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
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.
Amateur astronomer discovers first interstellar comet
It is two years since astronomers in Hawaii discovered the first object known to have approached the Sun from beyond our solar system. Given the Hawaiian name of ʻOumuamua, this appeared to be a reddish and elongated slab-shaped body of about the size of a skyscraper that passed 38 million km from the Sun before sweeping within 24 million km of the Earth. It came from roughly the current direction of the star Vega and headed away towards the Square of Pegasus, though it may take 20,000 years to leave the solar system completely.
Its small size meant that it was followed only faintly and for barely a month. Astronomers were surprised to notice no sign of cometary activity – no surrounding fuzzy coma and no tail – while suggestions that it was an alien probe prompted unsuccessful scans for any artificial radio emissions.
Now the second-known interstellar intruder has been sighted, and this one appears larger, brighter and is surely a comet. It was discovered photographically on 29 August from an observatory in Crimea by the amateur astronomer Gennady Borisov using a telescope he built himself. Initially called C/2019 Q4 (Borisov), or Comet Borisov for short, it was clearly speeding along a strongly hyperbolic path past the Sun, very unlike the elliptical or nearly parabolic orbits followed by all previous comets. Now it has been awarded the official interstellar designation of 2I/Borisov.
The comet was travelling at about 33 km per second as it entered the solar system from the direction of the constellation Cassiopeia, fast enough to cover the 4-light-years distance of the nearest star in under 40,000 years. Perihelion, its closest point to the Sun, occurs at 303 million km on 8 December, putting it still beyond the orbit of Mars, and it reaches its closest to the Earth at 293 million km twenty days later.
It is still faint, no better than magnitude 17, but may attain magnitude 14 near perihelion and, while it will never reach naked-eye or binocular visibility, is likely to be within telescopic range until at least the middle of next year. This gives plenty of time for astronomers to study a comet that probably formed elsewhere in the Milky Way galaxy at a different time and with possibly a different composition than those that formed alongside the Sun and Earth. October has Comet Borisov travelling south-eastwards to the west of the Sickle of Leo and passing within a Moon’s-breadth east of the star Regulus on the 24th.
Leo’s Sickle rises in the north-east in the early morning and stands some 30° high in the east before dawn as our southern sky is dominated by the glorious constellation of Orion. The pre-dawn also gives us a chance to spot Mars as it emerges from the Sun’s far side. The planet rises in the east one hour before the Sun on the 1st and two hours before sunrise on the 31st. Moving east-south-eastwards in Virgo, it shines only at magnitude 1.8 and lies 8° below the slender earthlit Moon on the 26th.
As the Sun tracks southwards by 11° during October, the sky at nightfall is changing only slowly. The Summer Triangle is still high in the south as darkness falls, although its three stars, Vega in Lyra, Deneb in Cygnus and Altair in Aquila, have shifted into the west by our star map times. By then, Pegasus, the upside-down flying horse with his nose near Delphinus the Dolphin, stands high in the south.
The sunrise/sunset times for Edinburgh change this month from 07:15/18:49 BST (06:15/17:49 GMT) on the 1st to 07:17/16:35 GMT on the 31st, following Summer Time’s end on the 27th. The Moon reaches first quarter on the 5th, full phase on the 13th, last quarter on the 21st and new on the 28th.
Like Mars, Venus is also coming into view from beyond the Sun, but this time into our evening twilight in the west-south-west. Although brilliant at magnitude -3.9, it stands a mere 3° high at sunset for Edinburgh and sets at present only 30 minutes later, so we need good weather and a clear horizon to catch it. On the 29th, look for it 2.8° below the sliver of the earthlit young Moon, only 3° illuminated. Mercury is fainter and even lower at sunset and not visible from Scotland.
Jupiter is well past its best as an evening object although it remains obvious low in the south-west at nightfall, sinking to set at Edinburgh’s south-western horizon at 21:14 BST on the 1st and as early as 18:34 GMT by the 31st. At magnitude -2.0 to -1.9 and 36 to 33 arcseconds in diameter, it lies close to the Moon on the 3rd and 31st.
