WHAT'S UP THIS MONTH - SEPTEMBER 2017
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THESE PAGES ARE INTENDED TO HELP YOU FIND YOUR WAY AROUND THE SKY
Click HERE for a downloadable PDF white chart - (Close PDF file to return here)
The chart above shows the night sky as it appears on 15th September at 21:00 (9 o'clock) in the evening British Summer Time (BST). As the Earth orbits the Sun and we look out into space each night the stars will appear to have moved across the sky by a small amount. Every month Earth moves one twelfth of its circuit around the Sun, this amounts to 30 degrees each month. There are about 30 days in each month so each night the stars appear to move about 1 degree. The sky will therefore appear the same as shown on the chart above at 10 o'clock BST at the beginning of the month and at 8 o'clock BST at the end of the month. The stars also appear to move 15º (360º divided by 24) each hour from east to west, due to the Earth rotating once every 24 hours.
The centre of the chart will be the position in the sky directly overhead, called the Zenith. First we need to find some familiar objects so we can get our bearings. The Pole Star Polaris can be easily found by first finding the familiar shape of the Great Bear ‘Ursa Major' that is also sometimes called the Plough or even the Big Dipper by the Americans. Ursa Major is visible throughout the year from Britain and is always easy to find. This month it is in the North West. Look for the distinctive saucepan shape, four stars forming the bowl and three stars forming the handle. Follow an imaginary line, up from the two stars in the bowl furthest from the handle. These will point the way to Polaris which will be to the north of overhead at about 50º above the northern horizon. Polaris is the only moderately bright star in a fairly empty patch of sky. When you have found Polaris turn completely around and you will be facing south. To use this chart, position yourself looking south and hold the chart above your eyes.
Planets observable this month: Saturn (early evening) with Uranus and Neptune later in the night .
EXPLORING THE NIGHT SKY THIS MONTH
The Southern Night Sky during September 2017 at 21:00 BST (9:00 pm)
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 or Nadir and is shown 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 Sagittarius (the Archer), Capricornus (the Goat), Aquarius (the Water Carrier), Piscis (the Fishes), Aries (the Ram) and Taurus (the Bull) is about to rise over the eastern horizon.
Just disappearing over the south western horizon is the constellation of Sagittarius (the Archer). It is really a southern constellation but we can see the upper part creep along the horizon during the summer. The central bulge of our galaxy is located in Sagittarius so the richest star fields can be found in the constellation along with many of the beautiful and interesting deep sky objects that we seek out.
The summer constellations are still prominent in the night sky lead by Hercules (the Hunter). Following Hercules is the Summer Triangle with its three corners marked by the bright stars: Deneb in the constellation of Cygnus, Vega in Lyra, and Altair in Aquila. The Summer Triangle is very prominent and can be used as the starting point to find our way around the night sky. The Milky Way (our Galaxy) flows through the Summer Triangle passing through Cygnus, down to the horizon in Sagittarius.
The Milky Way flows north from the Summer Triangle through the rather indistinct constellation of Lacerta (the Lizard), past the pentagon shape of Cepheus and on through the ‘W' shape of Cassiopeia (the Queen) .
At the top, centre of the chart above is the fairly faint constellation of Ursa Minor (the Little Bear) also called the Little Dipper by the Americans. Although Ursa Minor may be a little difficult to find in a light polluted sky it is one of the most important constellations. This is because Polaris (the North Star) is located in Ursa Minor. Polaris is the star that is located at the approximate point in the sky where an imaginary line projected from Earth's North Pole would point to. As the Earth rotates on its axis, the sky appears to rotate around Polaris once every 24 hours. This means Polaris is the only bright star that appears to remain stationary in the sky.
At the top right of the chart (North West) is the constellation of Ursa Major (the Great Bear). The saucepan shape of the constellation is often called the Plough in the UK but is also known as the Big Dipper in the USA. It does actually look remarkably like a saucepan. Four bright stars represent the pan and three stars represent the handle. An imaginary line drawn from the side of the ‘pan' opposite the handle points to Polaris (the Pole Star). See the chart above.
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.
EXPLORING THE SKY AROUND THE SUMMER TRIANGLE
The chart above shows the sky around the Summer Triangle. The term ‘Summer Triangle' was suggested by Sir Patrick Moore and has now become the best known feature of the summer night sky. The corners of the imaginary triangle are positioned on the 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 CONSTELLATION OF AQUILA (the Eagle)
The constellation of Aquila (the Eagle) is found at the bottom corner of the Summer Triangle. There are no interesting objects in Aquila but the one bright star, Altair, has a fainter star above and below it that makes it quite easy to find.
