Starwatch: the September night sky

The Sun is tracking southwards at its fastest for the year when it crosses the celestial equator at the autumnal equinox on the 23d. As darkness arrives earlier, the daily westwards shift of our evening sky appears to slow so that the Summer Triangle still lingers near the meridian at nightfall.

Our nights belong to the conspicuous planet Jupiter which rises in the ENE some 80 minutes before our map times and climbs high into the S by dawn. Brightening from mag -2.7 to -2.8 it is unmistakable as it creeps westwards in Aries. Telescopes show its parallel cloud bands, now restored to near-symmetry about the equator after one of them went awol last year. Jupiter’s disc is 47 arcsec wide when the planet lies alongside the Moon on the night of the 16th.

Mars rises in the NE at 01:30 BST on the 1st, stands 7° to the right of Pollux in Gemini and forms an “L” with the other (fainter) twin Castor as they climb to stand more than 30° high in the E before dawn. At mag 1.4, Mars is between Castor and Pollux in brightness and improves to mag 1.3 by the month’s end as it draws closer and tracks eastwards into Cancer. Look for its reddish light in line with Castor and Pollux on the 16th, above-left of the waning Moon on the 23rd and (with binoculars) only 0.7° above-right of the Praesepe star cluster on the 30th.

Venus and Saturn are too low in Britain’s W evening twilight to be seen, but Mercury is well placed before dawn. Between the 1st and the 9th, it rises 100 minutes before the Sun, climbs to stand 8° high in the E 40 minutes before sunrise, and brightens from mag 0.2 to -0.9. It also moves from 9° above-right of Regulus to pass only 0.7° above-left the star on the 9th. The small planet, now with Nasa’s Messenger probe in orbit, should be visible to the naked eye and may be followed for a further week before we lose it in the twilight.

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