Mars greets a rival and two new orbiters
The Summer Triangle, formed by the bright stars Vega, Deneb and Altair, still has pride of place high in our southern sky at nightfall. Mars and Saturn are visible on our September evenings, too, but we must look low in the south-west to catch them. Both are well past their best and less interesting telescopic targets than Jupiter which is now resplendent in the east before dawn.
Having swept 3°, or six Moon-widths, to the south of Saturn on 27 August, Mars has a trio of further notable encounters later in September. Two new spacecraft, NASA’s MAVEN and India’s MOM or Mangalyaan, are on course to enter orbit around Mars on the 21st and 24th respectively while the planet is due to pass 3° north of the enormous red supergiant star Antares in Scorpius on the 27th. The name Antares comes from the Ancient Greek for “rival to Mars” and, while they may indeed be similar in brightness by the month’s end, it will be fascinating to see how their colours compare.
Meanwhile, Mars, or rather the spacecraft in orbit around it, are due for a more challenging encounter when the icy nucleus of comet C/2013 A1 Siding Spring sweeps within some 130,000 km of the planet on 19 October. The operators of NASA’s Mars Reconnaissance Orbiter and Mars Odyssey, and of Europe’s Mars Express, are arranging to shield their craft from the worst of the dust storm that is likely to be accompanying the comet, and similar precautions may be needed for MAVEN and MOM.
In other space news, Europe’s Rosetta craft is now studying five potential landing sites for its Philae lander on the nucleus of Comet Churyumov-Gerasimenko. The landing is not due until November, but it is planned to choose a primary and a backup site this month as Rosetta closes to with 30 km of the nucleus.
By our star map times, the Summer Triangle lies just west of our meridian as it gives way to the stars of autumn led by the topsy-turvy winged horse Pegasus whose nose is marked by the star Enif. Use binoculars to look 4° north-west of Enif for the star cluster M15 which appears as a fuzzy blob less than half as wide as the Moon. In fact, it is one of the finest globular clusters in the sky and contains more than 100,000 stars at a distance in excess of 30,000 light years.
The Sun tracks 11.5° southwards in the sky during September and crosses the equator at 03:29 BST on the 23rd, the time of this year’s autumnal equinox. Sunrise/sunset times for Edinburgh change from 06:17/20:07 on the 1st to 07:13/18:51 on the 30th as the duration of nautical twilight at dawn and dusk falls from 89 to 80 minutes.
The Moon is at first quarter on the 2nd, full on the 9th, at last quarter on the 16th and new on the 24th. As the full moon nearest to the equinox, the one on the 9th is also our Harvest Moon and, since it comes less than a day after the Moon is closest to the Earth, it is yet another supermoon.
Saturn stands about 11° high in the south-west and only 0.3° above the northern tip of the crescent Moon as the evening twilight fades on 31 August, with Mars another 4° below and to their left.
On 27 September, the young Moon returns to lie 6° to the right of Saturn which, by then, is 4° lower in the sky and becoming hard to spot in the twilight. Both planets begin the period at magnitude 0.6, but Mars dims slightly to magnitude 0.8 by the 29th when it stands 5° below the Moon and 3° above Antares. It is also 20° to the left of Saturn and drops below Edinburgh’s horizon at 20:51 BST. Viewed through a telescope, Mars is only 6 arcseconds in diameter at midmonth, while Saturn is 16 arcseconds wide within rings that span 36 arcseconds and have their north face tilted 22° towards us.
After Mars and Saturn set, the sky is devoid of bright planets until Jupiter rises more than five hours later. True, Neptune and Uranus are binocular objects at magnitudes of 7.8 and 5.7 in Aquarius and Pisces respectively, but we need better charts to identify them.
There is no mistaking Jupiter, though. The conspicuous giant planet rises at Edinburgh’s east-north-eastern horizon at 03:29 on the 1st and by 02:07 on the 30th. climbing well clear of the eastern to south-eastern horizon by dawn. As it brightens slightly from magnitude -1.8 to -1.9, it also tracks 6° eastwards, below and away from the Praesepe or Beehive star cluster in Cancer. Look for the waning earthlit Moon 6° below and right of Jupiter before dawn on the 20th. Viewed through a telescope on that morning, the cloud-banded Jovian disk is 33 arcseconds across.
Venus is also a morning object and, although it remains brilliant at magnitude -3.9, it is sinking deeper into the twilight as it approaches conjunction on the Sun’s far side in October. On the 1st, it rises 87 minutes before the Sun and stands 14° below and left of Jupiter as it climbs 12° above our eastern horizon by sunrise. Jupiter soon leaves it behind, though, so by the 30th it rises 32 minutes before the Sun and is only 6° high at sunrise. Viewed telescopically, its almost full disk is only 10 arcseconds across.
The other inner planet, Mercury, moves to lie 26° east of the Sun on the 21st, but hugs the western horizon at sunset and is not observable from our latitudes.