ASTRONOMY GUIDE TO THE NIGHT SKY – September 2021
The chart above shows the night sky at 20:00 on 15th September 2021
Click on the chart to enlarge and click to the side of the chart to close
The chart above shows the night sky looking south at about 21:00 BST on 15th September. West is to the right and east to the left. The point in the sky directly overhead is known as the Zenith and is shown (in red) at the upper centre of the chart. The curved brown line across the sky at the bottom is the Ecliptic or Zodiac. This is the imaginary line along which the Sun, Moon and planets appear to move across the sky. The brightest stars often appear to form a group or recognisable pattern; we call these ‘Constellations’.
Constellations through which the ecliptic passes this month are: Libra (the Scales), Scorpio (the Scorpion) Sagittarius (the Archer), Aquarius (the Water Carrier), Capricornus (the Goat) and Pisces (the Fishes).
Prominent in the southern sky is the Summer Triangle that dominates the Summer Sky and was described in detail previous pages. The triangle is defined by three obvious bright stars: Deneb in the constellation of Cygnus, Vega in Lyra, and Altair in Aquila. The Milky Way (our Galaxy) flows through the Summer Triangle and passes through Aquila and Cygnus. The Summer Triangle is bigger than may be expected but once it has been found it is very easy to find again.
As the Summer Triangle is so easy to find it is very useful to use as a starting place for finding our way around the night sky. To the west (right) of the Summer Triangle and almost overhead is the constellation of Hercules (the Strong Man). Hercules has a distinctive distorted square shape, at its centre, called the ‘Keystone’. This is due to its resemblance to the centre stone of an arch or bridge. The jewel of Hercules is without doubt the Great Globular Cluster, Messier 13 (M13) that can be found in the western vertical imaginary line forming the ‘Keystone’. M13 is just visible using a good pair of 9 x 50 binoculars. The spherical cluster, of about a million stars can be seen using a 90mm f10 telescope but will look even more impressive when using a larger telescope.
To the west of Hercules and moving towards the western horizon is the bright orange coloured star called Arcturus in the constellation of Boötes. Arctaurus is a star similar to our Sun but more advanced and is developing into a Red Giant star that is nearing the end of its ‘life’ as a normal star. It has used almost all of its Hydrogen fuel and has expanded to become a Red Giant, 25 times the diameter of our Sun. At the moment it shines 115 times brighter than our Sun but it is destined to collapse and become a White Dwarf and Planetary Nebula.
To the East of the Summer Triangle is the constellation of Pegasus (the Winged Horse). The main feature of Pegasus is the square formed by the four brightest stars. This asterism (shape) is known as the Great Square of Pegasus. The square is larger than might be expected but once found is easier to find again. The square can be used to judge the seeing condition of the night sky. Under perfect conditions about ten stars can be seen inside the square this would indicate a very good night for observing. If three to five stars can be seen then conditions will still be good. If fewer or none can be seen then stick to looking at the Moon or planets. There is a very nice Globular cluster in Pegasus it is known as Messier 15 (M15). It is a lovely sight to see in a medium to large telescope.
The Ecliptic is low in the sky during the summer months so the Moon and planets appear close to the southern horizon. Saturn and Jupiter are well placed but due to their low altitude will not be at their best for observation this year. The thick, murky and turbulent air will cause the planets to appear quite unsteady.
Follow this link to see the full ‘Monthly What’s Up’ guide to the night sky:
To see a full version of this article and a guide to the night sky with charts, read the Newbury Astronomical Society (NAS) – Monthly Magazines for Beginners on the Beginners website at: www.naasbeginners.co.uk.
All meetings of the Newbury Astronomical Society have been cancelled due to the Coronavirus. However virtual meetings will continue on-line using Zoom. Check the website above.