Impressive conjunction before dawn for Mars and Saturn
The Sun climbs almost 10° northwards during April to bring us longer days and, let us hope, some decent spring-like weather at last. Our nights begin with Venus brilliant in the west and end with three other planets rather low across the south. Only Mercury is missing – after rounding the Sun’s near side on the 1st it remains hidden in Scotland’s morning twilight despite standing further from the Sun in the sky (27°) on the 29th than at any other time this year.
Edinburgh’s sunrise/sunset times change from 06:44/19:51 BST on the 1st to 05:32/20:50 on the 30th. The Moon is at last quarter on the 8th, new on the 16th, first quarter on the 22nd and full on the 30th.
Mars and Saturn rise together in the south-east at about 03:45 BST on the 1st and are closest on the following day, with Mars, just the brighter of the two, only 1.3° south of Saturn. Catch the impressive conjunction less than 10° high in the east-south-east as the morning twilight begins to brighten.
Both planets lie just above the so-called Teapot of Sagittarius but they are at very different distances – Mars at 166 million km on the 1st while Saturn is nine times further away at 1,492 million km.
Brightening slightly from magnitude 0.5 to 0.4 during April, Saturn moves little against the stars and is said to be stationary on the 18th when its motion reverses from easterly to westerly. Almost any telescope shows Saturn’s rings which are tipped at 26° to our view and currently span some 38 arcseconds around its 17 arcseconds disk.
Mars tracks 15° eastwards (to the left) and almost doubles in brightness from magnitude 0.3 to -0.3 as its distance falls to 127 million km. Its reddish disk swells from 8 to 11 arcseconds, large enough for telescopes to show some detail although its low altitude does not help.
Saturn is 4° below-left of Moon and 3° above-right of Mars on the 7th while the last quarter Moon lies 5° to the left of Mars on the next morning.
Orion stands above-right of Sirius in the south-west as darkness falls at present but has all but set in the west by our star map times. Those maps show the Plough directly overhead where it is stretched out of shape by the map projection used. We can extend a curving line along the Plough’s handle to reach the red giant star Arcturus in Bootes and carry it further to the blue giant Spica in Virgo, lower in the south-south-east and to the right of the Moon tomorrow night.
After Sirius, Arcturus is the second brightest star in Scotland’s night sky. Shining at magnitude 0.0 on the astronomers’ brightness scale, though, it is only one ninth as bright as the planet Jupiter, 40° below it in the constellation Libra. In fact, Jupiter improves from magnitude -2.4 to -2.5 this month as its distance falls from 692 million to 660 million km and is hard to miss after it rises in the east-south-east less than one hour before our map times. Look for it below-left of the Moon on the 2nd, right of the Moon on the 3rd, and even closer to the Moon a full lunation later on the 30th.
Jupiter moves 3° westwards to end the month 4° east of the double star Zubenelgenubi (use binoculars). Telescopes show the planet to be about 44 arcseconds wide, but for the sharpest view we should wait until it is highest (17°) in in the south for Edinburgh some four hours after the map times.
Venus’ altitude on the west at sunset improves from 16° to 21° this month as the evening star brightens from magnitude -3.9 to -4.2. Still towards the far side of its orbit, it appears as an almost-full disk, 11 arcseconds wide, with little or no shading across its dazzling cloud-tops. Against the stars, it tracks east-north-eastwards through Aries and into Taurus where it stands 6° below the Pleiades on the 20th and 4° left of the star cluster on the 26th. As it climbs into our evening sky, the earthlit Moon lies 6° below-left of Venus on the 17th and 12° left of the planet on the 18th.
The reason that we have such impressive springtime views of the young Moon is that the Sun’s path against the stars, the ecliptic, is tipped steeply in the west at nightfall as it climbs through Taurus into Gemini. The orbits of the Moon and the planets are only slightly inclined to the ecliptic so that any that happen to be towards this part of the solar system are also well clear of our horizon. Contrast this with our sky just before dawn at present, when the ecliptic lies relatively flat from the east to the south – hence the non-visibility of Mercury and the low altitudes of Mars, Saturn and Jupiter.
The evening tilt of the ecliptic means that, under minimal light pollution and after the Moon is out of the way, it may be possible to see the zodiacal light. This appears as a cone of light that slants up from the horizon through Venus and towards the Pleiades. Caused by sunlight reflecting from tiny particles, probably comet-dust, between the planets, it fades into a very dim zodiacal band that circles the sky. Directly opposite the Sun this intensifies into an oval glow, the gegenschein (German for “counterglow”), which is currently in Virgo and in the south at our map times – we need a really dark sky to see it though.
Diary for 2018 April
Times are BST.
1st 19h Mercury in inferior conjunction on Sun’s near side
2nd 13h Mars 1.3° S of Saturn
3rd 15h Moon 4° N of Jupiter
7th 14h Moon 1.9° N of Saturn
7th 19h Moon 3° N of Mars
8th 08h Last quarter
16th 03h New moon
17th 13h Saturn farthest from Sun (1,505,799,000 km)
17th 20h Moon 5° S of Venus
18th 03h Saturn stationary (motion reverses from E to W)
18th 15h Uranus in conjunction with Sun
22nd 23h First quarter
24th 05h Venus 4° S of Pleiades
24th 21h Moon 1.2° N of Regulus
29th 19h Mercury furthest W of Sun (27°)
30th 02h Full moon
30th 18h Moon 4° N of Jupiter
This is a slightly revised version, with added diary, of Alan’s article published in The Scotsman on March 31st 2018, with thanks to the newspaper for permission to republish here.
