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The Night Sky September 2021


Compiled by Ian Morison




This page, updated monthly, will let you know some of the things that you can look out for in the night sky.  It lists the phases of the Moon, where you will see the naked-eye planets and describes some of the prominent constellations in the night sky during the month.


New(ish)

The author's: Astronomy Digest

which, over time, will provide useful and, I hope, interesting articles for all amateur astronomers.   A further aim is to update and add new material to link with the books recently published by Cambridge University Press and which are described on the home page of the digest.   It now includes over 100 illustrated articles.



Image of the Month

Abell 3827


Image: NASA, ESA, HST and R. Massey

Over to the right of the 4 central elliptical galaxies in the Abel 3827 cluster there appears to be an unusual curved galaxy with three centres.   They are, however, three image of the same, more distant, galaxy whose light has been gravitationally lensed by the mass of the foreground galaxy cluster.   Such multiple images can provide information about the distribution of both normal and dark matter within the cluster.   The first gravitationally lensed galaxy was discovered in the 1970's in a radio survey using the Jodrell Bank, Mk1A (now Lovell) 76m radio telescope.

Highlights of the Month


September - a great month to view Jupiter.

Jupiter
Jupiter imaged by Damian Peach

This is a great month to observe Jupiter which will be visible during all the hours of darkness.   It lies in the southern part of the ecliptic and, sadly, will only reach an elevations of ~24 degrees when crossing the meridian.   An interesting observation is that the Great Red Spot appears to be diminishing in size.   At the beginning of the last century it spanned 40,000 km across but now appears to be only ~16,500 km across - less than half the size.   It used to be said that 3 Earths could fit within it, but now it is only one.   The shrinking rate appears to be accelerating and observations indicate that it is now reducing in size by ~580 miles per year.   Will it eventually disappear?

  The features seen in the Jovian atmosphere have been changing quite significantly over the last few years - for a while the South Equatorial Belt vanished completely (as seen in Damian's image) but has now returned to its normal wide state.   The diagram on right shows the main Jovian features as imaged by the author at the beginning of December 2012.

The image by Damian Peach was taken with a 14 inch telescope in Barbados where the seeing can be particularly good.   This image won the "Astronomy Photographer of the Year" competition in 2011.

See more of Damian Peach's images: Damian Peaches Website"





Jovian Features
Features in Jupiter's atmosphere - December 2013.




September: Look for the Great Red Spot on Jupiter

Great Red Spot
Observe the Great Red Spot
Image: NASA

This list gives the best late evening times during September to observe the Great Red Spot which should then lie on the central meridian of the planet and so face the Earth. The times are in UT.



3rd       22:47     22nd   23:28

10th     23:33     27th   22:36

15th     22:41     22nd  22:53

September - Find Neptune

Neptune
Neptune at opposition on September 11th
Image: Stellarium/IM

This month Neptune is at opposition and so will be visible throughout the night.   It lies in Aquarius below one of the circlets in Pisces and shines at magnitude +7.8 having a 2.4 arc second disc so binoculars or a telescope will be needed to spot it under a dark sky.   I hope the charts will help you find it - not so difficult as it lies close to a nice grouping of stars.   Of course, a well aligned computerised telescope will take you right there but, unless the seeing is exceptional, I suspect that the dark bluish disk will not be that obvious.

September, late evening: the Double Cluster and the 'Demon Star', Algol.

Algol
Algol and the Double Cluster.
Image: Stellarium/IM

Later in the month is a good time to look high in the Southeast towards the constellations of Cassiopea and Perseus.   Perseus contains two interesting objects; the Double Cluster between the two constellations and Algol the 'Demon Star'.   Algol in an eclipsing binary system as seen in the diagram below.   Normally the pair has a steady magnitude of 2.2 but every 2.86 days this briefly drops to magnitude 3.4.   Visible times of the eclipse are (in UT): on the 13th at 01:15 and the 15th at 22:41.

Double Cluster
Double Cluster imaged by IM and the Algol Light Curve



September - Find the globular cluster in Hercules and spot the "Double-double" in Lyra

M13
Use binoculars to find the globular cluster M13 in Hercules and the "Double-double" in Lyra
Image: Stellarium/IM

There are two very nice objects to spot with binoculars in the south-western sky after dark this month.   Two thirds of the way up the right hand side of the 4 stars that make up the "keystone" in the constellation Hercules is M13, the best globular cluster visible in the northern sky.   The 15 minute exposure image on right was taken by the author using a 127 mm APO refractor and SBIG 8.3 megapixel CCD camera.

