The Night Sky in October
The Harvest Moon is the name given to the full Moon occurring closest to the Northern Hemisphere’s autumn equinox. The full Moon on Sept 6 occurred 16 days before, while the full Moon of Oct 5 occurs 13 days after the equinox, and is therefore the Harvest Moon for 2017.
The title comes about because the difference between the daily rise times for the fuller phases of the Moon is minimised around the September equinox. Coinciding with the gathering of the harvest, the bright Moon appears to light the way, being present at similar times from one evening to the next.
On the morning of Oct 5, as the full Moon descends towards the western horizon, brilliant Venus can be seen rising in the east. Look carefully and you should be able to see Mars appearing just below. On the morning of Oct 6, Venus and Mars remain close, separated by two thirds of the apparent diameter of the Moon. This continues over the following mornings, both planets appearing to separate as they head east. As the Moon slips into its later phases, on the morning of Oct 18 there is a wonderful opportunity to see its thin waning crescent close to brilliant Venus in the dawn twilight.
It is around mid-month that the Moon moves out of the way and leaves the middle of the night dark enough to enjoy the stars and constellations. The most prominent pattern on view is that of the Great Square of Pegasus, a collection of four middle-bright stars outlining a large dark void in the sky. Emanating out of the north-east corner of the Great Square is the wedge-shaped pattern representing Andromeda, the Chained Princess.
She gets this title because as daughter of Queen Cassiopeia and King Cepheus, she became victim of her mother’s hubris. Cassiopeia claimed her daughter to be the most beautiful creature of all, and this upset the Nereids, sea nymphs who symbolised beauty. Poseidon, the god of the sea, sent the sea monster Cetus to ravage the land as punishment, and poor Andromeda was chained to a rock as sacrifice to Cetus to stop the destruction. Andromeda, Cetus, Cassiopeia and Cepheus are all depicted in the night sky, along with Perseus, the Greek hero who rescued Andromeda and took her for his wife.
In the sky, Andromeda contains Messier 31, the Andromeda Galaxy. This can be seen with the naked eye under modest dark sky conditions, appearing as an elliptical smudge. Locate it by extending the upper side of the Great Square to the east (left) for approximately the same distance again, bending slightly north as you go. This brings you to the star Mirach. Turn direction by 90 degrees until you reach fainter Mu Andromedae. Keep going a short distance to reach even fainter Nu Andromedae. M31 appears close to this star.
The new Moon on Oct 19 means that the middle of the month is a good time to try to see M31 with the naked eye.
On the evening of Oct 20 it may be possible to see the Moon as a very thin illuminated crescent low in the south-west approximately 30 minutes after sunset. Spotting such a thin lunar crescent can be very hard but a worthy challenge.
This early phase Moon is perfectly timed for the peak of the Orionid meteor shower which occurs on the night of Oct 21/22. Each Orionid meteor you see streaking across the sky results from a tiny fragment of Halley’s Comet vaporising in Earth’s atmosphere.