Saturn, one tenth as bright at magnitude 0.5 to 0.6, lies some 25° to the left of Jupiter. When it is close to the first quarter Moon on the 5th, its disk and rings span 17 and 38 arcseconds respectively. It is in Sagittarius low in the south at nightfall and sets in the south-west soon after our map times.
Neptune and Uranus are binocular brightness object of magnitudes 7.8 and 5.7 in Aquarius and Pisces respectively. There is little hope of locating them using our chart, but a web search, such as “Where is Uranus?”, should bring up information and a finder chart. Uranus, in fact, reaches opposition at a distance of 2,817 million km on the 28th when it stands directly opposite the Sun and appears as a tiny 3.7 arcseconds blue-green disk through a telescope.
Our Diary, below, records the peak dates for two of the October’s meteor showers, the Draconids on the 8th and the Orionids on the 22nd. Neither is among the year’s top showers, though both can yield rates of 20 or more meteors per hour under ideal conditions. The Draconids are active from the 6th to the 10th with slow meteors that diverge from a radiant near the Head of Draco, the quadrilateral of stars below and left of the D of DRACO on our north map. Unfortunately, the light of the bright gibbous Moon will hinder observations before the Moon sets in the early morning.
The Orionids, like May’s Eta Aquarids shower, are caused by meteoroid debris from Comet Halley. They last throughout the month and into early November but are expected to be most prolific on the nights of the 22nd and 23rd when their fast meteors diverge from a point that lies around 10° north-east (above-right) of Betelgeuse at Orion’s shoulder. That point passes high in the south before dawn but is just rising in the east-north-east at our map times, so no Orionids appear before then. As with those other swift meteors, the Perseids of August, many of the brighter Orionids leave glowing trains in their wake.
Diary for 2019 October
Times are BST until the 27th and GMT thereafter.
3rd 21h Moon 1.9° N of Jupiter
5th 18h First quarter
5th 22h Moon 0.3° S of Saturn
8th 07h Peak of Draconids meteor shower
13th 22h Full moon
17th 23h Moon 2.9° N of Aldebaran
20th 05h Mercury furthest E of Sun (25°)
21st 14h Last quarter
22nd Peak of Orionids meteor shower
22nd 06h Moon 1.0° N of Praesepe
23rd 19h Moon 3° N of Regulus
26th 18h Moon 5° N of Mars
27th 02h BST = 01h GMT End of British Summer Time
28th 04h New moon
28th 08h Uranus at opposition at distance of 2,817 million km
29th 14h Moon 4° N of Venus
30th 08h Mercury 2.7° S of Venus
31st 14h Moon 1.3° N of Jupiter
This is an extended version, with added diary, of Alan’s article published in The Scotsman on September 30th 2019, with thanks to the newspaper for permission to republish here.
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.
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.
InSight probe to land on bright evening planet Mars
The Summer Triangle, still high in the south at nightfall, shifts to the west by our map times as our glorious winter constellations climb in the east. Taurus with the Pleiades and its leading star Aldebaran (close to the Moon on the 23rd) stands well clear of the horizon while Orion is rising below and dominates our southern sky after midnight.
In the month that should see NASA’s InSight lander touch down on its surface, the planet Mars continues as a prominent object in the south at nightfall. Venus springs into view as a spectacular morning star but we must wait to see whether the Leonids meteor shower, which has produced some storm-force displays in the past, gives us any more than the expected few meteors this year.
InSight is due to land on the 26th on a broad plain called Elysium Planitia that straddles Mars’ equator. There it will place an ultra-sensitive seismometer directly onto the surface and cover it with a dome-like shell to shield it from the noise caused by wind and heat changes. This should be able of detect marsquakes and meteor impacts that occur all around Mars. Other InSight experiments will hammer a spike up to five metres into the ground to measure Mars’ heat flow, and further investigate the planet’s interior structure by using radio signals to track tiny wobbles in its rotation.
Until recently, Mars has remained low down as it performed a loop against the stars in the south-western corner of Capricornus. That loop, resulting entirely from our changing vantage point as the Earth overtook Mars and came within 58 million km on 31 July, took Mars more than 26° south of the sky’s equator and 3° further south than the Sun stands at our winter solstice.