Just to the left of the bright star Vega in Lyra is the multiple star system Epsilon Lyrae often called the double-double.   With binoculars a binary star is seen but, when observed with a telescope, each of these two stars is revealed to be a double star - hence the name!

M13
M13 imaged by Ian Morison in May 2021





September 10th - Venus and the Moon

Venus
Venus and a 4 day old crescent Moon.
Image: Stellarium/IM

If clear after sunset on September 10th, a thin crescent Moon will lie up to the left of Venus as seen in the accompanying image.   Binoculars may well be needed to cut through the Sun's glare, but please do not use them until after the Sun has set.

September 17th - Jupiter, Saturn and the Moon

Jupiter
Jupiter, Saturn and a waxing gibbous Moon.
Image: Stellarium/IM

Low in the south in the late evening, Jupiter and Saturn will lie above a 12 day old waxing, gibbous, Moon.

September 18th - just after sunset: Mars and Mercury.

Mars
A conjunction of Mars and Mercury.
Image: Stellarium/IM

If it is clear and given a VERY low horizon towaards the West-Northwest at sunset, it should be possible to spot Mars close by Mercury.   Binoculars will almost certainly be needed, but please do not use them until after the Sun has set.

September 25th - late evening : Saturn and Jupiter above a waxing Moon .

Jupiter
Venus and a thin crescent Moon.
Image: Stellarium/IM

In the late evening of the 25th and looking south, the Moon, just after first quarter, will be seen below Saturn and Jupiter.

September 16th and 28th: Mons Piton and Cassini

Moon
Location of Mons Piton:IM

Mons Piton and the crater Cassini
Best seen on the dates above when the terminator is close, Mons Piton is an isolated lunar mountain located in the eastern part of Mare Imbrium, south-east of the crater Plato and west of the crater Cassini.   It has a diameter of 25 km and a height of 2.3 km.   Its height was determined by the length of the shadow it casts.   Cassini is a 57km crater that has been flooded with lava.   The crater floor has then been impacted many times and holds within its borders two significant craters, Cassini A, the larger and Cassini B.   North of Mons Piton can be seen a rift through the Alpine Mountains (Montes Alpes).   Around 166 km long it has a thin rille along its center.   I have never seen it but have been able to image it as seen in the lunar section.

Mons Piton
Mons Piton and Cassini

A Messier Object imaged with the Faulkes Telescope: M57 in Lyra

Ring Nebula, M57
The Ring Nebula, M57
Image:Danial Duggan
Faulkes Telescope North.

Planetary Nebula M82, imaged by Daniel Duggan.
This image was taken using the Faulkes Telescope North by Daniel Duggan - for some time a member of the Faulkes telescope team.   Lying at at distance of 2.3 thousand light years in the constellation Lyra, it is the remnant of a star like our Sun.   The core of the star has contracted down to an object about the size of the Earth supported by electron degeneracy pressure and is seen in the centre of the object.   The outer parts of the star were blasted out into space forming the "ring" (or torus) that we see.   Though showing very well in images, the central "star", called a "White Dwarf" is hard to see visually.

Learn more about the Faulkes Telescopes and how schools can use them: Faulkes Telescope"


















Observe the International Space Station

The International Space Station
The International Space Station and Jules Verne passing behind the Lovell Telescope on April 1st 2008.
Image by Andrew Greenwood

Use the link below to find when the space station will be visible in the next few days. In general, the space station can be seen either in the hour or so before dawn or the hour or so after sunset - this is because it is dark and yet the Sun is not too far below the horizon so that it can light up the space station. As the orbit only just gets up the the latitude of the UK it will usually be seen to the south, and is only visible for a minute or so at each sighting. Note that as it is in low-earth orbit the sighting details vary quite considerably across the UK. The NASA website linked to below gives details for several cities in the UK. (Across the world too for foreign visitors to this web page.)

Note: I observed the ISS three times recently and was amazed as to how bright it has become.

