WHAT'S UP THIS MONTH - SEPTEMBER 2021

(Link to What's Up August 2021)

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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) 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 8 o'clock BST at the beginning of the month and at 10 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 quite easy to find. This month it is high 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 in the evening sky: Venus early evening with Saturn, Jupiter, Neptune and Uranus later.

THE SOUTHERN NIGHT SKY THIS MONTH

The night sky looking south at about 22:00 BST on 15th September

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 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 is described in detail on the following 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 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 f 10 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.

 

EXPLORING THE SKY AROUND THE SUMMER TRIANGLE

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Chart showing 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 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 that denotes the upper point of the Summer Triangle and represents the Swan's tail. The wings spread out 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.

Albireo 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 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. This type of double star is much rarer than a pair of stars that are associated and 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 parallelogram 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 just our eyes or small binoculars because it really does look like an ‘arrow'. It is composed of three stars that look like the shaft of the arrow and two stars that resemble the flight feathers.

The constellation of Sagitta

The real beauty of Sagitta is how it looks using small 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 Dwarf G lobular Cluster that does need a medium to large sized telescope to see well.

VULPECULA (The Fox)

Vulpecular is a quite indistinct constellation located in the Summer Triangle, see the chart above. The pattern of stars that denotes Vulpecular is just three rather faint stars that do not resemble anything like the fox that it is supposed to represent. However it is host to 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

It is thought that as the original star began to collapse to form a White Dwarf the surrounding gas was in influenced by either a strong magnetic force in the star or the presence of one or more planets around the star. This may have caused the gas to be formed into the two lobes that we can see.

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 a 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.

The Coathanger in Vulpecula

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 a 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.

 

THE AUTUMN EQUINOX – 22nd SEPTEMBER 2021

The Spring and Autumn Equinoxes and Summer and Winter Solstices

As astronomers we have a rather confusing view of the sky around us due to the tilt of Earth's axis. There are some very noticeable effects that we take for granted. The first is: how much the position of the Sun in the sky changes from summer to winter.

The reason we have the seasons is because Earth's axis of rotation is tilted 23.4º relative to the axis of the Solar System. This has had a major effect on the evolution of life on Earth and on our view of the sky. Looking at Earth's tilt from a different perspective, Earth's axis is tilted 66.6º from the plane (or equator) of the Solar System. This gives Earth somewhat odd view of space around us including the movements of the Sun, Moon and planets. The first thing we need to do is understand how this tilt works.

The diagram above shows how Earth orbits the Sun and how Earth's axis is always tilted in the same direction. In fact an imaginary line projected into space, from the north of Earth's axis, points into space at a point in the constellation of Ursa Minor (the Little Bear) very close to the star Polaris. This is why we also call Polaris ‘the North Star' or ‘the Pole Star'. The tilt causes the Sun to appear to move up and down in the sky over the progress of one orbit (our year) and produces our seasons.

If we start Earth's orbit at the left position on the diagram above the position of Britain, in the northern hemisphere is tilted towards the Sun and it will be midsummer. When Earth has completed half of its orbit, Britain will be tilted away from the Sun as shown in the right position on the diagram so it will be midwinter. We call the times when the Sun appears at its highest point in the sky the ‘Summer Solstice' and at when at its lowest the ‘Winter Solstice'.

The imaginary line around Earth where the Sun will appear directly overhead on midsummer day, in the northern hemisphere, is called the ‘Tropic of Cancer'. The lowest point at midwinter is called the ‘Tropic of Capricorn'.

Another affect caused by the tilted Earth is that our sky appears to move up and down as Earth's Equator effectively moves up and down from summer to winter. As we have seen Earth continues to rotate with its axis pointing in the same direction. However the point where the Sun appear directly overhead at midday moves north and south.

The apparent movement of Earth's Equator

Our summer occurs when the Equator moves up to the Tropic of Cancer position or down to The Tropic of Capricorn for winter. Britain will be closest to the Equator on 21st June so the Sun will appear at its highest point in the sky at midday on 21st June, the Summer Solstice.

Half way between the Summer and Winter Solstices the Sun will be directly overhead on Earth's actual equator as it appears to be moving south or north. We call these times the ‘Autumn Equinox' and ‘Spring Equinox'. The Autumn Equinox occurs on 22nd or 23rd September and the Spring (Vernal) Equinox occurs on 20th or 21st March. These are the official ‘meteorological' starts of Autumn and Spring.

At the time of the Equinoxes the length of our day and night are the same (12 hours). On midwinter day 21st December the Night will be 16 hours long and the day will be just 8 hours long. On Midsummer Day 21st June the day was 16 hours long and the night was 8 hours long.