Now, though, Mars is climbing east-north-eastwards on a track that will take it further north than the Sun ever gets by the time it disappears into Scotland’s night-long twilight next summer. One by-product of this motion is that Mars’ setting time is remarkably constant over the coming months, being (for Edinburgh) within 13 minutes of 23:42 GMT from now until next May.
This month sees Mars leave Capricornus for Aquarius and shrink as seen through a telescope from 12 to 9 arcseconds as it recedes from 118 million to 151 million km. Its path, indicated on our southern chart, carries it 0.5° (one Moon’s breadth) north of the multiple star Deneb Algedi, the goat’s tail, on the 5th. It almost halves in brightness, from magnitude -0.6 to 0.0, but its peak altitude above Edinburgh’s southern horizon early in the night improves from 16° to 25°, though by our map times it is sinking lower towards the south-west.
Mars is not our sole evening planet since Saturn shines at magnitude 0.6 low down in the south-west at nightfall. It is only a degree below-right of the young Moon on the 11th and sets more than 90 minutes before our map times. The two most distant planets, Neptune and Uranus, are also evening objects and may be glimpsed through binoculars at magnitudes 7.9 and 5.7 in Aquarius and Aries respectively.
Edinburgh’s sunrise/sunset times vary from 07:19/16:32 on the 1st to 08:17/15:45 on the 30th. The Moon is new on the 7th, at first quarter and below-right of Mars on the 15th, full on the 23rd and at last quarter on the 30th.
Jupiter is hidden in the solar glare as it approaches conjunction on the Sun’s far side on the 26th. Mercury stands furthest east of the Sun (23°) on the 6th but is also invisible from our northern latitudes.
Venus, though, emerges rapidly from the Sun’s near side into our morning twilight where it stands to the left of the star Spica in Virgo. Shining brilliantly at magnitude -4.1, the planet rises in the east-south-east only 29 minutes before the Sun on the 1st. By the 6th, though, it rises 80 minutes before sunrise and stands 8° below and right of the impressively earthlit waning Moon. Venus itself is 58 arcseconds wide and 4% illuminated on that morning, its slender crescent being visible through binoculars. By the 30th, Venus rises four hours before the Sun, climbs to stand 23° high in the south-south-east at sunrise and appears as a 41 arcseconds and 25% sunlit crescent.
It is just as well that my previous note led on the usually neglected Draconids meteor shower because observers, at least those under clear skies, were thrilled to see it provide perhaps the best meteor show of 2018. For just a few hours around midnight on 8-9th October, the sky became alive with slow meteors at rates of up to 100 meteors per hour or more.
Leonid meteors arrive this month between the 15th and 20th, with the shower expected to hit its usually-brief peak at around 01:00 on the 18th. Although they flash in all parts of the sky, they diverge from a radiant point in the so-called Sickle of Leo which rises in the north-east before midnight and climbs high into the south before dawn. No Leonids appear before the radiant rises, but even with the radiant high in a dark sky we may see fewer than 20 per hour – all of them very swift and many of the brighter ones leaving glowing trains in their wake.
Leonid meteoroids come from Comet Tempel-Tuttle which orbits the Sun every 33 years and was last in our vicinity in 1998. There has not been a Leonids meteor storm since 2002 and we may be a decade or more away from the next one, or are we?
Diary for 2018 November
2nd 05h Moon 2.1° N of Regulus
6th 16h Mercury furthest E of Sun (23°)
7th 16h New moon
11th 16h Moon 1.5° N of Saturn
15th 15h First quarter
16th 04h Moon 1.0° S of Mars
18th 01h Peak of Leonids meteor shower
23rd 06h Full moon
23rd 22h Moon 1.7° N of Aldebaran
26th 07h Jupiter in conjunction with Sun
26th 20h InSight probe to land on Mars
27th 09h Mercury in inferior conjunction on Sun’s near side
27th 21h Moon 0.4° S of Praesepe
30th 00h Last quarter
This is a slightly revised version, with added diary, of Alan’s article published in The Scotsman on October 31st 2018, with thanks to the newspaper for permission to republish here.
