WHAT'S UP THIS MONTH - SEPTEMBER 2019

(Link to What's Up October 2019)

(Link to What's Up June 2019)

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THESE PAGES ARE INTENDED TO HELP YOU FIND YOUR WAY AROUND THE SKY

The chart above shows the whole 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: Jupiter, Saturn and Neptune and Uranus later in the evening.

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EXPLORING THE NIGHT SKY THIS MONTH

The Southern Night Sky during September 2019 at 21:00 BST

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. It is usually best to look towards the south when observing the night sky. There are two good reasons for this. First we see the Sun, Moon and planets as they appear to move along an imaginary line that we call the Ecliptic (or Zodiac) which is shown at the bottom of the chart. The Sun, Moon and planets are always seen in the south and are never seen towards the north.

Secondly we are able to see more of the sky pass through our view to the south during the night. The point in the sky directly overhead is known as the Zenith and is shown at the centre of the chart. Due to the axis of rotation of Earth being tilted at 23.4º, compared to the axis of Earth's orbit around the Sun, the sky appears to rotate around the star Polaris, shown at the middle top of the chart above. Consequently we see all the sky from the southern horizon, up through the Zenith and to Polaris pass through our view (this is 113. 4º of elevation). This compares to just 66.6º of elevation from the northern horizon up to Polaris.

The brightest stars often appear to form groups or recognisable patterns; we call these ‘Constellations'. Constellations through which the ecliptic passes this month are Sagittarius (the Archer), Capricornus (the Goat), Aquarius (the Water Carrier), Pisces (the Fishes), Aries (the Ram) and Taurus (the Bull) just off to the left and about to rise over the eastern horizon. So the Sun, Moon and planets can appear to be located in these ‘Constellations of the Zodiac'.

Just disappearing over the south western horizon is the constellation of Sagittarius (the Archer). It is really a southern hemisphere 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 this constellation along with many of the beautiful and interesting deep sky objects that we can seek out.

The summer constellations are prominent in the night sky and are 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. See the following pages. 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 (a mythical Queen) .

At the top of the chart above is the fairly faint constellation of Ursa Minor (the Little Bear) also called the Little Dipper in the USA. 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.

Just off the chart to the top right (North West) is the constellation of Ursa Major (the Great Bear). The saucepan shape of the constellation of Ursa Major 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. Ursa Major is ‘circumpolar' this means it never disappears below the horizon and is always in the sky. 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 ‘whole sky' chart at the beginning of this What's Up.

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 IN AND AROUND THE SUMMER TRIANGLE

The constellations of the Summer Triangle

There is still time to search out the Summer Triangle and the interesting objects in and around it. 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. See the chart above.

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 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 which 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 are spread out to each side and the long neck of the swan stretches on to Albireo.

An illustration of Cygnus (the Swan).

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 may not be a true pair they may just happen to be in the same line of sight. Although the blue star is much bigger and brighter than the golden star it is a lot further away from us. 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 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 detectors 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 rhombus shape that is better known as the ‘Lozenge'

To the south east of the very bright star Vega is the 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. It is in fact a dying star that was 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 seen using normal 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.

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. M71 is a rather sparse, small and faint globular cluster. It 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 quite 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). It had its outer stars stripped away by the powerful gravity of the Milky Way. There about 100 Globular Clusters around our Galaxy.

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 just be seen 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.

Messier 27 (M27) a planetary Nebula in Vulpecular

THE COATHANGER (asterism)

Between the small constellations of Vulpecula and Sagitta is a rather delightful ‘Asterism' (pattern of stars) known as the Coat Hanger. It is best seen using binoculars or a small telescope (using a low power eyepiece). It is located about half way between the ‘tail feathers' of Sagitta (the Arrow) and the western (right) star of the three ‘brightest' stars in Vulpecular (the Fox).

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The location of the Coathanger in Vulpecular
The Coathanger asterism in Vulpecular

The Coat Hanger (also known as C399) is sparse Open Cluster of ten 5th to 7th magnitude stars in a pattern that does resemble an up-side-down Coat Hanger. It is easily seen using a pair of 8x50 binoculars and looks very pleasing to the eye. There are other smaller stars in the cluster but the main interest is the Coat Hanger shape.

To find the Coathanger, first locate the two tail feather stars of the Arrow then slowly sweep the binoculars up and to the west (right) and the Coat Hanger should come into view. The asterism is too large to fit into the field of view of most telescopes but looks good in the finderscope.

DELPHINUS (the Dolphin)

Just to the east (left) 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 seen using binoculars.

 

COSTELLATIONS OF THE MONTH HERCULES AND BOÖTES

 

The constellations of Hercules and Boötes

To the west (right) of the Summer Triangle is the constellation of Hercules. Hercules is the great strongman from Greek mythology. The constellation can be a bit difficult to find but once identified is easier to find again. The central distorted square of the Hercules asterism (shape) is referred to as the ‘Keystone' due to its resemblance to the central stone of an arch or bridge.

Hercules has two Messier objects within its bounds and these are both Globular Clusters. These clusters are generally spherical clusters of between 10,000 and 1 million stars. They are thought to be the central core of smaller galaxies that have had their outer stars torn away in close encounters with our Giant Spiral Galaxy that we call the Milky Way. M13 is the brightest ‘Globular' to be seen from the UK. It can be seen using binoculars but looks beautiful when using a telescope. M92 is further away and therefore appears smaller and requires a telescope to see it. See the chart above.

Messier 13(M13)

Messier 92(M92)

Globular clusters are thought to be the cores of small galaxies that have ventured too close to Giant Spiral Galaxies like our Milky Way. The outer stars of these smaller galaxies have been stripped away, by the gravity of the giant spiral, leaving the dense core clusters of between 100,000 and a million stars. There are about 100 Globular Clusters in a halo around the Milky Way.

