September wasn’t a particularly good month weatherwise but on the 5th, Mars and the Moon were good to see between the rolling clouds. Mars has continued its retrograde motion and is now below the lower arm in Pisces (more about that later) while Jupiter and Saturn have continued westward in the evening sky. Poor weather prevented the viewing of the setting Sun on the equinox so will have to wait till the spring equinox in March to fix due West.
I was rather dismissive of two of the watery zodiacal constellations, Aquarius and Capricornus, in last month’s blog and as a ‘fishy’ constellation features this month it is probably time to say something about stellar magnitudes. It is OK looking at a stick presentation of a constellation in a diagram but they don’t look like that in the sky! Originally the brightness of a star was classified on a scale of 1 to 6, 1 being the brightest and 6 being just visible to the unaided eye. (Note the the bigger the number the dimmer the star. In the modern scientific era measurements have shown that a difference in magnitude of 1 means the brightness differs by a factor of 2.5, ie a magnitude 2 star is two and a half times as bright as a magnitude 3 star and a magnitude 1 star is one hundred times brighter than a magnitude 6 star. Really bright objects have a negative magnitude). But that was over two thousand years ago in the Middle East with clear skies and no light pollution. What can we expect to see today at a latitude of about 50 degrees North with the associated weather that brings and the light pollution from modern towns and cities.
If we face South and look above us just past our zenith we see again the reassuring sight of the ‘W’ shape of Cassiopeia. The three brightest stars Caph, Schedar and Navi are close to magnitude 2 and clearly visible while epsilon Cas on the extreme left is magnitude 3.4 and considerably dimmer but easily visible if conditions are reasonable. (See diagram below).
If we drop down to the horizon towards the right we locate as we did last month the Great Square of Pegasus with the star Alpheratz at magnitude 2.1 and Algenib about half as bright with the other two stars of the square in between. The four stars stand out because they are in a fairly empty part of the sky. Now for the tricky part. Again as we did last month, starting from Alpheratz we look for the two curved strings of stars which make up Andromeda. The lower string isn’t too bad because from Alpheratz; delta Andromeda, Mirach and Almach have magnitudes 3.3, 2.1 and 2.2 respectively so no problem. However the higher curved string of stars from Alpheratz; pi Andromeda, mu Andromeda and 51 Andromeda have magnitudes of 4.3, 3.9 and 3.6 respectively. We are now in a situation where poor atmospherics and light pollution become critical if the stars are to be visible to the unaided eye. For comparison, if you are trying to locate the Andromeda galaxy, M31, it has a magnitude of 3.4.
If you are struggling to see the fainter stars even in a clear sky you need to leave the comforts of your home and find a more rural dark sky site. Sorry!
That’s all my excuses made now so we can return to sky gazing. Below Andromeda and to the south east of the Great Square of Pegasus lies the constellation Pisces- The Fish. Supposedly two fish, one the Circlet and the other the group of stars to the East of Alpheratz, tied together with a ribbon. I use the word ‘lies’ advisedly because unfortunately only two stars in Pisces are brighter that magnitude 4 and even then, only just, so it is unlikely that you will see anything if you are in your back garden! If any readers follow their ‘stars’ in the newspapers or were born under the star sign Pisces perhaps now is the time to consign astrology to the rubbish bin. Why did I bother to mention Pisces? At present the planet Mars is in Pisces and at magnitude -2.3 it is more than a hundred times brighter than a magnitude 3 star and outshines anything nearby. It will have a close approach with the Moon on the 3rd October just three days after the full Moon. It will be at its closest to the Earth on the 6th October and at opposition (on the far side of the Earth from the Sun) on the 14th October so visible all night. Now that is something to look forward to.
The diagram below has more named stars than usual not because they are bright but because I used them in the text to explain the variation we see in stellar magnitudes and again I have omitted some minor star groupings to help with clarity.
With the idea of stellar magnitudes firmly in mind let us look at three further constellations. To the southeast of the lower string of Andromeda and due East of the Great Square of Pegasus is another zodiacal constellation, Aries- The Ram. (Remember you probably cannot see anything in Pisces apart from Mars). Aries contains two brightish stars, Hamal at magnitude 2.0 and Sheratan at magnitude 2.7 which are readily seen but there is not much more. How you make the shape of a ram from that I do not know. However Aries has a claim to fame in that it was the location of the spring equinox about two thousand years ago and that event is still called the ‘first point in Aries’ even though it is now in Pisces.
Between Aries and Andromeda is the constellation Triangulum- The Triangle. It has the great redeeming feature that it is what it says on the tin- a triangle! However it doesn’t have any stars brighter than magnitude 3 but because of its compact nature it is readily recognisable if seen.
Finally to the northeast of Aries and Andromeda and southeast of Cassiopeia is the fairly prominent constellation Perseus- another hero from Greek mythology. It contains the stars Mirfak and Algol both around magnitude 2 and another five stars around magnitude 3 or brighter.