The constellation of Aquila (the Eagle)
THE CONSTELLATION OF CYGNUS (the Swan)
The constellation of Cygnus (the Swan) is located at the top of the Summer Triangle. The brightest star in Cygnus is Deneb and it denotes the upper point of the Summer Triangle and represents the Swan's tail. The wings spread from the star Sadr and the head is marked by Albireo. Deneb is one of the largest and brightest stars in our vicinity in our galaxy (the Milky Way) and is classified as a Supergiant. It is about 25 times more massive than our Sun and has a diameter 60 times that of our Sun. It is located 3000 light years away. As it is so much larger than our Sun it consumes its Hydrogen fuel much faster and consequently shines 60,000 times brighter.
The constellations of Cygnus and Lyra
Cygnus (the Swan) does actually resemble the swan it is supposed to represent. We start at the bright star Deneb which marks the tail of the swan. From the fairly bright star Sadr the wings of the swan are spread out to each side and the long neck of the swan stretches on to the star Albireo.
Albirio can be seen as a beautiful double star when viewed through a telescope. One star is bright and gold in colour the other is fainter and distinctly blue. This is not a true pair of stars they just happen to be in the same line of sight. Although the blue star is much bigger and brighter than the golden coloured star it is a lot further away from us so it looks fainter. This type of double star is much rarer than a pair of stars that are associated, linked by their common gravity and orbiting a common centre of gravity.
The double star Albireo in Cygnus
THE CONSTELLATION OF LYRA (the Harp)
The constellation of Lyra (the Harp) is located to the west (right) of Cygnus but is much smaller. The most obvious feature of Lyra is the very bright star Vega that is located at the top right corner of the Summer Triangle. Vega is the fifth brightest star in our sky with a magnitude of 0.4. It is located at a distance of 25.3 light years from us and is thought to be 3.2 times the diameter of our Sun and 58 times brighter. Inferred telescopes on the IRAS satellite have detected a ring of dust around Vega that may indicate planets are forming around the star.
The constellation of Lyra (small harp)
The main asterism (shape) of Lyra is composed of a line of three stars with Vega in the centre and a group of four fainter stars that form a trapezium shape that is better known as the ‘Lozenge'
To the south east of the very bright star Vega is a lozenge shaped asterism comprised of four stars . Between the two lower stars: Sulafat and Sheliak is the Messier object M57. This is a ‘Planetary Nebula' which has nothing to do with a planet. These objects are called Planetary Nebulae because they looked like a planet when viewed through the more primitive telescopes of the 18 th century. It is in fact a dying star that was originally similar to our Sun but older. The star had used most of its Hydrogen fuel and expanded to form into a Red Giant. After passing though that red giant phase it gently collapsed to become a White Dwarf. The very thin outer mantle of the red giant drifted away into space as the star collapsed. The white dwarf is now surrounded by a bubble of gas and dust. It looks like a small ‘smoke ring' when seen through a telescope but can't be see using binoculars.
Messier 57 (M57) the Ring Nebula
There are two other constellations that are located within the Summer Triangle. They are both small and comprised of relatively faint stars but are worth seeking out using binoculars.
THE CONSTELLATION OF SAGITTA (the Arrow)
Sagitta is good fun to find using binoculars because it really does look like an ‘arrow'. It is composed of three stars that look like the shaft of an arrow and two stars that resemble the flight feathers.
The constellation of Sagitta
The real beauty of Sagitta is how it looks using binoculars but it does host one messier object this is M71 also known as NGC 6838. This is a rather nice but small and faint globular cluster that does need a medium sized telescope to see well .
A telescope will show Messier 71 (M71) in Sagitta. It is not the most spectacular Globular Cluster but does look nice in a medium sized telescope.
Messier 71 (M71) in Sagitta
A Globular Cluster is thought to be the core of a small galaxy that has ventured too close to our large spiral galaxy the Milky Way and had its outer stars stripped away by the powerful gravity of the Milky Way. Some of these small galaxies have been pulled right into the Milky Way and are in the process of being absorbed. There about 100 Globular Clusters in a halo around our Galaxy.
THE CONCTELLATION OF VULPECULAR (the Fox)
The constellation of Vulpecula
Vulpecular is a quite indistinct constellation located in the Summer Triangle, see the chart above. It has a Bright planetary nebula (M27) that can be seen as a small smudge of light using a good pair of binoculars. It is also known as the Dumbbell Nebula but looks more like a butterfly. It is a similar object to M57 but has two interesting lobes. These may have developed because the star had a powerful magnetic field or may have had one or two large planets that disturbed the nebula as it was forming. A telescope is required to see it well.
