Brilliant Venus plunges into the evening twilight
Stargazers will be hoping for better weather as Orion and the stars of winter depart westwards in our evening sky, Venus dives into the evening twilight and around the Sun’s near side, while all the other bright planets are on view too. Indeed, Venus has the rare privilege of appearing as both an evening star and a morning star over several days, provided our western and eastern horizons are clear.
Orion still dominates our southern sky at nightfall as Leo climbs in the east and the Plough balances on its handle in the north-east. The Sun’s northwards progress and our lengthening days mean that by nightfall at the month’s end Orion has drifted lower into the south-west, halfway to his setting-point in the west. He is even lower in the west-south-west by our star map times when it is the turn of Leo to reach the meridian and the Plough to be almost overhead.
Leo’s leading star, Regulus, sits at the base of the Sickle of Leo, the reversed question-mark of stars from which meteors of the Leonids shower stream every November. The star Algieba in the Sickle (see chart) appears as a glorious double star through a telescope. Its components are larger and much more luminous than our Sun and lie almost 5 arcseconds apart, taking some 510 years to orbit each other. The pair lie 130 light years away and are unrelated to the star less than a Moon’s breadth to the south which is only half as far from us.
The Sun travels northward across the equator at 10:28 GMT on the 20th, the moment of the vernal (spring) equinox in our northern hemisphere. On this date, nights and days are of roughly equal length around the globe. Sunrise/sunset times for Edinburgh change from 07:04/17:47 GMT on the 1st to 06:46/17:49 BST (05:46/18:49 GMT) on the 31st after we set our clocks forwards to BST on the morning of the 26th. The lunar phases change from first quarter on the 5th to full on the 12th, last quarter on the 20th and new on the 28th.
Look for the young earthlit Moon well to the left of the brilliant magnitude -4.6 Venus on the 1st when telescopes show the planet’s dazzling crescent to be 47 arcseconds in diameter and 16% sunlit. Venus’ altitude at sunset plummets from 29° in the west-south-west on that day to only 7° in the west on the 22nd as its diameter swells to 59 arcseconds and the phase shrinks to only 1% – indeed, a few keen-sighted people might be able to discern its crescent with the naked eye and this is certainly easy to spot through binoculars.
Venus dims to magnitude -4.0 by the time it sweeps 8° north of the Sun and only 42 million km from the Earth at its inferior conjunction on the 25th. This marks its formal transition from the evening to the morning sky, but because it passes so far north of the Sun as it does every eight years or so, Venus is already visible in the predawn before we lose it in the evening. In fact, it is 7° high in the east at sunrise on the 22nd, and it only gets better as the month draws to its close.
Before Venus exits our evening sky, it meets Mercury as the latter begins its best spell as an evening star this year. On the 20th, the small innermost planet lies 10° to the left of Venus, shines at magnitude -1.2 and sets at Edinburgh’s western horizon 78 minutes after the Sun. By the 29th, it is 10° high forty minutes after sunset and shines at magnitude -0.4, easily visible through binoculars and 8° to the right of the very young Moon.
Mars, near the Moon on the 1st and again on the 30th, dims from magnitude 1.3 to 1.5 this month as it tracks from Pisces into Aries. By the month’s end, it lies to the left of Aries’ main star Hamal and sets at our map times. It is now more than 300 million km away and its disk, less than 5 arcseconds across, is too small to be of interest telescopically.
The Moon has another encounter with the Hyades star cluster on the night of the 4th-5th, hiding several of its stars but setting for Scotland before it reaches Taurus’ main star Aldebaran. The latter, though, is occulted later as seen from most of the USA. The Moon passes just below Regulus on the night of the 10th-11th and meets the planet Jupiter on the 14th.
Jupiter, conspicuous at magnitude -2.3 to -2.5, rises in the east at 21:37 GMT on the 1st and only 31 minutes after Edinburgh’s sunset on the 31st. Now edging westwards above the star Spica in Virgo, it is unmistakable as it climbs through our south-eastern sky to cross the meridian in the small hours and lie in the south-west before dawn. Its disk, 43 arcseconds wide at mid-month, shows parallel cloud bands through almost any telescope, while its four moons may be glimpsed through binoculars as they orbit from one side to the other.
Saturn, the last of the night’s planets, rises in the south-east at 03:44 GMT on the 1st and almost two hours earlier by the 31st. Improving very slightly from magnitude 0.5 to 0.4 during March, it is the brightest object about 10° above the south-south-eastern horizon before dawn. Look for it 4° below-left of the Moon on the 20th.
This is a slightly-revised version of Alan’s article published in The Scotsman on February 28th 2017, with thanks to the newspaper for permission to republish here.
