The Night Sky in October, 2016

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1 The Night Sky in October, 2016 At the beginning of October, the Sun will rise at 07:12 BST and set at 18:45 BST. By the end of the month it will rise at 07:07 GMT and set at 16:38 GMT. So let s hope for some clear, dark and warm Autumnal nights for stargazing! There will be a New Moon on October 1 st and a Full Moon on October 16 th, so the beginning and end of the month will be the best for stargazing. The star chart below shows the sky in the East and Southeast at 06:00 BST on October 1 st. Here you can see the planet Mercury just rising in the East. It is in the constellation of Leo the Lion which is easy to spot since the head of the lion looks like a backward question mark. At the top right of the chart you can see the bottom half of the great winter constellation of Orion the Hunter. You can just see two of the stars of Orion s belt Alnitak and Alnilam. Note that Orion is followed by his hunting dogs Canis Minor with its bright star Procyon and Canis Major with its bright star Sirius. Sirius is the brightest star in Earth s night sky. The 2 nd brightest is Canopus in the Southern hemisphere. Sirius is actually a binary star. Sirius A is a bright bluish white star which means it is quite young about 200 million years old in fact. It has a temperature of 8,858 degrees compared with our sun which is 6,000 degrees. Sirius B is a white dwarf star which was left when a red giant star collapsed. These two stars, A and B, orbit around each other forming a rotating binary star. Sirius is quite close to us astronomically speaking. It is 8.6 light years away where a light year is the distance light can travel in one year and is 6,000,000,000,000 (6 trillion) miles! It is interesting to compare its brightness with that of Mercury. Sirius has a magnitude of whereas Mercury s magnitude is Remember that magnitudes are like golf handicaps - the lower the magnitude the brighter the object. So Sirius significantly outshines the planet Mercury! What about the other planets? Neptune was at opposition last month and can still be seen low in the South around midnight but binoculars are needed. Jupiter is on its way back into our skies but is not

2 favourable to observe at the moment. Venus, Mars and Saturn are low down in the sky and also not favourable to view. But, as I mentioned in my last newsletter, Uranus is at opposition on October 15 th. This means it is at the opposite side of the earth from the sun and so will reflect the maximum amount of sunlight from its disc. Unfortunately, there will be a full moon when the planet is at opposition so the star chart below shows the sky in the South at midnight on October 20 th. Note what a rich part of the sky this is! You can see that Uranus is in the constellation of Pisces the Fish. To the right you can see a circle of stars which form the head of the fish. This is a famous asterism which is known as the circlet. Note that an asterism is a distinctive pattern of stars which do not form a constellation. The most famous asterism in northern skies is the Plough which is part of the constellation of Ursa Major the Great Bear. Above the circlet is the Great Square of Pegasus. This is a magnificent constellation to view in the Autumn. The star in the top left of the square is called Alpheratz. Leading from it are two lines of stars which form the constellation of Andromeda. Above this is the Andromeda Galaxy (M31) with its two companion galaxies M110 and M32. To the left of the chart you can just see the constellation of Taurus the Bull and to the right of it the Pleiades star cluster. There are a lot of photo opportunities in this part of the sky! If you have very dark skies you may just be able to see Uranus with the naked eye. It is a good target for binoculars but a telescope is required to see its greenish-blue disc and some of its moons. The first image below shows you what Uranus will look like at midnight on October 20 th. Note that Uranus has 27 moons in total and many of them are named after Shakespearean characters! There is something else of interest in the above star chart. Near the bottom of the chart and below Uranus you can see an object called Ceres. This was once known as a large asteroid but it is now classified as a dwarf planet. Ceres is 946 km in diameter and is one of the five dwarf planets the others being Pluto, Haumea, Eris and Makemake. Ceres is the smallest of these and is only a tiny dot

3 when seen from earth. However, it is at opposition on October 21 st when you should be able to see it with binoculars. The second image below shows you what Ceres will look like through a large telescope. October is the month when we expect to see the Orionid meteor shower. This occurs when the earth passes through the cloud of dust left by the most famous comet of all Halley s comet. This comet visits earth s skies once every years. It was last in our skies in 1986 and won t be back again until 2061! But at least we can see the bits of it left behind! When these come into our atmosphere, they burn up and we see the streaks of light we call meteors.

4 The Orionids are called this because the radiant (the point in the sky from which all the meteors seem to emanate) is near the constellation of Orion. The shower will be visible from October 2 nd until November 7 th but the peak will occur on the night of October 21 st. About meteors per hour are expected. The star chart below shows the sky in the East at 23:55 on October 21 st. You can see that a waning gibbous moon is in the way and will spoil the view a bit. You should be able to see the meteors from 9pm onwards but they will be better after midnight. Now to the Southern hemisphere!

5 What's Up in the Southern Hemisphere? The star chart below shows the sky in the West over Sydney at 22:00 on October 1 st As you can see, you have the pleasure of being able to observe the planet Saturn close to the giant red star Antares in the constellation of Scorpius. Vertically above Saturn you can also see Mars in the constellation of Sagittarius. It is surrounded by the rich number of Messier objects which are to be found in that region of the sky. You can also see the constellation of Crux the Southern Cross in the Southwest in the bottom left of the chart. Above this and to the right are the two pointer stars Hadar and Rigil Kentaurus (also known as alpha and beta Centauri). The star chart below shows the sky in the Northeast over Sydney also at 22:00 on October 1 st. You also can see the planet Uranus in the constellation of Pisces with the circlet above it. To the left is the Great Square of Pegasus with the Andromeda constellation and the Andromeda galaxy below it.

6 So will you be able to see the Orionids? The next star chart shows the sky in the East over Sydney at 23:59 on October 21 st. You can see that Orion has just risen above the Eastern horizon so you should be able to see the Orionids after midnight. Also to the right you can see Orion s hunting dog Canis Major with its bright star Sirius. This is the brightest star in the whole of the sky. However, you have the interesting opportunity to compare its brightness with that of Canopus. This is the second brightest star in the sky and you can see it in the top right of the star chart. Canopus has a magnitude of whereas Sirius s magnitude is Remember that the lower the magnitude, the brighter the star so Sirius is significantly brighter. To the left of the chart you can also see that the constellation of Taurus the Bull has risen. Just above the giant red star Aldebaran (the red eye of the bull) you can see a V-shaped cluster of stars called the Hyades. To the left of this you can see the beautiful Pleiades. So you also have plenty of photo opportunities! Well that is all until next month. Meanwhile happy stargazing! Dark skies! Valerie Calderbank FRAS