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.
This is a slightly-revised version of Alan’s article published in The Scotsman on September 1st 2016, with thanks to the newspaper for permission to republish here.
Mars shines brightly at opposition in Virgo
Six years have passed since Mars was as close and bright as it is this month, but two other planets outshine it and a fourth, Saturn, will soon be at its best for the year. There are also two of 2014’s four eclipses but, as with the second pair in October, neither is of much interest for observers in Scotland.
For the moment, our evening sky retains a flavour of stellar feast we enjoyed over the winter. Orion is still on show in the south-west at nightfall below the conspicuous planet Jupiter. Orion’s Belt now lies almost parallel to the horizon, a line along it pointing to the left towards Sirius, our brightest nighttime star, and to the right towards Aldebaran and the Pleiades in Taurus. By our star map times, though, Orion has all but sunk below our western horizon.
Jupiter, however, continues as our brightest evening object bar the Moon. As it slips 3.5° or seven Moon-widths eastwards in the middle of Gemini during April, it fades a little between magnitude -2.2 and -2.0 and its telescopic diameter shrinks from 38 to 35 arcseconds. The earlier in the night that we catch it, the higher it stands and the sharper the view of its cloud-banded disk. By our map times Jupiter is some 30° high in the west and on its way to setting in the north-west four hours later.
The month begins with impressive views of the young earthlit Moon in the west at nightfall. It is only 5% illuminated on the 1st as it stands 14° high forty minutes after sunset. Look for it below the Pleiades on the 2nd, below the Aldebaran-Pleiades line on the 3rd and 6° below Jupiter on the 6th as it nears first quarter.
Mars reaches opposition on the 8th when it lies 93 million km away and shines at magnitude -1.5 so that its orange-red beacon rivals Sirius in brightness if not in colour. By definition, it stands opposite the Sun in the sky so that we find it climbing from the eastern horizon as the evening twilight fades to pass 28° high on Edinburgh’s meridian two hours after our map times. As the arrow on our south map shows, Mars tracks 10° westwards in Virgo during April, from 5° above the magnitude 1.0 Spica today to lie 1.6° below-left of the famous binary star Porrima as the month ends.
Often the day of opposition is when a planet is closest to us but Mars is approaching the Sun in its orbit and is 450,000 km closer to us on the 14th than on the 8th. Through a telescope, its ochre disk is 15 arcseconds wide and shows dusky markings and the dwindling white smudge of its north polar ice cap, tipped about 22° towards us.
The full Moon lies below Mars on the evening of the 14th and is approaching Spica as it sets for Edinburgh at 06:08 BST on the 15th. Only 14 minutes before this, and while it is less than 2° above the west-south-western horizon in the twilight, it begins to enter the outer shadow of the Earth, the penumbra. Sadly, we have no hope of seeing any dimming of the lunar disk before it sets. Observers in the Americas are much better placed to view the resulting total eclipse of the Moon which is total from 08:07 until 09:26 BST (03:07 to 04:25 EDT).
Sunrise/sunset times for Edinburgh change from 06:44/19:51 BST on the 1st to 05:32/20:50 on the 30th while the duration of nautical twilight at dawn and dusk stretches from 84 to 105 minutes. After first quarter on the 7th, the Moon is full during the eclipse on the 15th, at last quarter on the 22nd and new on the 29th when a small area of Antarctica and perhaps a few penguins experience an annular eclipse of the Sun. A partial solar eclipse is visible from Australia and the southern Indian Ocean.
On course to reach opposition in May, Saturn rises at Edinburgh’s east-south-eastern horizon at 23:31 on the 1st and only 36 minutes after sunset by the 30th, climbing to pass 18° high on the meridian four hours after our map times. Improving from magnitude 0.3 to 0.1, it edges westwards in Libra and draws ever closer to the Moon overnight on the 16th-17th when Saturn’s disk is 18 arcseconds wide while its stunning rings span 42 arcseconds.
Mercury is hidden in the dawn twilight until it passes around the Sun’s far side on the 26th. Venus, brilliant as a morning star, rises in the east-south-east seventy minutes before sunrise on the 1st and in the east only 51 minutes before the Sun on the 30th. Dimming from magnitude -4.3 to -4.1, its gibbous disk shrinks from 22 to 17 arcseconds in diameter.
It is less than a month since results from NASA’s WISE spacecraft appeared to rule out any Jupiter or Saturn-sized planet lurking unseen in the outermost solar system. Now we learn that a new dwarf planet, dubbed 2012 VP113, has been found to have an orbit that comes no closer to the Sun than 80 times the Earth’s distance, further than any other known object in the solar system. Thought to be a ball of rock and ice perhaps 450 km wide, it may be six times further away at its farthest, and take perhaps 5,000 years to complete each orbit.
Surprisingly, 2012 VP113’s orbit is similarly orientated to those of some other remote bodies, including the only other comparable object, Sedna. There is speculation that this is because they are influenced by a larger undiscovered world, perhaps a super-Earth, even further out.