Draconid meteors glide away from the Dragon’s head
Mars dominates our southern evening sky but most of the other bright planets are poorly placed this month. Even so, our October nights are full of interest, from the Summer Triangle in the evening to the star-fest around Orion before dawn.
Although Mars dims from magnitude -1.3 to -0.6, its reddish light remains prominent as it moves from low in the south-south-east at nightfall to the south-south-west at our map times and onwards to set in the south-west a little before 01:00 BST (midnight GMT). As its distance grows from 89 million to 118 million km, and its diameter shrinks from 16 to 12 arcseconds, the planet speeds through Capricornus to climb 6° northwards and that much higher in our sky. Catch it to the left of the Moon on the 17th and below-right of the Moon on the 18th.
The Sun tracks 11° southwards as Edinburgh’s sunrise/sunset times change from 07:15/18:48 BST (06:15/17:48 GMT) on the 1st to 07:17/16:35 GMT on the 31st. The Moon is at last quarter on the 2nd, new on the 9th, at first quarter on the 16th, full (the Hunter’s Moon) on the 24th and back at last quarter on the 31st.
Our charts show the Plough in the north as it moves below Polaris, the Pole Star. Mizar, in the Plough’s handle, forms a famous double star with the fainter Alcor – the pair being separated by about one third the diameter of the Moon. Once held as a (not very rigorous) test of eyesight, they were dubbed “The Horse and Rider”.
Both lie 83 light years (ly) from us although we can’t be certain that they are tied together by gravity. In any case, we are not talking about just two stars, for Alcor has a faint companion and most telescopes show Mizar to be a binary star – the first to be discovered telescopically in the 17th century. Spectroscopes reveal that each of Mizar’s components is itself binary, so Mizar and Alcor, if they are truly associated, together form a sextuplet star system.
Mizar is the same brightness, magnitude 2.2, as Eltanin which lies 14° to the right of Vega and very high in the west at nightfall, falling into the north-west overnight. It is the brightest star in Draco and a member of a quadrilateral that marks the head of the Dragon whose body and tail twist to end between the Plough and Polaris. It lies 154 ly away but is approaching the Sun and will pass within 28 ly in another 1.5 million years to become the brightest star in Earth’s night sky.
Meteors from the Draconids shower diverge from a radiant point that lies close to Draco’s head (see our north map) between the 7th and 10th. Don’t expect a major display – perhaps no more than 10 meteors per hour, though all of them are very slow as they glide away from the radiant. The shower’s peak is due in a moonless sky around midnight on the 8th-9th and is worth checking because some years surprise us with strong displays and the shower’s parent comet, Comet Giacobini-Zinner, was visible through binoculars when it swept within 59 million km last month.
A better-known comet, Halley, is responsible for the meteors of the Orionids shower which lasts from the 16th to the 30th and has a broad but not very intense peak of fast meteors between the 21st and 24th. The radiant point, between Orion and Gemini, rises in the east-north-east soon after our map times and passes high in the south before dawn. Sadly, the peak coincides with the full moon, so don’t expect much of a show.
From high in the south at nightfall, the Summer Triangle (Vega, Deneb and Altair) tumbles into our western sky by the map times. By then, the less impressive and rather empty Square of Pegasus is in the south and Taurus and the Pleiades star cluster are climbing in the east. Orion rises below Taurus over the next two hours and crosses the meridian as the night ends.
Neptune and Uranus, now well placed in the evening, may be located through binoculars using better charts than I can provide here. A web search, for example for “Neptune finder chart”, should help. Neptune shines at magnitude 7.8 and lies in Aquarius at a distance of 4,342 million km on the 1st. Uranus is 2,824 million km away in Aries, near its border with Pisces, when it stands opposite the Sun in the sky (opposition) on the 24th. Although the full Moon stands close to it on that day, its magnitude of 5.7 makes it just visible to the unaided eye under a good dark and moonless sky.
October should see the launch of the European Space Agency’s BepiColombo mission to Mercury, but the planet itself is too low in our evening twilight to be seen. Venus sweeps around the Sun’s near side at inferior conjunction on the 26th and remains hidden in the Sun’s glare.