Further to the west (right) from Hercules is the ‘kite' shape of Boötes (the Hunter). The bright and noticeably orange star Arcturus is easy to find and shows us where Boötes is. Arcturus is a Red Giant star that has a diameter 25 times that of our Sun and is about 115 times as bright as our Sun. It is more advanced in its 'life time' than our Sun and has expanded in diameter and volume to become a Red Giant. It is now producing more heat and energy but as its energy is now spread over a greater surface area so its surface temperature is cooler.

Although not so bright, the other stars in Boötes do form the shape of a traditional ‘diamond' shaped kite. Arcturus is located at the bottom of the ‘kite' shape where the tail would be attached. See the chart above.

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THE SOLAR SYSTEM SEPTEMBER 2019

MERCURY will not be observable this month as it will be too close to the Sun. It will be in conjunction with the Sun (passing just above the Sun) on 4th September. See the chart below.

Mercury, Venus and Mars in the south at midday

VENUS will not be observable this month as it will be too close to the Sun. It was in conjunction with the Sun (passed just above the Sun) on 14th August. See the chart above.

MARS will not be observable this month as it will be too close to the Sun. It will be in conjunction with the Sun (passing just above the Sun) on 2nd September. See the chart above.

JUPITER is still in a good position for observation but is very low in the sky. See the chart below. It will be in the murky and turbulent air close to the southern horizon. The king of the planets was at Opposition (directly opposite to the Sun in the sky) and at its best on the10th June. It is now starting to move towards the western horizon and will set at 23:00 at the beginning of the month and at 21:40 by the end of the month. A small telescope will reveal its cloud markings and will allow the movement of the four moons to be followed. See the special 'Notes about Oppositions, and Conjunctions' below.

Jupiter and Saturn on 15th September at 20:00

SATURN will be in the in the south east as the sky darkens and follows Jupiter along the ecliptic. It will be at its best (due South) at about 20:30. Saturn is also very low and in the murky and turbulent air close to the southern horizon. Saturn was at Opposition (directly opposite to the Sun in the sky) and at its best on the 10th July and is still in a good position for observing. It will require a small telescope 75mm to 100mm and a magnification of about 100x to see the rings. The moon Titan should be visible and up to 4 or 5 other moons using a larger telescope.

URANUS rises in the East at about 20:30 this month. The Ice Giant Planet will reach opposition (due south at midnight – 24:00 GMT) on 28th October when it will be at its best position for observation this year. It will be visible later in the evening using a small telescope as a slightly fuzzy blue, star like, object. A larger telescope with a magnification of 100x or more will show it as a small blue/green disc. See the chart below.

Uranus, Neptune and Saturn in the south at 21:00

NEPTUNE will be at opposition (due south at midnight – 01:00 BST) on 10th September and at its best position for observation this year. A medium sized telescope (100mm to 150mm) will be needed to show Neptune as a small blue/green disc using a magnification of 150x or more but it is small and difficult to find. See the chart above.

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Notes about Oppositions, and Conjunctions

OPPOSITION When one of the outer (Superior) planets is directly opposite to the Sun in the Sky as seen from Earth and Earth is overtaking the planet

CONJUCTION When two (or more) objects are seen in the same direction in the sky

TRANSIT When one object passes in front of another larger object

OCCULTATION When one object is obscured by a larger object passing in front

The table below shows the Sidereal Period of the planets (time taken for the planet to actually complete one orbit of the Sun) and the Synodic Period (the time between successive oppositions seen from Earth as Earth overtakes the planet).

Planet

Sidereal Period

Earth times

Synodic Period (days)

Mercury

88 days

116

Venus

225 days

584

Earth

1.0 year

-

Mars

1.9 years

780

Jupiter

11.9 years

399

Saturn

29.5 years

378

Uranus

84.0 years

370

Neptune

164.8 years

368

Pluto

248.5 years

367

The Inferior Planets (Mercury and Venus) orbit inside Earth's orbit so they pass in front and behind the Sun as seen from Earth. These conjunctions are called Inferior Conjunction (when the planet passes in front of the Sun) and Superior Conjunction (when the planet passes behind the Sun). Unlike the Superior Planets (those with orbits outside Earth's orbit) Mercury and Venus cannot have an Opposition. This is because they cannot align with the planet and the Sun directly 180° on either side of Earth as it overtakes the planet. See the diagram below.

The Planets aligned at Opposition and Conjunction

The Synodic Period is the time taken for a Planet to return to the same position in the sky. From the chart above we can see that although Mercury orbits the Sun in 88 days, it takes 116 days to return to the same point in the sky as seen from Earth due to relative movements of the two planets. This means although Mercury completes four orbits for every orbit of Earth (365 divided by 88 = 4.15) we only see (365 divided by 116 = 3.15) Inferior and Superior conjunctions each year. So the inner planets do not have Oppositions but have two Conjunctions.

The outer planets take over a year to reach successive Oppositions. Mars takes the longest (2.14 years) because it moves the furthest forward on its own orbit. It therefore takes Earth longer to catch up and overtake Mars. The further a planet is out from the Sun the slower it appears to move around the Sun. Earth catches up and overtakes the planets further out quicker because they have not moved so far along their own orbit. We actually see the four outermost planets a little further to the east each year compared to the background stars. So Earth catches up with the outer planets a little more than a year after its last Opposition:

Jupiter About 33 days later

Saturn About 13 days later

Uranus About 5 days later

Neptune About 2½ days later

THE SUN

There may still be some occasional sunspots to see even though the active phase of the Solar Cycle is now over.

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

First Quarter will be on 6th September

Full Moon will be on 14th September

Last Quarter will be on 22nd September

New Moon will be on the 28th September

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