In the northern hemisphere the North Pole of Earth's axis is tilted towards the Sun during the summer. This gives the effect of a point on the surface, such as the UK, appearing closer to the equator of the Solar System, called ‘the Ecliptic'. As a consequence the Sun will appear much higher in the sky during the summer with the Moon and planets appearing lower at night.

During the winter nights the Ecliptic appears very high in the night sky and low during the day. In the northern hemisphere the North Pole of Earth's axis is tilted away from the Sun during the winter season. As the Moon and planets appear to move along the Ecliptic they are seen through less of Earth's atmosphere so they appear clearer and steadier. During the summer they appear lower and close to the horizon in dirty and turbulent air

The summer days are warmer because when the Sun is higher so it is shining almost straight down on to the ground. However in the winter the UK is tilted away from the Sun so the light and heat lands on the earth where the ground is effectively sloping away from the Sun. Therefore the light and heat is distributed over longer area due to the slope. So with the light spread over a larger area there will be less heat per unit of area and it will be colder.

 

THE SOLAR SYSTEM - SEPTEMBER 2021

The outer planets in the evening at 21:00 on 15th September

The chart above shows the location of the planets along the Ecliptic. The sky has been darkened to make the planets visible. The planets to the west of the Sun (right) will be visible late evening and early morning sky before sunrise. The planets to the east of the Sun (left) will be visible in the early evening sky after sunset.

MERCURY will be very close the Sun after sunset. Experts may be able to find it in the bright evening sky but it will require a clear view to the western horizon.

VENUS will be visible in the early evening sky as soon as possible after sunset. It will be easy to find but will it require a clear view to the western horizon. Venus is emerging from its excursion behind the Sun when it was in ‘Conjunction' with the Sun. It will appear at its smallest diameter and will be fully illuminated because it is still beyond the Sun from our point of view.

Venus, Mercury and Mars at sunset on 15th September

MARS has now moved out of view and will not appear in the evening sky again until September 2022. It can be seen very close to the Sun in the chart above but it will be too close to the Sun and too small in diameter to be observable.

JUPITER will be rising in the east at about 18:30 and will be visible in the south east by 19:30. Jupiter was at opposition and its best on 20th August. Jupiter will look brilliant and interesting on any clear night but some nights there may be something special to see. The computer generated chart below shows the moons spread out to both sides of Jupiter at midnight on the 2nd September. This spread is always lovely to see but at this time Io will appear to be passing close to Ganymede.

Moons Io and Ganymede close at midnight 2nd Sept

Jupiter and Saturn can be seen in a smaller telescope of the sort that a beginner to astronomy might have been advised to buy as a first telescope by experienced astronomers.

SATURN will be rising in the east at about 18:00 but will be more difficult to observe than Jupiter in the turbulent air close to the horizon. Saturn rises before Jupiter in the east and was at its best for this year on 2nd August when it was at opposition.

Saturn and its brightest moons

A high magnification must be used and on a good clear and still night when the view will be very rewarding. The previous charts show the sort of things that can be expected using a beginner's first telescope. The view through a larger aperture telescope, with a longer effective focal length will allow more surface detail to be seen.

As Saturn is twice as far away as Jupiter it looks smaller and fainter. At twice the distance of Jupiter the amount of sunlight (per unit of area) reaching Saturn is a quarter of what Jupiter receives. Also the amount of light that we see reflected back from Saturn is also four times less that reflected by Jupiter. In total this means that Saturn not only appears much smaller but is also more than 16 times fainter (4 x 4) than Jupiter.

So that is the bad news but this is compensated for by the magnificent ring system of Saturn. The ring does add a little to the brightness and can be seen even using a smaller telescope.

The movement of Jupiter's moons can be predicted by using a Planetarium Application on a computer. The free to download application called Stellarium is very good for doing this. We will be able to predict when a moon will pass in front (transit) or behind the planet (occultation). There will always be something interesting going on.

Jupiter with the shadow of Io 29th August 00:38 BST imaged by Steve Harris

URANUS will be observable this month and will be best at 04:00 when it will be due south and at its highest point above the horizon but is small and faint at +5.7.

NEPTUNE will be just visible this month to the east of Jupiter (see the chart). It is small a difficult to see at only 2.4 arc-seconds in diameter and magnitude +7.7.

 

THE SUN

The Sun rises at about 06:20 at the beginning of the month and 06:52 by the end. It sets at 19:35 at the beginning of the month and 18:50 at the end of the month. It reached its highest point in the sky on 21st June which was the Summer Solstice and will be at the Autumn Equinox on the 22nd September. There have been a few small Sunspots during July and August.

Sunspots seen on 29th August 2021 image from SOHO

 

THE MOON PHASES DURING SEPTEMBER

New Moon will be on 7th September

First Quarter will be on 13th September

Full Moon will be on 21st September

Last Quarter will be on 29th September

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