Jupiter is bright (magnitude -1.8) but less than 8° high in the south-west at sunset as the month begins. One of our last chances of spotting it in our bright evening twilight comes on the 11th when it lies 4° below-left of the young earthlit Moon.
Saturn, magnitude 0.5 and edging eastwards in Sagittarius, stands less than 10° high above Edinburgh’s south-south-western horizon as the sky darkens and sets in the south-west some 45 minutes before our map times. Look for it to the left of the Moon on the 14th.
Diary for 2018 October
Times are BST until the 28th
2nd 11h Last Quarter
9th 00h Peak of Draconids meteor shower
9th 05h New moon
11th 22h Moon 4° N of Jupiter
15th 04h Moon 1.8° N of Saturn
16th 19h First quarter
18th 14h Moon 1.9° N of Mars
21st – 24th Peak of Orionids meteor shower
24th 02h Uranus at opposition at distance of 2,824m km
24th 18h Full moon
26th 15h Venus in inferior conjunction on Sun’s near side
28th 02h BST = 01h GMT End of British Summer Time
31st 17h GMT Last quarter
This is a slightly revised version, with added diary, of Alan’s article published in The Scotsman on September 29th 2018, with thanks to the newspaper for permission to republish here.
Summer Triangle stars as autumn evenings begin
We may be edging towards autumn, but the Summer Triangle, the asterism formed by the bright stars Vega, Altair and Deneb, looms high in the south as night falls and shifts into the high south-west by our star map times later in the evening. Vega, almost overhead as the night begins, is the brightest of the three and lies in the small box-shaped constellation of Lyra the Lyre.
The next brightest, Altair in Aquila the Eagle, stands lower in the middle of our southern sky and, at 16.7 light years (ly), is one of the nearest bright stars to the Sun – eight light years closer than Vega. Flanking Altair, like the two sides of a balance, are the fainter stars Alshain (below Altair) and Tarazed (above) whose names come from “shahin-i tarazu”, the Arabic phrase for a balance.
Deneb, 25° from Vega, lies very high in the south-east at nightfall and overhead at our map times. It marks the tail of Cygnus the Swan which is flying overhead with wings outstretched and its long neck reaching south-westwards to Albireo, traditionally the swan’s beak. Although it is the dimmest corner-star of the Triangle, Deneb is one of the most luminous stars in our galaxy. Current estimates suggest that it shines some 200,000 time more brightly than our Sun from a distance of perhaps 2,600 ly, but its power and distance are hard to measure and the subject of some controversy.
Also controversial is the nature of Albireo. Even small telescopes show it as a beautiful double star in which a brighter golden star contrasts with a dimmer blue one. The mystery concerns whether the pair make up a real binary, with the two stars locked in orbit together by gravity, or whether this is just the chance alignment of two stars at different distances. Now measurement by the European Space Agency’s Gaia spacecraft appear to confirm the chance alignment theory.
The Milky Way, the band of countless distant stars in the plane of our galaxy, flows through the Summer Triangle and close to Deneb as it arches across our evening sky. Scan it through binoculars to glimpse a scattering of other double stars and star clusters.
One interesting stellar group is the so-called Coathanger which lies 8°, a little more than a normal binocular field-of-view, south of Albireo. It is also easy to locate one third of the way from Altair to Vega. Its line of stars, with a hook of stars beneath, gives it the appearance of an upside-down coat hanger. For decades this was regarded as a true star cluster, whose stars formed together, and its alternative designations as Brocchi’s Cluster and Collinder 399 reflect this. In 1998, though, results from the Hipparcos satellite, Gaia’s predecessor, proved that the Coathanger’s stars are at very different distances so that it, like Albireo, is simply a fortuitous chance alignment.
The Sun sinks 11.5° southwards during September to cross the sky’s equator at 02:54 BST on the 23rd. This marks our autumnal equinox and, by one definition, the beginning of autumn in the northern hemisphere. Sunrise/sunset times for Edinburgh change from 06:17/20:07 BST on the 1st at 07:13/18:51 on the 30th. The Moon is at last quarter on the 3rd, new on the 9th, at first quarter on the 17th and full on the 25th.