Find details of sighting possibilities from your location from: Location Index

See where the space station is now: Current Position


The Moon

3rd Quarter Moon
The Moon at 3rd Quarter. Image, by Ian Morison, taken with a 150mm Maksutov-Newtonian and Canon G7.
Just below the crator Plato seen near the top of the image is the mountain "Mons Piton".   It casts a long shadow across the maria from which one can calculate its height - about 6800ft or 2250m.
`
new moon first quarter full moon third quarter
September 7th September 13th September 20th September 29th

Some Lunar Images by Ian Morison, Jodrell Bank Observatory: Lunar Images

A World Record Lunar Image

World record Lunar Image
The 9 day old Moon.

To mark International Year of Astronomy, a team of British astronomers have made the largest lunar image in history and gained a place in the Guinness Book of Records!   The whole image comprises 87.4 megapixels with a Moon diameter of 9,550 pixels.  The resolution of ~0.4 arc seconds allows details as small as 1km across to be discerned!   The superb quality of the image is shown by the detail below of Plato and the Alpine Valley.  Craterlets are seen on the floor of Plato and the rille along the centre of the Alpine valley is clearly visible.  The image quality is staggering! The team of Damian Peach, Pete lawrence, Dave Tyler, Bruce Kingsley, Nick Smith, Nick Howes, Trevor Little, David Mason, Mark and Lee Irvine with technical support from Ninian Boyle captured the video sequences from which 288 individual mozaic panes were produced.   These were then stitched together to form the lunar image.

Plato and the Alpine valley
Plato and the Alpine Valley.

Please follow the link to the Lunar World Record website and it would be really great if you could donate to Sir Patrick Moore's chosen charity to either download a full resolution image or purchase a print.









The 8 day old Moon

Lunar Image
The 8 day old Moon imaged by Ian Morison.

This image was taken by the author on a night in March 2018 when the Moon was at an elevation of ~52 degrees and the seeing was excellent.   This enabled the resolution of the image to be largely determined by the resolution of the 200 mm aperture telescope and the 3.75 micron pixel size of the Point Grey Chameleon 1.3 megapixel video camera.   The use of a near infrared filter allowed imaging to take place before it was dark and also reduced the effects of atmospheric turbulence.   The 'Drizzle' technique developed by the Hubble Space Telescope Institute (HSTI) was used to reduce the effective size of the camera's pixels to allow the image to be well sampled.   Around 100 gigabytes of data, acquired over a 2 hour period, was processed to produce images of 54 overlapping areas of the Moon which were then combined to give the full lunar disk in the free 'stitching' program Microsoft ICE.   A further HSTI development called 'deconvolution sharpening' was then applied to the image.   The Moon's disk is ~6,900 pixels in height and has a resolution of 0.6 to 0.7 arc seconds.   Interestingly, as seen in the inset image, the rille lying along the centre of the Alpine Valley is just discernable and this is only ~0.5 km wide!   [Due to size limitations the large image is 2/3 full size.]









The Planets

 A montage of the Solar System
A montage of the Solar System. JPL / Nasa

Jupiter

Jupiter
A Cassini image of Jupiter . Nasa

Jupiter. As September begins, Jupiter may be seen throughout the night shining at magnitude -2.85 and having an angular size of 48.83 arc seconds.   By month's end its magnitude will have reduced slightly to -2.72 and its angular size to 46.42 arc seconds.   On the night of the first/second, Jupiter will have an elevation of ~24 degrees when crossing the meridian at ~00:17 BST.   As the month progresses it will transit sooner - by 22:21 BST by month's end.   Happily, this year Jupiter has climbed up the ecliptic somewhat so the atmosphere will not hinder our view of the solar system's giant planet as much as it has in the last two years.



Saturn

Saturn
The planet Saturn. Cassini - Nasa

Saturn precedes Jupiter into the sky, rising before sunset at the beginning of the month and setting at 3 am.   It then shines with a magnitude of +0.31 with its disk 18.3 arc seconds across and the rings spanning some 43 arc seconds.   By month's end, it has a reduced brightness of magnitude +0.47 and a 17.65 arc second disk.   In the middle of the month it will be best seen towards the south at ~22:29 BST when it will have an elevation of just ~19 degrees.


