The Night Sky in October
Back in August, the European Southern Observatory confirmed the detection of a planet in the habitable zone of the Sun’s nearest stellar neighbour, Proxima Centauri. Named Proxima b, this Earth-sized planet orbits Proxima at a distance of 4.7 million miles or one-twentieth the Earth-Sun distance. Here it receives just 2% of the visible light we experience from the Sun on Earth. This twilight illumination increases to 65% in the infra-red but is 400 times more powerful at x-ray wavelengths. Proxima itself is a flare star, so despite its tantalising proximity, Proxima b has the potential to be a harsh world. Proxima Centauri is a member of the Alpha Centauri triple star system and not visible from the UK. It is best seen from the Southern Hemisphere.
Closer to home, the Juno spacecraft is now in polar orbit around Jupiter and is already sending back incredible images. It’s now settling into its 37-orbit science phase with the promise of more amazing discoveries ahead. Jupiter is currently visible low in the east just before sunrise, appearing close to Mercury between 10-12 October.
Further afield, a showpiece of the The Pleiades or Seven Sisters cluster autumn sky is the Pleiades or Seven Sisters open cluster. During October this can be seen in the east-southeast around midnight and on a cold, dark night it sparkles like diamonds on black velvet. The cluster is 444 light years away and contains around 500, mostly faint, stars. There are more than seven brighter ones visible to the naked eye and it can be entertaining to try to count them. The Pleiades formed during the Cretaceous period on Earth and will probably exist as a cluster for another 250 million years.
Perseus, the Greek Hero, lies to the north of the Pleiades, his shape resembling a lower case Greek letter Pi. Beta Persei or Algol is an interesting star because it dips in brightness every 2 days 20 hours and 49 minutes. The dip is noticeable to the naked eye, lasting for around 10 hours. This is an example of an eclipsing-binary with two unequal brightness stars separated by 5.8 million miles in close orbit around their common centre of gravity. The orbit is such that from Earth we see the stars partially eclipse one another. When part of the dim one is hidden by the bright one, the dimming is very slight. However, when the dim one partially covers the bright one, the dip is very noticeable. In Greek mythology Perseus cut off the head of the Gorgon Medusa. In the sky he is shown carrying the head with Algol representing Medusa’s winking eye.
During October we get a reminder of Halley’s Comet thanks to the Orionid meteor shower which peaks on the 21st. This is caused by Earth passing through the dust debris strewn around Halley’s orbit. An almost last quarter Moon will cause some interference with this year’s peak but it’s still worth going out to have a look. Orionid meteors appear to emanate from a location close to the star Betelgeuse in Orion.
There’s no moonlight interference at the start and end of the month. This is because the Moon is new on 1 October and then again on 30 October. A second new Moon in the month is known as a ‘Black Moon’. The 30th is also significant because it marks the end of daylight savings in the UK, the clocks going back by 1 hour at 2am.