Venus is brilliant at magnitude -4.4 and 45° from the Sun on the 1st but it is only 4° above Edinburgh’s west-south-western horizon at sunset and sets 35 minutes later as its evening apparition as seen from Scotland comes to an end.
The other inner planet, Mercury, is prominent but low in the east-north-east before dawn until about the 14th. Glimpse it at magnitude -1.1 when it lies 1° above-left of Regulus in Leo on the 6th and 9° below-left of the impressively earthlit waning Moon on the 8th.
Jupiter is conspicuous but very low in the south-west at nightfall, sinking to set in the west-south-west one hour before our map times. Look for it below-right of the Moon on the 13th.
Saturn and Mars are in the far south of our evening sky. Saturn, the fainter of the two at magnitude 0.4 to 0.5, stands above the Teapot of Sagittarius and is just below and right of the Moon on the 17th when a telescope shows that its rings span 38 arcseconds around its 17 arcseconds disk. It sets in the south-west some 70 minutes after our map times.
Mars stands more than 25° east (left) of Saturn, tracks 7° eastwards and northwards in Capricornus and stands near the Moon on the 19th and 20th. It is easily the brightest object (bar the Moon) in the sky at our map times though it more than halves in brightness from magnitude -2.1 to -1.3. As its distance increases from 67 million to 89 million km, its ochre disk shrinks from 21 to 16 arcseconds. The dust storm that blanketed the planet since June has now died down.
Finally, we have a chance to spot the Comet Giacobini-Zinner as it tracks south-eastwards past the bright star Capella in Auriga, low in the north-east at our map times but high in the east before dawn. The comet takes only 6.6 years to orbit the Sun and should appear in binoculars as a small oval greenish smudge only 0.9° to the right of Capella on the evening of the 2nd when it is 60 million km away. Moving at almost 2° per day, it passes less than 7° north-east of Elnath in Taurus (see chart) on the morning of the 11th, just a day after it reaches perihelion, its closest (152 million km) to the Sun.
Diary for 2018 September
Times are BST
2nd 10h Venus 1.4° S of Spica
3rd 03h Moon 1.2° N of Aldebaran
3rd 04h Last quarter
6th 11h Saturn stationary (motion reverses from W to E)
7th 04h Moon 1.1° S of Praesepe in Cancer
7th 19h Neptune at opposition
8th 23h Moon 0.9° N of Mercury
9th 19h New moon
10th 08h Comet Giacobini-Zinner closest to Sun (152 million km)
14th 03h Moon 4° N of Jupiter
16th 14h Mars closest to Sun (206,661,000 km)
17th 00h First quarter
17th 17h Moon 2.1° N of Saturn
20th 08h Moon 5° N of Mars
21st 03h Mercury in superior conjunction
23rd 02:54 Autumnal equinox
25th 04h Full moon
30th 09h Moon 1.4° N of Aldebaran
This is a slightly revised version, with added diary, of Alan’s article published in The Scotsman on August 31st 2018, with thanks to the newspaper for permission to republish here.
Perseid meteor shower peaks in planet-rich sky
The persistent twilight that has swamped Scotland’s night sky since May is subsiding in time for us to appreciate four bright evening planets and arguably the best meteor shower of the year.
The Perseid shower returns every year between 23 July and 20 August as the Earth cuts through the stream of meteoroids that orbit the Sun along the path of Comet Swift-Tuttle. As they rush into the Earth’s atmosphere at 59 km per second, they disintegrate in a swift streak of light with the brighter ones often laying down a glowing train that may take a couple of seconds or more to dissipate.
The shower is due to peak in the early hours of the 13th at around 02:00 BST with rates in excess of 80 meteors per hour for an observer under ideal conditions – under a moonless dark sky with the shower’s radiant point, the place from which the meteors appear to diverge, directly overhead. We should lower our expectations, however, for although moonlight is not a problem this year, most of us contend with light pollution and the radiant does not stand overhead.
Even so, observable rates of 20-40 per hour make for an impressive display and, unlike for the rival Geminid shower in December, we don’t have to freeze for the privilege. Indeed, some people enjoy group meteor parties, with would-be observers reclining to observe different parts of the sky and calling out “meteor!” each time they spot one. Target the night of 12th-13th for any party, though rates may still be respectable between the 9th and 15th.
