The Night Sky in October
British Summer Time (BST) comes to an end at 2am on Oct 25, clocks being set back by one hour to mark a return to Greenwich Mean Time (GMT).
A full Moon on Oct 1 is the first of two this month. The second occurs on Oct 31. The Moon’s phase-to-phase cycle is about 29.5 days (29 days, 12 hours, 44 minutes and 2.8 seconds, if you want to be precise). This is the Moon’s synodic month and is of such a length that the phases of the Moon drift in synchronisation with the calendar months. If the drift causes a full Moon to occur right at the start of a month, there’s a good chance another can be squeezed in before the month is over.
You may hear the term “blue Moon” attributed to the second of October’s full Moons. This is an incorrect term created from a bit of misreporting in 1946. The correct definition of a blue Moon is from the Maine Farmers’ Almanac. This states that a blue Moon is the third full Moon in a sequence of four which may occur in a season. As this is harder to understand than “the second full Moon in a month” the incorrect definition has gained popular traction.
During the 2020s autumn season, full Moons occur on Oct 1 and 31 and Nov 30. The full Moon of Dec 30 occurs after the December solstice and is technically the first full Moon of winter. Ironically, using the correct definition, there is no blue Moon this autumn.
A full Moon represents the Moon at opposition. This term is used to describe an object in the opposite part of the sky to the Sun. Mars, too, is at opposition this month, on Oct 13. The Red Planet is closest to Earth on Oct 6. On this date Mars, 38.6 million miles away, appears at its brightest and largest when seen through a telescope. To the naked eye the planet appears like an intense orange-hued star, almost half way up the sky around midnight GMT (1am BST).
It’s currently summer for the Martian southern hemisphere and a telescopic view shows the now rather depleted southern polar cap. The planet’s tilt is such that the north polar cap is hidden from view, but a veil of cloud is currently forming over it and through a telescope this can be seen creeping around the northern edge.
Look towards the north-east early in the evening to see late autumn stars. This includes the beautiful Pleiades open cluster, part of Taurus the Bull. The Pleiades formed within the last 100 million years and lies at a distance of 444 light years. Like all open clusters, its component stars will eventually disperse into the background of the Milky Way.
The Pleiades are sometimes called the Seven Sisters. You may think this indicates how many members you can see with the naked eye, but with good eyesight, a steady, dark night will reveal nearly twice that number. Binoculars reveal more.
In the period running up to midnight, an old friend makes an appearance over the eastern horizon. Orion the Hunter is a constellation traditionally seen at its best in late autumn and winter months. The main star in the north-east of the constellation is red supergiant Betelgeuse, an old, massive star running out of fuel. Betelgeuse is a useful marker for the radiant, or apparent point of origin, of the Orionid meteor shower, which peaks on the night of Oct 20-21. This is one of two annual meteor showers caused when Earth passes through the debris stream around the orbit of comet P1/halley. It is best witnessed from midnight BST through to 05:45 BST.