TT



Mercury

Mercury.
Messenger image of Mercury Nasa


Mercury might just be spotted very low in the west at the very start of the month having a magnitude of -0.05 and an angular size of 6 arc seconds.   Binoculars may well be needed, but please do not use them before the Sun has set





Mars

Mars showing Syrtis major.
A Hubble Space Telescope image of Mars.
Jim Bell et al. AURA / STScI / Nasa

Mars starts the month with a magnitude of +1.79 and an angular size of 3.57 arc seconds.   It may then just be possible to spot it after sunset, down to the right of Venus, a little north of west at an elevation of ~3 degrees.   Even then it may be lost in the Sun's glare and binoculars will certainly be needed - but please do not use them until after the Sun has set.




Venus

Venus
Venus showing some cloud structure

Venus. At the start of September, Venus, at magnitude -4.04 and having an angular size of ~15 arc seconds, will only have an elevation of ~10 degrees at sunset looking towards the southwest.   It may well be lost in the Sun's glare until around 20:18 when it will have an elevation of ~6 degrees.   By month's end, its elevation at sunset will have reduced slightly to ~9 degrees with its magnitude increasing -4.25.   Venus will grace the evening sky for the rest of this year and reaches its greatest elongation east from the Sun on October 29th but will be highest in the evening sky at the beginning of December.   As the ecliptic is at a shallow angle to the horizon in the latter part of the year, it will never get to a high elevation after dark during this apparation.   See highlight above.





The Stars

The Evening September Sky

September Sky
The September Sky in the south - early Sept:~11pm, late Sept:~10pm

This map shows the constellations seen towards the south in late evening. To the south in early evening moving over to the west as the night progresses is the beautiful region of the Milky Way containing both Cygnus and Lyra. Below is Aquilla. The three bright stars Deneb (in Cygnus), Vega (in Lyra) and Altair (in Aquila) make up the "Summer Triangle". East of Cygnus is the great square of Pegasus - adjacent to Andromeda in which lies M31, the Andromeda Nebula. To the north lies "w" shaped Cassiopeia and Perseus.

The constellations Lyra and Cygnus

Cygnus and Lyra
Lyra and Cygnus

This month the constellations Lyra and Cygnus are seen almost overhead as darkness falls with their bright stars Vega, in Lyra, and Deneb, in Cygnus, making up the "summer triangle" of bright stars with Altair in the constellation Aquila below. (see sky chart above)

Lyra

Lyra is dominated by its brightest star Vega, the fifth brightest star in the sky. It is a blue-white star having a magnitude of 0.03, and lies 26 light years away. It weighs three times more than the Sun and is about 50 times brighter. It is thus burning up its nuclear fuel at a greater rate than the Sun and so will shine for a correspondingly shorter time. Vega is much younger than the Sun, perhaps only a few hundred million years old, and is surrounded by a cold,dark disc of dust in which an embryonic solar system is being formed!

There is a lovely double star called Epsilon Lyrae up and to the left of Vega. A pair of binoculars will show them up easily - you might even see them both with your unaided eye. In fact a telescope, provided the atmosphere is calm, shows that each of the two stars that you can see is a double star as well so it is called the double double!

The Double Double
Epsilon Lyra - The Double Double

Between Beta and Gamma Lyra lies a beautiful object called the Ring Nebula. It is the 57th object in the Messier Catalogue and so is also called M57. Such objects are called planetary nebulae as in a telescope they show a disc, rather like a planet. But in fact they are the remnants of stars, similar to our Sun, that have come to the end of their life and have blown off a shell of dust and gas around them. The Ring Nebula looks like a greenish smoke ring in a small telescope, but is not as impressive as it is shown in photographs in which you can also see the faint central "white dwarf" star which is the core of the original star which has collapsed down to about the size of the Earth. Still very hot this shines with a blue-white colour, but is cooling down and will eventually become dark and invisible - a "black dwarf"! Do click on the image below to see the large version - its wonderful!

M57 - The Ring Nebula
M57 - the Ring Nebula
Image: Hubble Space telescope

M56 is an 8th magnitude Globular Cluster visible in binoculars roughly half way between Alberio (the head of the Swan) and Gamma Lyrae. It is 33,000 light years away and has a diameter of about 60 light years. It was first seen by Charles Messier in 1779 and became the 56th entry into his catalogue.

M56 - Globular Cluster
M56 - Globular Cluster

Cygnus

Cygnus, the Swan, is sometimes called the "Northern Cross" as it has a distinctive cross shape, but we normally think of it as a flying Swan. Deneb,the arabic word for "tail", is a 1.3 magnitude star which marks the tail of the swan. It is nearly 2000 light years away and appears so bright only because it gives out around 80,000 times as much light as our Sun. In fact if Deneb where as close as the brightest star in the northern sky, Sirius, it would appear as brilliant as the half moon and the sky would never be really dark when it was above the horizon!