Cassini begins Grand Finale at Saturn
This month brings the final truly dark night skies for Scotland until mid-July or later. Our dwindling nights are dominated by Jupiter, bright and unmistakable as it passes about 30° high in our southern evening sky and sinks to the western horizon before dawn. Venus is brighter still but easily overlooked as it hovers low in our brightening eastern dawn twilight. Saturn is also best as a morning planet, though it rises at our south-eastern horizon a few minutes before our May star map times.
Saturn creeps westwards from the constellation Sagittarius into Ophiuchus this month and brightens a little from magnitude 0.3 to 0.1, making it comparable with the brightest stars visible at our map times – Arcturus, Capella and Vega. The ringed planet, though, climbs to only 12° high in the south by the time morning twilight floods our sky, which is too low for crisp telescopic views of its stunning rings. On the morning of the 14th, as Saturn stands only 3° below-right of the Moon, its rotation-squashed globe measures 18 arcseconds in diameter while its rings stretch across 41 arcseconds and have their northern face tipped at 26° to our view.
Saturn’s main moon, Titan, takes 16 days to orbit the planet and is an easy telescopic target on the ninth magnitude. It stands furthest west of the disk (3 arcminutes) on the 3rd and 19th and furthest east on the 11th and 27th.
The Cassini probe is now into the final chapter, its so-called Grand Finale, of its epic exploration of the Saturn system. On 22 April, it made its 127th and last flyby of Titan, while on 26 April it dived for the first time through the gap between the planet and its visible rings, successfully returning data from a region it has never dared to explore before. Cassini’s new orbit sees it make another 21 weekly dives until, come 15 September, its almost-20 years mission ends with a fiery plunge into the Saturnian atmosphere.
The Sun’s northwards progress during May, to within only 1.4° of its most northerly point at the summer solstice, changes the sunrise/sunset times for Edinburgh from 05:29/20:52 BST on the 1st to 04:36/21:46 on the 31st. The Moon reaches first quarter on the 3rd, full on the 10th, last quarter on the 19th and new on the 25th.
This crescent Moon on the 1st lies in the west, between the stars Pollux in Gemini and Procyon in Canis Minor, lower to its left, while on the 2nd it is 4° below-left of the Praesepe star cluster in Cancer, best viewed through binoculars. It lies near Regulus in Leo on the 3rd and 4th, and appears only 1.2° above the conspicuous Jupiter on the 7th.
The giant planet lies 10° above-right of Virgo’s leading star Spica and edges 2° to the west-north-west this month, drawing closer to the celebrated double star Porrima whose two equal stars orbit each other every 169 years but appear so close together at present that we need a good telescope to divide them.
Following its opposition on 7 April, Jupiter recedes from 678 million to 724 million km during May, dimming slightly from magnitude -2.4 to -2.2 as its diameter shrinks from 43 to 41 arcseconds. Any telescope should show its changing cloud-banded surface while its four main moons may be glimpsed through binoculars, although sometimes one or more disappear as they transit in front of the disk or are hidden behind it or in its shadow.
Some 30° above and to the left of Jupiter is the orange-red giant star Arcturus in Bootes the Herdsman. At magnitude -0.05, this is (just) the brightest star in the northern celestial hemisphere ahead of Capella in Auriga, low in the north-north-west at our map times, and Vega in Lyra, climbing in the east. It is also one of the closer stars to the Sun, but it is only a temporary neighbour for it is speeding by the solar system at 122 km per second at a distance of 36.7 light years. Even so, it takes 800 years to move a Moon’s breadth across our sky. It is also a corner star of a rarely-heralded asterism dubbed the Spring Triangle – the other vertices being marked by Spica and Regulus.
A useful trick for finding Arcturus is to extend a curving line along the handle of the Plough which passes overhead during our spring evenings but is always visible somewhere in our northern sky. That line, still pending, leads to Arcturus and then onwards to Spica. The traditional mnemonic for this is “Arc to Arcturus, spike to Spica” but, given current circumstances, we might amend this to “Arc to Arcturus, jump to Jupiter”.
Venus rises 65 minutes before the Sun on the 1st and climbs to stand 9° high at sunrise. By the 31st, these figures change only a little to 75 minutes and 10°, so it is far from obvious as a morning star, even though it blazes at magnitude -4.5 to -4.3. Through a telescope, it shows a crescent whose sunlit portion increases from 27% to 48% while its diameter shrinks from 38 to 25 arcseconds. Early rises, or insomniacs, can see it left of the waning Moon on the 22nd.
