Sky Diary

The Sky at Night

The constellation Perseus follows Andromeda to its highest point in the evening skies of December.  Perseus was the hero of the epic Greek tale of the rescue of Andromeda from the sea monster, Cetus, and her return to her grateful parents Cassiopeia and Cepheus.  These constellations are grouped quite closely together in the sky with Perseus’ steed, Pegasus, in the south-west during December evenings.
Perseus is one of the classic 48 constellations listed by Ptolemy in the 2nd century.  The constellation does not contain any particularly bright stars but is easy to spot as an inverted ‘Y’ between Andromeda and Auriga, marked by the bright star Capella.  The brightest star of Perseus is Mirfak, a supergiant star which is rather bigger and hotter than our Sun and is at a distance of 510 light years.  Better known, but a bit less bright than Mirfak, is Algol, the Demon Star, given this name because of its variable brightness.  The brightness of Algol dips regularly every 2.86 days for about 10 hours then returns to its previous brightness.  The mechanism for this very precise change in brightness was correctly proposed by John Goodricke in 1783.  He suggested that a dark body was passing in front of the star and it was proven to be correct in 1881 and that the dark body was a less bright companion star eclipsing the brighter star.

Perseus is also the location of a fine double star cluster, simply knows as the Double Cluster, which are open clusters within out Milky Way Galaxy at a distance of 7,600 light years.  Both clusters contain a few hundred bright, hot stars and are believed to be young with an age of only about 13 million years.  They make a fine sight in a pair of binoculars and can be seen by the unaided eye from a dark location.  Swing your binoculars gently from the top of Perseus to the W of Cassiopeia and the Double Cluster will be hard to miss.

Last month I mentioned the favourable but rather sparse meteor shower, the Leonids.  December brings us one of the best meteor showers of the year which will be very favourable because of the 26 day old Moon at the time of the maximum of this shower.  The Geminid meteor shower can be seen between the 8th and 17th December but maximum occurs at 2am on the 14th December.  Around that time there could be more than 100 meteors per hour and these tend to be quite slow with a high proportion of bright meteors.  The source of the particles which produce the meteors appears to be an asteroid, 3200 Phaethon, rather than a comet.  It has an orbit of the Sun which takes 1.4 years and has been called a ‘rock comet’ which may account for the larger size of the particles shed by it and the relatively slow and bright meteors.  This meteor shower is worth watching, so choose a nice dark location and make sure you are prepared for cold conditions!

The Planets

  • Mercury will be low in the south-east from around the 20th December, rising one and a half hours before the Sun by the end of the month.
  • Venus is not visible throughout December.
  • Mars rises at 3.30am, about 4 hours before the Sun and can be seen towards the south at around 7am, to the right of Jupiter which is significantly brighter and more noticeable.
  • Saturn is not visible this month.
  • The Moon is full on the 3rd, at last quarter on the 10th, new on the 18th and at first quarter on the 26th November.

Mills Observatory is open from 4pm – 10pm, Monday to Friday and 12.30pm – 4pm Saturday and Sunday.  At Christmas the observatory will be closed from the 24th to the 26th December. There will be planetarium shows on Fridays 1st and 8th December at 7pm, 7.45pm, 8.30pm and 9.15pm.  Booking for these events is essential on 01382 435967 and there is a charge of £1 for adults and 50p for children.  With the exception of these events there is no charge for entry to the observatory.

The winter series of monthly astronomical talks continues on Saturday 16th December from 2 – 3pm with a talk about alien surface simulation. These talks are more suitable for adults and older children. There is no charge for these talks but booking at 01382 435967 is essential.

Sky at Night Chart