Harvest moon eclipsed on the 16th
Two eclipses and a couple of notable space exploration milestones make September an interesting month for astronomers. I’ll postpone until the close of this note, though, my thoughts on the exciting news that Proxima Centauri, the closest star to our Sun, has a planet which is probably rocky, slightly larger than the Earth and in the star’s so-called habitable zone where liquid water might exist.
The first eclipse, an annular or “ring” eclipse of the Sun, occurs on the 1st with the Moon too distant to hide the Sun completely. Instead, a dazzling ring of sunlight remains visible along a narrow path that stretches across Central Southern Africa into the Indian Ocean. Surrounding areas enjoy a partial solar eclipse but nothing is seen as far north as Europe
Of greater interest for us is a penumbral eclipse of the Moon on the 16th during which the Moon passes through the southern outer part of the Earth’s shadow, the penumbra. The event lasts from 17:55 to 21:54 BST although, as seen from Edinburgh, the Moon only rises in the east at 19:29. Maximum eclipse occurs 25 minutes later, at 19:54, when all but the southern 9% of the Moon is within the penumbra. Little darkening of the disk may be noticeable, except near the northern edge which is closest to the Earth’s umbra where all direct sunlight is extinguished.
Since this full moon is the one closest to the autumnal equinox, due at 15:21 BST on the 22nd, it is also called the harvest moon. The tradition is that the bright moon stands at a similar altitude in the eastern sky over several evenings at this time, so permitting the harvesting hours to be extended.
The Sun tracks 11.5° southwards during September to cross the celestial equator at the equinox when day and night have approximately equal lengths around the Earth. Sunrise/sunset times for Edinburgh change from 06:18/20:06 BST on the 1st to 07:14/18:50 on the 30th. The moon is new on the 1st, at first quarter on the 9th, full on the 16th, at last quarter on the 23rd and new again on 1 October.
Jupiter is now lost from view as it nears conjunction on the Sun’s far side on the 26th. It leaves Venus as an evening star, but even though Venus is brilliant at magnitude -3.9 it stands less than 5° above Edinburgh’s horizon at sunset and sets itself within the next 45 minutes. Catch it, if you can, in the west as September begins, shifting to the south-west by the month’s end.
Mars, Saturn and the star Antares in Scorpius form a triangle low in the south-west as darkness falls at present, with Saturn above Antares and Mars a few degrees to their left. Saturn is magnitude 0.5 while Mars is brighter and noticeably reddish, though it fades from magnitude -0.3 to 0.1 as it speeds 18° eastwards and further away. By month’s end, its motion brings it onto our chart and close to the so-called Teapot of Sagittarius, just setting in the south-west.
Look for the Moon close to Saturn on the 9th and above Mars on the 10th when, if viewed telescopically, the two planets appear 16 and 10 arcseconds wide respectively, with Saturn’s wide-open rings spanning 37 arcseconds.
Mercury begins its best morning appearance of the year late in the month. From the 24th onwards, it rises in the east more than 95 minutes before the Sun and reaches more than 8° high forty minutes before sunrise. It is furthest west of the Sun (18°) on the 28th and is magnitude -0.5 when it lies alongside the slender earthlit Moon on the 29th.
Just a day later, on the 30th, Europe’s Rosetta spacecraft is destined to end its mission when it collides with Comet Churyumov-Gerasimenko, the rubber-duck shaped body it has been orbiting and investigating since August 2014. The collision will be gentle but radio contact and data-collection is likely to be lost as the craft settles on the comet’s surface.
Earlier in the month, during a month-long launch window beginning on the 8th, NASA’s OSIRIS-REx spacecraft is due to embark on its seven-years mission to collect and return samples from the surface of Bennu, a small asteroid which has been given an outside chance of having a catastrophic impact with the Earth late in the next century.
Proxima Centauri lies at a distance of only 4.25 light years but is much too dim to be seen without a telescope, A small red dwarf star, it is less than 15% as massive and wide as our Sun and has less than 0.2% of the Sun’s energy output. Also called Alpha Centauri C, it was discovered in 1915 by the Edinburgh-born astronomer Robert Innes and lies 15° to the east of the Southern Cross in a part of the sky we never see from Britain. It is thought to form a triple star system with Alpha Centauri A and B, a tight binary of more Sun-like stars that lie 2° away in the sky.
The newly discovered world has been dubbed Proxima b but it is something of a stretch to call it Earth-like. It orbits its star in a year of 11.2 Earth-days at a distance of less than 8 million km where it is blasted by X-rays from dramatic flares that we see erupting on Proxima’s surface – far from ideal for life. It is also probably tidally locked – keeping its same face towards the star – and we do not even know (yet!) that it has water, never mind life.