WHAT'S UP THIS MONTH - SEPTEMBER 2018

(Link to What's Up June 2018)

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

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: Jupiter (early evening) Saturn, Mars, Uranus and Neptune.

 

EXPLORING THE NIGHT SKY THIS MONTH

The Southern Night Sky during September 2018 at 22:00 BST

The chart above shows the night sky looking south at about 22:00 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 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), Pisces (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 (the Milky Way) is located in Sagittarius so the richest star fields can be found in the constellation. Also many interesting deep sky objects are here along with the planets Saturn and Mars this year.

The summer constellations are still prominent in the night sky lead by Hercules (the strong man). 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 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 ‘Pole' or ‘North Star') is located in Ursa Minor. Polaris is the star that is located at the approximate position 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 as Earth rotates every 24 hours.

To west of the Summer Triangle is the constellation of Hercules (the strong man). The main feature forming the asterism (shape) of Hercules is the misshapen square at its centre known as the ‘Keystone' due to its resemblance to the central stone of an arch. Located in the right vertical side of the ‘Keystone' is the most impressive ‘Globular Cluster' known as Messier 13 (M13). This can be seen in a modest telescope as a beautiful ball of about a million stars.

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.

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).

The Coathanger star cluster 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.

 

COSTELLATION OF THE MONTH - HERCULES

 

The constellation of Hercules

The chart above shows the constellation of Hercules and its location to the west (right) of the Summer Triangle. Hercules is the great strongman from Greek mythology. He is illustrated in the picture below (up-side-down), as he appears in the sky, with a club held above his head. The ‘Keystone' asterism (shape) can be a little difficult to identify in a light polluted sky but easy to find again.

An illustration of the constellation of Hercules

The jewel of Hercules is without doubt is the Great Globular Cluster, Messier 13 (M13). M13 can be found in the western (right) vertical imaginary line of the ‘Keystone'. It is just visible using a good pair of 9 x 50 binoculars. The cluster, of about a million stars, can be seen using a 90mm f 10 telescope but will look even more impressive when using a larger telescope.

The Great Globular Cluster in Hercules

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.

 

THE SOLAR SYSTEM SEPTEMBER 2018

MERCURY is an early morning object rising in the east at 04:00 at the beginning of the month. It will be possible but very difficult to see in the beginning of September. It is low in the east as the sky begins to brighten. Mercury will be moving back towards the Sun during the month and will move into Superior conjunction (passing behind the Sun) on 21 st September. See the chart below.

Mercury in the east at sunrise 06:00

VENUS is moving back towards the Sun and will just be visible above the western horizon soon after the Sun sets. It sets at about 19:30 so will need a clear view towards the western horizon after the Sun has set at about 18:40. See the chart below. Venus is moving into Inferior conjunction (passing in front of the Sun) on 26th October. Although Venus has regular conjunctions with the Sun it rarely actually passes across the surface of the Sun as a ‘Transit' or passes directly behind the Sun in an ‘Occultation'. It usually appears to pass above or below the Sun due to the slight tilt in its orbital plane. This is same for the other planets. See the special ‘Notes about Oppositions, and Conjunctions below'.

Mars, Saturn, Jupiter and Venus on 1st September

MARS will be well placed this month for observing, in the constellation of Sagittarius but is very low over the southern horizon positioned in turbulent and smoggy air. .The Red Planet passed through ‘Opposition' on 27th July so is still close to Earth. It appears fairly large at 20.0 arc-seconds in diameter and is still very bright at magnitude -2.0. See the chart above.

Unfortunately the view of Mars will not be good this year despite the two planets being relatively close together. The Red Planet will be positioned very low in the sky and just above the southern horizon. This means we will be looking through the Maximum amount of our own atmosphere. This is also where there is the most turbulence due to heat rising from the ground and where the mist and smog is at its worst.

A small telescope will show the planet as a bright orange disc with the white ice cap just visible. A larger telescope will show the ice cap in more detail and some surface features may be visible. These are in the form of darker regions that used to be thought (even up to the 1960's) to possibly be vegetation on the surface. Picking out detail may be difficult this year due to the position of Mars being so low in the sky.

Mars imaged in 2010 by Steve Harris

Mars has two very small moons called Phobos and Deimos but these will not be visible in amateur's telescopes.

JUPITER is moving into conjunction with the Sun on 26th November so it is starting to be difficult to observe as it is low in the west. See the chart above. It is still easy to see but detail is starting to be difficult to make out.

Jupiter imaged on 6th July 2018 by Steve Harris

SATURN is well positioned in the south, in the constellation of Sagittarius but low in the sky and in turbulent, smoggy air close to the horizon. A small telescope will show the ring system but a larger telescope will be required to show it well. Saturn has an axial tilt of 27.6° but is always tilted in the same direction as it orbits the Sun. Therefore as we look out from our position, close to the Sun, Saturn's ring appears to tilt up and down as Saturn orbit s the Sun every 30 years . This year the ring system appears almost fully open, it was at its maximum tilt last year 2017. However the view is likely to be rather poor as shown in the image shown below.

Saturn imaged on 16th August 2018 by Steve Harris

URANUS will be in a good observable position this month. It is moving towards its opposition on 23rd October when it will be due south at midnight (01:00 BST). Uranus will be quite high in the south east as soon as the sky is dark. A good pair of 9x50 binoculars will reveal a slightly fuzzy blue, star like, object. A telescope at a magnification of 100x will show it as a small blue/green disc.

Uranus, Neptune and Mars at 01:00 BST 7th September

NEPTUNE will be at opposition (due south at midnight – 01:00 BST) on 7th September so at its best position for observation this year. See the chart above. 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. The chart above shows Neptune at its Opposition on 7th September.

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 the odd small sunspots 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

Last Quarter will be on 3rd September

New Moon will be on the 9th September

First Quarter will be on 16th September

Full Moon will be on 25th September

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