Messier 27 (M27) a planetary Nebula in Vulpecular
THE CONSTELLATION OF DELPHINUS (the Dolphin)
Just to the east (right) of the lower part of the Summer Triangle is the lovely little constellation of Delphinus (the Dolphin). It is small but can be seen easily with the unaided eye from a dark area when there is a clear sky.
Delphinus (the Dolphin)
The asterism (shape) of Delphinus is comprised of a four stars that form a neat diamond shape and a fifth star a short distance from the diamond shape that completes the dolphin's body and tail. With a little imagination it does look remarkably like a dolphin leaping out of the water. It looks even more striking when using binoculars.
THE CONSTELLATON OF SAGITTARIUS (the Archer)
Our Milky Way Galaxy can be seen on a clear dark night looking like a cloud passing through the Summer Triangle and down the southern horizon. We see the brightest part of the Milky Way just above the horizon in the constellation of Sagittarius. Sagittarius straddles the horizon with its lower half hidden below the horizon.
Sagittarius (the Archer)
This part of the Milky Way in Sagittarius is brighter because we are looking towards the centre of the Galaxy in that direction. Saturn and the bright red star Antares can also be seen in this direction. There are also many beautiful and interesting Messier objects around Sagittarius. A lovely Open Cluster called M11 the Wild Duck Cluster, in the small constellation of Scutum, can be seen using binoculars and iseven more beautiful using a telescope.
THE PLANETS THIS MONTH
MERCURY is an early morning object that rises in the east at 04:00 at the beginning of the month and at 04:45 at the end of the month. Its earliest rising time will be in the middle of the month when Mercury will be at its greatest apparent distance from the Sun. It will be very difficult to see so low in the east as the sky begins to brighten.
Mercury, Venus and Mars in the early morning sky
VENUS is moving back towards the Sun and will be visible above the eastern horizon before the Sun rises. Venus rises at about 03:00 so will need a clear view towards the eastern horizon before the sky begins to brighten at about 04:30. See the chart above.
MARS will be rising east just an hour before the Sun rises. The Red Planet appears small at just 3.5 arc-seconds in diameterand not very bright at magnitude +1.8. Mars will be very difficult to see just above the eastern horizon especially as the sky brightens. See the chart above.
JUPITER will be moving into conjunction with the Sun on 26th September so will not be visible this month. See the chart below. The sky has been darkened on the chart to show the location of the planets.
Chart showing Jupiter and Saturn at sunset on 15th September
SATURN will be in the south east as the Sun is setting and the sky begins to darken. The Ringed Planet appears small at 16.5 arc-seconds in diameter but is quite bright at magnitude +0.5. It is now moving towards the western horizon so will have to be found as soon as it is dark because it sets at 21:45. A small telescope will show Saturn's largest moon and a larger telescope may reveal two or three more. Observation will be difficult this month because Saturn is very low and creeping into the bright twilight sky in the west.
Saturn imaged on 17th June 2017
URANUS will be in a good observable position this month. It will be rising in the east as soon as the sky is dark. It will be better placed later in the evening. Using a good pair of 10x50 binoculars a slightly fuzzy blue, star like, object can be seen. A telescope at a magnification of 100x will show it as a small blue/green disc.
Uranus, Neptune and Saturn in the south at 21:00
NEPTUNE will be visible in the south as soon as the sky is dark. It will be at opposition (due south at midnight – 01:00 BST) on 2nd September so at its best position for observation this year. A telescope will be needed to show Neptune as a small blue/green disc using a magnification of 100x but it is small and difficult to find.
There are still some sunspots to see even though the active phase of the Solar Cycle is drawing to a close. The image below (from SOHO) shows a large sunspot and a group of smaller ones during early July this year.
A large group of sunspots as seen on 11th July 2017
The Sun rises at 05:10 at the beginning of the month and at 06:00 by the end of the month. It will be setting at 18:45 at the beginning and 17:40 by the end of the month. Sunspots and other activity on the Sun can be followed live and day to day by visiting the SOHO website at: http://sohowww.nascom.nasa.gov/ .
THE MOON PHASES IN SEPTEMBER
Full Moon will be on the 6th September
First Quarter will be on 13th September
New Moon will be on 20th September
Last Quarter will be on 28th September
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