The star, Albireo, which marks the head of the Swan is much fainter, but a beautiful sight in a small telescope. This shows that Albireo is made of two stars, amber and blue-green, which provide a wonderful colour contrast. With magnitudes 3.1 and 5.1 they are regarded as the most beautiful double star that can be seen in the sky.

Alberio
Alberio: Diagram showing the colours and relative brightnesses

Cygnus lies along the line of the Milky Way, the disk of our own Galaxy, and provides a wealth of stars and clusters to observe. Just to the left of the line joining Deneb and Sadr, the star at the centre of the outstretched wings, you may, under very clear dark skys, see a region which is darker than the surroundings. This is called the Cygnus Rift and is caused by the obscuration of light from distant stars by a lane of dust in our local spiral arm. the dust comes from elements such as carbon which have been built up in stars and ejected into space in explosions that give rise to objects such as the planetary nebula M57 described above.

There is a beautiful region of nebulosity up and to the left of Deneb which is visible with binoculars in a very dark and clear sky. Photographs show an outline that looks like North America - hence its name the North America Nebula. Just to its right is a less bright region that looks like a Pelican, with a long beak and dark eye, so not surprisingly this is called the Pelican Nebula. The photograph below shows them well.

The North American Nebula
The North American Nebula

Brocchi's Cluster An easy object to spot with binoculars in Gygnus is "Brocchi's Cluster", often called "The Coathanger",although it appears upside down in the sky! Follow down the neck of the swan to the star Alberio, then sweep down and to its lower left. You should easily spot it against the dark dust lane behind.

The Coathanger
Brocchi's Cluster - The Coathanger

The constellations Pegasus and Andromeda

Pegasus and Andromeda
Pegasus and Andromeda

Pegasus

The Square of Pegasus is in the south during the evening and forms the body of the winged horse. The square is marked by 4 stars of 2nd and 3rd magnitude, with the top left hand one actually forming part of the constellation Andromeda. The sides of the square are almost 15 degrees across, about the width of a clentched fist, but it contains few stars visibe to the naked eye. If you can see 5 then you know that the sky is both dark and transparent! Three stars drop down to the right of the bottom right hand corner of the square marked by Alpha Pegasi, Markab. A brighter star Epsilon Pegasi is then a little up to the right, at 2nd magnitude the brightest star in this part of the sky. A little further up and to the right is the Globular Cluster M15. It is just too faint to be seen with the naked eye, but binoculars show it clearly as a fuzzy patch of light just to the right of a 6th magnitude star.

Andromeda

The stars of Andromeda arc up and to the left of the top left star of the square, Sirra or Alpha Andromedae. The most dramatic object in this constellation is M31, the Andromeda Nebula. It is a great spiral galaxy, similar to, but somewhat larger than, our galaxy and lies about 2.5 million light years from us. It can be seen with the naked eye as a faint elliptical glow as long as the sky is reasonably clear and dark. Move up and to the left two stars from Sirra, these are Pi amd Mu Andromedae. Then move your view through a rightangle to the right of Mu by about one field of view of a pair of binoculars and you should be able to see it easily. M31 contains about twice as many stars as our own galaxy, the Milky Way, and together they are the two largest members of our own Local Group of about 3 dozen galaxies.

M 31 - The Andromeda Nebula
M31 - The Andromeda Nebula

M33 in Triangulum

If, using something like 8 by 40 binoculars, you have seen M31 as described above, it might well be worth searching for M33 in Triangulum. Triangulum is

the small faint constellation just below Andromeda. Start on M31, drop down to Mu Andromedae and keep on going in the same direction by the same distance as you have moved from M31 to Mu Andromedae. Under excellent seeing conditions (ie., very dark and clear skies) you should be able to see what looks like a little piece of tissue paper stuck on the sky or a faint cloud. It appears to have uniform brightness and shows no structure. The shape is irregular in outline - by no means oval in shape and covers an area about twice the size of the Moon. It is said that it is just visible to the unaided eye, so it the most distant object in the Universe that the eye can see. The distance is now thought to be 3.0 Million light years - just greater than that of M31.

M33
M33 in triangulum - David Malin