The Night Sky in September
The main planets are conspicuous by their absence this month, embedded either in the evening or morning twilight. Two exceptions are Uranus and Neptune but as Uranus is on the limit of naked eye visibility and Neptune requires optical assistance to see, they aren’t always obvious. Neptune is at opposition on Sept 5, when it’ll be opposite the Sun in the sky and visible all night long.
Jupiter and Saturn belong to the evening sky. Jupiter appears bright but is very low in the west-southwest shortly after sunset. Saturn is dimmer and, being further east, is visible for longer against darker skies. It currently appears to be balancing on the knee of Ophiuchus, the Serpent Bearer, and from the UK is rather low. If it is clear on Sept 26, look for the waxing crescent Moon low in the southwest after sunset. Saturn is the brightest object near the Moon. The other planets appear clustered in the morning sky, creating an ever-changing combination of close passes with each other and the background stars. On Sept 5, Mars appears close to the bright star Regulus in Leo. Five days later, on Sept 10, Mercury appears close to the same star. On Sept 16 and 17, Mercury catches up with Mars and both planets appear very close. On Sept 18, Mercury, Mars, a waning crescent Moon and Venus appear in a line. Finally, on Sept 20, it is Venus and Regulus that appear close to each other.
This month also marks the end of the Cassini spacecraft’s historic mission to Saturn. The probe will be made to descend into the planet’s atmosphere on Sept 15. Cassini was launched in 1997 and has been sending back amazingly detailed data and images of this beautiful world ever since it entered orbit in 2004. In December of that year Cassini released the Huygens probe, which had been piggybacking a ride. The Huygens probe eventually descended through the atmosphere of Saturn’s giant moon, Titan, landing on its surface on Jan 14 2005.
On Sept 22 the Sun’s centre crosses the projection of Earth’s equator in space. This equinox marks the movement of the Sun from the northern half of the sky into the southern. For those living in the northern hemisphere, this is known as the autumn equinox. An equinox marks the time of year when the length of night is changing at its fastest rate. For the Northern Hemisphere this means the nights are rapidly increasing in length, which is great news for stargazing. The most prominent star pattern visible in the south near to midnight is the Great Square of Pegasus, part of Pegasus the Flying Horse. The northeast (upper-left as seen from the UK) star in the square doesn’t actually belong to Pegasus at all, being Alpheratz, the alpha star of Andromeda the Chained Princess.
The Great Square of Pegasus represents the torso of the flying horse. His head is marked by a squat, down-pointing, isosceles triangle southwest of the Great Square.
If you follow the western (left side from the UK) side of the Great Square south, you’ll arrive at the low and bright star Fomalhaut, the brightest in Piscis Austrinus, the Southern Fish. This star is surrounded by several rings of debris and in 2008 it was announced that the Hubble Space telescope had imaged an exoplanet here. However, further study suggested this was nothing more than a condensed dust cloud. In 2012 it was announced that Fomalhaut b, as it became known, was probably a gravitationally bound collection of rubble; a cross between a dust cloud and a planet.