The shower takes its name from the fact that its radiant point lies in the northern part of the constellation Perseus, see the north map, and climbs from about 30° high in the north-north-east as darkness falls to very high in the east before dawn. Note that Perseids fly in all parts of the sky – it is just their paths that point back to the radiant.
Records of the shower date back to China in AD 36 and it is sometimes called the Tears of St Lawrence after the saint who was martyred on 10 August AD 258, though it seems this title only dates from the nineteenth century.
Sunrise/sunset times for Edinburgh change this month from 05:17/21:20 BST on the 1st to 06:15/20:10 on the 31st. The Moon is at last quarter on the 4th, new on the 11th, at first quarter on the 18th and full on the 26th.
A partial solar eclipse on the 11th is visible from the Arctic, Greenland, Scandinavia and north-eastern Asia. Observers in Scotland north of a line from North Uist to the Cromarty Firth see a thin sliver of the Sun hidden for just a few minutes at about 09:45 BST. Our best place to be is Shetland but even in Lerwick the eclipse lasts for only 43 minutes with less than 2% of the Sun’s disk hidden at 09:50. To prevent serious eye damage, never look directly at the Sun.
Vega in Lyra is the brightest star overhead at nightfall and marks the upper right corner of the Summer Triangle it forms with Deneb in Cygnus and Altair in Aquila. Now that the worst of the summer twilight is behind us, we have a chance to glimpse the Milky Way as it flows through the Triangle on its way from Sagittarius in the south to Auriga and the star Capella low in the north. Other stars of note include Arcturus in Bootes, the brightest star in our summer sky, which is sinking in the west at the map times as the Square of Pegasus climbs in the east.
Of the quartet of planets in our evening sky, two have already set by our map times. The first and brightest of these is Venus which stands only 9° high in the west at Edinburgh’s sunset on the 1st and sets itself 68 minutes later. By the 31st, these numbers change to 4° and 35 minutes, so despite its brilliance at magnitude -4.2 to -4.4, it is becoming increasingly difficult to spot as an evening star. It is furthest east of the Sun (46°) on the 17th.
Jupiter remains conspicuous about 10° high in the south-west as darkness falls and sets in the west-south-west just before the map times. Edging eastwards in Libra, it dims slightly from magnitude -2.1 to -1.9 and slips 0.6° north of the double star Zubenelgenubi on the 15th. A telescope shows it to be 36 arcseconds wide when it lies below-right of the Moon on the 17th.
The two planets low in the south at our map times are Mars, hanging like a prominent orange beacon only some 7° high in south-western Capricornus, and Saturn which is a shade higher above the Teapot of Sagittarius almost 30° to Mars’ right. Mars stood at opposition on 27 July and is at its closest to the Earth (57.6 million km) four days later. A planet-wide dust storm has hidden much of the surface detail on its small disk which shrinks during August from 24 to 21 arcseconds as its distance increases to 67 million km. Although Mars dims from magnitude -2.8 to -2.1, so it remains second only to Venus in brilliance. Catch the Moon near Saturn on the 20th and 21st and above Mars on the 24th.
Finally, we cannot overlook Mercury which is a morning star later in the period. Between the 22nd and 31st, it brightens from magnitude 0.8 to -0.7, rises more than 90 minutes before the Sun and stands around 7° high in the east-north-east forty minutes before sunrise. It is furthest west of the Sun (18°) on the 26th.
Diary for 2018 August
Times are BST
4th 19h Last quarter
9th 01h Mercury in inferior conjunction on Sun’s near side
11th 11h New moon and partial solar eclipse
13th 02h Peak of Perseids meteor shower
14th 15h Moon 6° N of Venus
17th 12h Moon 5° N of Jupiter
17th 19h Venus furthest E of Sun (46°)
18th 09h First quarter
21st 11h Moon 2.1° N of Saturn
23rd 18h Moon 7° N of Mars
26th 13h Full moon
26th 22h Mercury furthest W of Sun (18°)
28th 11h Mars stationary (motion reverses from W to E)