September sky at night — click on the image below to see a larger version

September is marked on our mental calendars as the end of summer, but astronomically speaking the year is much more advanced than memories of warm back-at-school days might suggest. The Sun crosses the celestial equator at 16.44 BST on the 22nd, and this point thus marks the autumnal equinox and the halfway point between midsummer and midwinter.

The Moon provides a guided tour to a series of more obscure highlights this month. As September begins, its waxing crescent will be visible alongside Venus just after sunset. Venus is very low but at magnitude -3.8 it should be bright enough to pick out during twilight. Mars and Mercury are here too, but are so faint that all but the most eagle-eyed (or imaginative!) observers will need optical aid to find them against a bright sky.

By the morning of the 13th, the Moon will be passing through the normally obscure constellation of Capricornus, the “sea goat”. This region of the sky contains many watery constellations, but the invention of a sea goat, although it dates back to at least Babylonian astronomy, has always seemed something of a stretch to me. Nevertheless, Capricornus is currently presided over by Neptune himself, and at about 2.30 BST on the 13th the Moon will cover, or occult, the outermost of the giant planets. A close passage with Uranus follows on the 14th, when the Moon is almost full.

At the equinox just over a week later, the last quarter Moon will lie just four degrees to the northwest of the open cluster M35 in the constellation of Gemini. This fine open cluster was first discovered by the French astronomer, Jean-Philippe de Cheseaux who is otherwise remembered only for his discovery of two rather obscure comets and his attempt to pin down biblical chronology using astronomical references in the Book of Daniel. If such research doesn’t distract you, then M35 is an excellent target for binoculars, which will reveal the cluster of young, and therefore blue, stars that form its heart.

To see the cluster best, wait a few days for the Moon to move away from the area and then find the cluster by following the line of stars that lead away from Castor, the brighter of the two twins. M35 lies just above the uppermost star in the chain, Eta Geminorum.

By the 25th the Moon, now a waning crescent, will have moved into the neighbouring constellation of Cancer, and will lie just below M44, the Beehive Cluster. Like M35, the Beehive is an open cluster, and it is brighter, being visible to the naked eye as a nebulous patch set among the stars that make up the crab’s body. This object, one of the closest of clusters at a distance of not much more than 500 light-years from us, fits perfectly into the field of view of a small pair of binoculars.

The Moon enters Leo a day or so later, and on the 27th makes its closest approach to Saturn, now appearing in the morning sky. The ringed planet reaches conjunction on the 4th, but by the end of the month is rising two hours earlier than the Sun. The rings already appear extremely thin, but will become even harder to spot as we head towards the crossing of the ring-plane by the Earth in September 2009.

Returning to the evening sky, Jupiter in the southern constellation of Sagittarius is the undoubted star for the show, due south at dusk. Being bright, at magnitude -2.3 and low down it is sure to attract attention, both from the astronomically inclined and those who might report the presence of alien spacecraft.

Its four brightest moons — Io, Ganymede, Europa and Callisto — were discovered by Galileo with optical equipment far inferior to modern binoculars. To get more than a fleeting glimpse, find a tripod or other support and give your eyes time to pick out the point-like satellites from the glare of their parent planet. Once you have managed that, come back on each clear night to watch them travel on their orbits, and interact in their never-ending series of transits, eclipses and occultations.