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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
This is a slightly revised version, with added diary, of Alan’s article published in The Scotsman on November 30th 2018, 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)
This is a slightly revised version, with added diary, of Alan’s article published in The Scotsman on July 31st 2018, with thanks to the newspaper for permission to republish here.
Dust storm rages on Mars as it stands closest since 2003
Mars comes closer to the Earth in July than at any time since its once-in-60,000-years record approach in 2003. It is just our luck that a dust storm that began a month ago now engulfs the entire planet so that the surface markings may now be glimpsed only through a patchy reddish haze.
Both current Mars rovers, Opportunity and Curiosity, are also affected. This is the most intense storm to impact Opportunity since it landed in 2004 and the vehicle has shut down because it lost power as the dust hid the Sun and coated its solar panels. It is hoped that, after the storm subsides, friendly gusts of wind will waft the dust from the panels and Opportunity will revive. If not, this would mark the end of a remarkable mission which had been planned, initially, to last for only 90 days. Its sister rover, Spirit, succumbed in 2010 after becoming stuck in soft soil.
Meanwhile, the more advanced Curiosity rover has been operating since 2012. Being nuclear powered, it is less vulnerable to the dust but its cameras are recording a dull reddened landscape beneath dusty orange skies.
For watchers in Edinburgh, Mars rises in the south-east just before midnight at the beginning of July and is conspicuous at magnitude -2.2 but only 11° high in the south during morning twilight. Look for it 4° below the Moon on the 1st as Mars moves westwards in the constellation of Capricornus.
Mars reaches opposition on the 27th when it stands opposite the Sun, rises during our evening twilight and is highest in the south in the middle of the night. By then it blazes at magnitude -2.8, making it second only to Venus in brilliance, and stands 58 million km away. A telescope shows it to be 24 arcseconds wide, with its southern polar cap tilted 11° towards us. Because Mars is edging inwards in its relatively elongated orbit, it is actually around 100,000 km closer to us on the 31st.
As Mars rises at its opposition on the 27th it once again lies below Moon, but this time the Moon is deep in eclipse as it passes almost centrally through the Earth’s shadow. The total phase of the eclipse, the longest this century, lasts from 20:30 to 22:13 BST and it is in the middle of this period, at 21:22, that the Moon rises for Edinburgh. By 22:13, and weather permitting, it may be possible to see the Moon’s dull ochre disk 5° high in the south-east. From then until 23:19, the Moon emerges eastwards from the Earth’s dark umbral shadow, and at 00:29 it is free of the penumbra, the surrounding lighter shadow.
The Earth stands at its furthest from the Sun for 2018 (152,100,000 km) on the 6th. Edinburgh’s sunrise/sunset times change from 04:31/22:01 on the 1st to 05:15/21:22 on the 31st. The Moon is at last quarter on the 6th and new on the 13th when a partial solar eclipse is visible to the south of Australia. First quarter on the 19th is followed by full moon and the total lunar eclipse on the 27th.
Our chart shows the corner stars of the Summer Triangle, Vega in Lyra, Altair in Aquila and Deneb in Cygnus, high in the south to south-east as the fainter corner stars of the Square of Pegasus are climbing in the east. The Plough stands in the middle of our north-western sky and the “W” of Cassiopeia is similarly placed in the north-east.
Venus sets before our chart times but is brilliant in the west at nightfall. It brightens from magnitude -4.0 to -4.2 but is sinking lower from night to night as it tracks southwards relative to the Sun. It passes 1.1° north of the star Regulus in Leo on the 9th as the much fainter planet Mercury (magnitude 0.4) stands 16° below-right of Venus. The little innermost planet stands furthest east of the Sun (26°) on the 12th but is a challenge to glimpse in the twilight this time around.
Venus lies to the left of the young earthlit Moon on the 15th, below-right of the Moon on the 16th and, by month’s end, stands less than 10° high at sunset before setting itself some 70 minutes later.
Jupiter lingers as a conspicuous evening object in the south-south-west at nightfall, sinking to set in the west-south-west one hour after our map times. Moving very little against the stars of Libra, it dims slightly from magnitude -2.3 to -2.1 and shows a 39 arcseconds disk when it lies below-left of the Moon on the 20th.
Saturn reached opposition on June 27 and is at its best at our star map times, albeit low in the south at a maximum altitude of less than 12° for Edinburgh. At magnitude 0.0 to 0.2, it is creeping westwards above the Teapot of Sagittarius where it lies near the Moon on the 24th and 25th. Its disk and wide-open rings appear 18 and 41 arcseconds wide respectively.
Our noctilucent, or “night-shining”, cloud season is now in full swing with sightings of several displays of these high-altitude blue-white clouds since late-May and further ones expected until August.
Often with a wispy cirrus-like appearance, noctilucent clouds are composed of ice-crystals at heights near 82 km and glimmer above our northern horizon where they catch the sunlight long after our more usual lower-level clouds are in darkness. Their nature is still something of a mystery but it may not be coincidental that the first definite record of them dates only as far back as 1885, just two years after the cataclysmic eruption of the Krakatoa volcano in Indonesia.
Diary for 2018 July
Times are BST
1st 03h Moon 5° N of Mars
6th 09h Last quarter
6th 18h Earth farthest from Sun (152,100,000 km)
9th 21h Venus 1.1° N of Regulus
11th 05h Jupiter stationary (motion reverses from W to E)
12th 06h Mercury furthest E of Sun (26°)
13th 04h New moon and partial solar eclipse S of Australia
14th 23h Moon 2.2° N of Mercury
16th 04h Moon 1.6° N of Venus
19th 21h First quarter
21st 01h Moon 4° N of Jupiter
25th 07h Moon 2.0° N of Saturn
27th 06h Mars at opposition at distance of 58 million km
27th 21h Full moon and total lunar eclipse
27th 22h Moon 7° N of Mars
29th Main peak of Delta Aquarids meteor shower
31st 09h Mars closest to Earth (57,590,000 km)