisces and well up in the south at nightfall, it stands above the Moon on the 12th. Our maps show it sinking in the south-west and it sets in the west before midnight.
Venus, its brilliance dimming only slightly from magnitude -4.5 to -4.3, stands furthest west of the Sun (47°) on the 6th and is low down (and getting lower) in our south-eastern predawn sky. Look for it below and left of the waning Moon on the 1st with the second-brightest planet, Jupiter at magnitude -1.8, 18° below and to Venus’s left. As Venus tracks east-south-eastwards against the stars, it sweeps 2.4° north of Jupiter in an impressive conjunction on the morning of the 22nd while the 31st finds it 8° left of Jupiter with the earthlit Moon directly between them.
Saturn, magnitude 0.6, might be glimpsed at the month’s end when it rises in the south-east 70 minutes before sunrise but Mercury is lost from sight is it heads towards superior conjunction on the Sun’s far side on the 30th.
China hopes that its Chang’e 4 spacecraft will be the first to touch down on the Moon’s far side, possibly on the 3rd. Launched on December 7 and named for the Chinese goddess of the Moon, it needs a relay satellite positioned beyond the Moon to communicate with Earth.
Meantime, NASA’s New Horizons mission is due to fly within 3,500 km of a small object a record 6.5 billion km away when our New Year is barely six hours old. Little is known about its target, dubbed Ultima Thule, other than that it is around 30 km wide and takes almost 300 years to orbit the Sun in the Kuiper Belt of icy worlds in the distant reaches of our Solar System.
Diary for 2019 January
1st 06h New Horizons flyby of Ultima Thule
1st 22h Moon 1.3° N of Venus
2nd 06h Saturn in conjunction with Sun
3rd 05h Earth closest to Sun (147,100,000 km)
3rd 08h Moon 3° N of Jupiter
4th 03h Peak of Quadrantids meteor shower
6th 01h New moon and partial solar eclipse
6th 05h Venus furthest W of Sun (47°)
12th 20h Moon 5° S of Mars
14th 07h First quarter
17th 19h Moon 1.6° N of Aldebaran
21st 05h Full moon and total lunar eclipse
21st 16h Moon 0.3° S of Praesepe
22nd 06h Venus 2.4° N of Jupiter
23rd 02h Moon 2.5° N of Regulus
27th 21h Last quarter
30th 03h Mercury in superior conjunction
31st 00h Moon 2.8° N of Jupiter
31st 18h Moon 0.1° N of Venus
This is a slightly revised version, with added diary, of Alan’s article published in The Scotsman on December 31st 2018, with thanks to the newspaper for permission to republish here.
Comet sweeps near Earth as meteors streak from Gemini
December brings our longest and perhaps most interesting nights of the year. The two stand-out planets are Mars in the evening and Venus before dawn, the latter now as brilliant as it ever gets and the source of a flurry of recent UFO reports. We may also enjoy the rich and reliable Geminids meteor shower and Comet Wirtanen looks set to be the brightest comet of the year.
The comet’s progress is plotted on our charts, beginning low in the south near the Cetus-Eridanus border on the 1st and sweeping northwards and eastwards through Taurus to Auriga and beyond. A small comet with an icy nucleus possibly less than 1 km wide, Wirtanen was discovered in 1948 and orbits the Sun every 5.4 years between the Earth and Jupiter. It was the original destination of the European Space Agency’s Rosetta mission before delays forced the probe to target Comet Churyumov–Gerasimenko instead.
Comet Wirtanen reaches perihelion, its closest to the Sun and just beyond the Earth’s orbit, on the 12th. It is nearest the Earth on the 16th, passing only 11.6 million km away in the tenth closest approach of any observed comet since 1950. On that evening it lies 4° east (left) of the Pleiades and may appear as a large fuzzy ball lacking any obvious tail.
Predictions of its appearance at that time vary, but I suspect that its total brightness may be around the fourth magnitude, a little brighter than the fainter stars plotted on our charts. While this would normally put it well within naked-eye range, the fact that it is so close to the Earth is likely to mean that its light is spread out over an area even wider than the Pleiades. Unless we have a good dark sky, we may struggle to see its extended glow, and it is a pity that the gibbous Moon (63% sunlit) will also hinder observations before midnight. Only a week later, on the evening of the 23rd, it lies only 1° east-south-east of the bright star Capella but will be fading in still brighter moonlight.