Mercury stands below and left of Venus but remains swamped by our dawn twilight. It is furthest west of the Sun (26°) on the 18th. Still visible, but destined soon to disappear into our evening twilight, is Mars. Shining at a lowly magnitude 1.6, it lies 7° above-right of Aldebaran as the month begins and tracks between the Bull’s horns as Taurus sinks below our north-western horizon in the early evening.
This is a slightly-revised version of Alan’s article published in The Scotsman on May 1st 2017, with thanks to the newspaper for permission to republish here.
A quarter-century for the Hubble Space Telescope
Just 25 years ago, scientists worldwide were celebrating the successful launch of the Hubble Space Telescope. We soon learned, though, that its precisely-figured 2.4-metre mirror had been built to the wrong shape, and we had to wait another three years before corrective optics could be installed to correct its blurred vision. Since then, Hubble has been returning research and a gallery of stunning images that have transformed our understanding of the Universe.
Its findings impact on every area of astronomy, and every distance-scale, from the farthest and earliest galaxies to the processes of star formation and images of objects in our solar system in unprecedented detail. It has also been a key player in the discovery that the entire Universe is expanding at an increasing rate because of a mysterious entity dubbed dark energy.
It is now six years since a shuttle visited to service it for the final time, and its instruments will eventually fail. Its orbit is also decaying because of the tiny atmospheric drag at its current altitude of 545 km, and it may spiral to destruction within another decade or so.
However, we expect that Hubble will still be alive when its successor, the James Webb Space Telescope, the JWST, is launched, hopefully in 2018. With a segmented 6.5-metre mirror, and working between visible and infrared wavelengths, this should build on Hubble’s legacy. The UK Astronomy Technology Centre at Edinburgh’s Royal Observatory has leading roles in the consortium from Europe and NASA that has built one of the JWST’s three main instruments, the Mid-InfraRed Instrument or MIRI.
As the Sun climbs another 7° higher at noon during May, Edinburgh’s days lengthen by almost two hours, although we lose much more than this of nighttime darkness. On the 1st, the Sun is more than 12° below Edinburgh’s horizon, and the sky effectively dark, for a little more than five hours, but by the month’s end this shrinks to only 32 minutes. More accurately, the sky would be dark for these periods were it not for the moonlight at the start and end of the month.
Sunrise/sunset times for Edinburgh vary from 05:30/20:51 BST on the 1st to 04:37/21:45 on the 31st while the Moon is full on the 4th, at last quarter on the 11th, new on the 18th and at first quarter on the 25th.
The conspicuous star Arcturus in Bootes is climbing in the east at nightfall to dominate the high southern sky by our map times although it pales by comparison with the planets Jupiter and Venus which lie further to the west.
Below and right of Arcturus is Virgo and the closest giant cluster of galaxies, the Virgo Cluster. Located some 54 million light years away, and one of Hubble’s earliest targets, it contains up to 2,000 galaxies, more than a dozen of which are visible through small telescopes under a dark sky. Its centre lies roughly midway between the stars Vindemiatrix in Virgo, and Leo’s tail-star Denebola (see map).
Another planet, Saturn, shines at magnitude 0.0 and almost rivals Arcturus in brightness when it reaches opposition at a distance of 1,341 million km on the 23rd. It is then best placed on the meridian in the middle of the night, though it stands only 15° above Edinburgh’s horizon so that telescopic views of its rings and globe, 42 and 18 arcseconds wide respectively, may be hindered by turbulence in our atmosphere.
Currently 1.2° north of the double star Graffias in Scorpius, Saturn creeps westwards into Libra by the day of opposition. The rings have their northern face tilted 24° towards us at present and although this will increase to 26° next year, Saturn itself slides another 2° further south. Catch Saturn to the right of the Moon on the 5th-6th.
This is the best time this year to glimpse Mercury in our evening sky. Until the 11th, it stands 10° or more above the west-north-western horizon forty minutes after sunset before it sinks to set more than two hours later. It dims from magnitude -0.3 on the 1st to 1.0 on the 11th and may be followed through binoculars for just a few more days as it sinks lower and fades to magnitude 1.7 by the 15th. Mercury stands furthest from the Sun (21°) on the 7th and passes around the Sun’s near side at inferior conjunction on the 30th.
The brilliant evening star Venus improves from magnitude -4.1 to -4.3 and is unmistakable in the west at sunset, sinking to set in the north-west after 01:00. From between the Horns of Taurus at present, it tracks eastwards into Gemini to stand 1.7° above-right of the star cluster M35 on the 9th (use binoculars) and end the month 4° to the south of Pollux in Gemini. Venus approaches from 148 million to 113 million km during the period as its gibbous disk swells from 17 to 22 arcseconds and its sunlit portion falls from 67% to 53%.
Jupiter still outshines every star, but is fainter than Venus and stands above and well to its left, their separation in the sky plummeting from 50° on the 1st to 21° on the 31st. Look for Jupiter in the south-west at nightfall at present and much lower in the west by our map times. This month it fades a little from magnitude -2.1 to -1.9 and tracks 3° eastwards to the east of the Praesepe star cluster in Cancer (use binoculars). The planet lies above the crescent Moon and 833 million km away on the 23rd when a telescope shows its cloud-banded disk to be 35 arc seconds across.