Scotland’s Sky in September, 2016

Harvest moon eclipsed on the 16th

The maps show the sky at 23:00 BST on the 1st, 22:00 on the 16th and 21.00 on the 30th. An arrow depicts the motion of Mars from the 21st. (Click on map to enlarge)

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 22^nd, 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 1^st to 07:14/18:50 on the 30^th. The moon is new on the 1^st, at first quarter on the 9^th, full on the 16^th, at last quarter on the 23^rd and new again on 1 October.

Jupiter is now lost from view as it nears conjunction on the Sun’s far side on the 26^th. 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 9^th and above Mars on the 10^th 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 24^th 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 28^th and is magnitude -0.5 when it lies alongside the slender earthlit Moon on the 29^th.

Just a day later, on the 30^th, 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 8^th, 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.

Alan Pickup

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.