The Sun reaches its most southerly point at the winter solstice at 22:23 GMT on the 21st as sunrise/sunset times for Edinburgh change from 08:19/15:44 GMT on the 1st to 08:42/15:40 on the 21st and 08:44/15:48 on the 31st. The Moon is new on the 7th, at first quarter on the 15th, full on the 22nd and at last quarter on the 29th.
Our charts show Andromeda and its Galaxy high in the south as Orion stands proudly in the south-east below Taurus and the Pleiades. Castor lies above Pollux in Gemini in the east and is close to the point in the sky that marks the radiant of the Geminids meteor shower.
The Geminids always produce an abundance of slow bright meteors which streak in all parts of the sky as they diverge from the radiant. The latter climbs to pass high in the south at around 02:00 and sinks into the west before dawn. The shower is active from the 8th to the 17th with the night of 13th-14th expected to be the best as meteor rates build to a peak at around dawn. An observer under an ideal dark sky with the radiant overhead may count upwards of 100 meteors per hour making the Geminids the highest-rated of our annual showers, though most of us under inferior skies may glimpse only a fraction of these.
Mars shines brightly some 25° high in the south as night falls for Edinburgh at present and is almost 10° higher by the month’s end after moving east-north-eastwards from Aquarius into Pisces. Our maps have it sinking in the south-west on its way to setting in the west before midnight. Although the brightest object in its part of the sky, it dims from magnitude 0.0 to 0.5 as it recedes from 151 million to 189 million km. When Mars stands above the Moon on the 14th, a telescope shows its ochre disk to be only 8 arcseconds across.
Saturn, magnitude 0.6, hangs just above our south-western horizon at nightfall as December begins but is soon lost in the twilight. Our other two evening planets, Uranus and Neptune, are visible through binoculars at magnitudes of 5.7 and 7.9 in Pisces and Aquarius respectively. Mars acts as an excellent guide on the evening of the 7th when Neptune stands about one quarter of a Moon’s breadth below-right of Mars.
Venus, now at its best as a dazzling morning star, rises in the east-south-east four hours before the Sun and climbs towards the south by dawn. This month it dims slightly from magnitude -4.7 to -4.5 as it tracks away from Virgo’s brightest star Spica in Virgo into the next constellation of Libra. Telescopes shows its crescent shrink from 40 to 26 arcseconds in diameter. Look for Venus below-left of the Moon on the morning of the 3rd and to the Moon’s right on the 4th.
Mercury is set to become as a morning star very low in the south-east and is soon to be joined by the even brighter Jupiter. Mercury rises more than 100 minutes before the Sun from the 5th to the 24th and stands between 5° and 9° high forty minutes before sunrise. It shines at magnitude 0.8 when it lies 7° below-left of the impressively earthlit Moon on the 5th, and triples in brightness to magnitude -0.4 by the 24th.
Jupiter, conspicuous at magnitude -1.8, emerges from the twilight and moves from 9° below-left of Mercury on the 11th to pass 0.9° south of Mercury on the 21st.
Diary for 2018 December
3rd 19h Moon 4° N of Venus
5th 21h Moon 1.9° N of Mercury
7th 07h New moon
7th 15h Mars 0.04° N of Neptune
9th 06h Moon 1.1° N of Saturn
12th 23h Comet Wirtanen closest to Sun (158m km)
14th 08h Peak of Geminids meteor shower
14th 23h Moon 4° S of Mars
15th 11h Mercury furthest W of Sun (21°)
15th 12h First quarter
16th 13h Comet Wirtanen closest to Earth (11.6m km) and 3.6° SE of Pleiades
20th 02h Jupiter 5° N of Antares
21st 08h Moon 1.7° N of Aldebaran
21st 15h Mercury 0.9° N of Jupiter
21st 22:23 Winter solstice
22nd 18h Full moon
23rd 18h Comet Wirtanen 0.9° SE of Capella
25th 05h Moon 0.3° S of Praesepe in Cancer
29th 10h Last quarter