Something to look out for
As mentioned above Mars is going to be the major attraction in the night sky this month so don’t miss it and see if you can follow its retrograde motion to the first week in November. ( I used my binoculars to locate eta Pisces and epsilon Pisces).
If you want to see a ‘falling star’ your best chance will be on the 21st October when the Orionid meteor shower is at its peak.
At the end of the month there are two lunar close approaches to look out for. The Moon and Jupiter on the 22nd and the Moon and Mars on the 29th. Clear skies and happy viewing.
My attempts to see the Perseid meteor shower were thwarted by cloud cover but the close approach of the planets Jupiter and Saturn with the Moon at the beginning of the month was good to see. At the time of writing there has been a lot of cloud cover so I’m not very optimistic about seeing the close approach at the end of August.
I hope you are keeping your eye on Cassiopeia on its journey west because it is approaching its optimal viewing position and with the Plough low in the northern sky, Cassiopeia is better placed to help us find our way among the stars. Also it is just lovely to look at!
The diagram below seems to contain a lot this month but some of it you are already familiar with and I have omitted some minor star groupings to help with clarity.
As usual we start facing south and look up to the zenith and just short of it and to the right hand side we see the bright star, Deneb, the tail of Cygnus the swan and along with Vega and Altair we quickly pick out the Summer Triangle. From the line joining Deneb and Altair turn left by about 45 degrees to look east of south and you will spot the asterism, the Great Square of Pegasus, which stands out not because of the brightness of its stars but because it is away from the Milky Way and there are few stars visible in this area. This asterism is part of the constellation Pegasus (the winged horse in Greek mythology). Again it is difficult to imagine a horse and no obvious signs of wings. Pegasus is quite a large constellation but its other stars do not stand out as much as the square. The square isn’t actually a square and to add insult to injury the star, Alpheratz, at the top of the square isn’t part of Pegasus! However on a positive note, the Great Square of Pegasus is easily picked out and is another good signpost to help us find our way around the skies.
Alpheratz, is part of the constellation Andromeda (the princess, daughter of the mythological Queen Cassiopeia and King Cepheus) and being the brightest star is also referred to as a And (alpha Andromedae). The main features of Andromeda are two curved strings of relatively faint stars meeting at Alpheratz and it is readily found because of its association with Pegasus. The constellation Andromeda is home to one of the most famous objects in the sky- the Andromeda galaxy also known as M31. It is marked on the diagram with a red cross and labelled M31. The Andromeda galaxy is the nearest large galaxy to Earth and is similar in many ways to our Milky Way galaxy and is the only one visible to the unaided eye in the northern hemisphere. To locate it for observing, (your eyesight needs to be pretty good), start at Alpheratz and by star hopping jump to the second pair of stars along the curved strings and the Andromeda galaxy will be to the right hand side at a distance approximately equal to the distance between the two stars. There is no rush as Andromeda will be in a good position right through till November. Now some mind boggling statistics- the distance to Andromeda is about two and a half million light years which means that the light entering your eyes from Andromeda set out two and a half million years ago, about the time the first members of the genus Homo appeared on Earth using stone tools and long before Homo sapiens arrived on the scene! Andromeda is the most distant object you can see with the unaided eye but you will need a dark site with no light pollution and clear skies. Don’t expect to see something like the images shown in the gallery of the WMA website, you will have to settle for something which might be described as a smudge or fuzzy star but that doesn’t detract from the sense of achievement. Good luck!
If we drop down from the Great Square of Pegasus along the diagonal from Alpheratz to the horizon we find another zodiacal constellation, Aquarius (the Water Carrier). Unfortunately to the unaided eye Aquarius has no bright stars and is of an indistinct pattern. Since antiquity it has been seen as a figure pouring water from a jug but I am obviously lacking in imagination.
However I remember a time in the late sixties when any radio channel you switched to was likely to be filling the air with the song ‘Aquarius’ from the musical ‘Hair’ or from a version by an American pop group. It was a song to cheer you up and had the memorable line, ’This is the dawning of the age of Aquarius’. This might lead us in to discussing ‘the precession of the equinoxes’ but luckily this has already been done in a recent blog by Gordon Dennis (Dennis, July 2020).
Now follow the line from Vega through Altair down to the horizon and you will find the right hand edge of another zodiacal constellation, Capricornus (the Sea Goat, associated with many myths from ancient times) just to the south east of Aquarius. This constellation is relatively small and the second faintest zodiac constellation so doesn’t have much to offer the casual observer and I can’t recall a pop song called Capricornus. However, precession is a slow process so there might be one by the year 4750 give or take a few hundred years!
Let’s have a grand tour. Find the Great Square of Pegasus and follow the left hand side of the square upwards past Andromeda on your left till you see Cassiopeia. From the star Navi go across the top of the constellation Cepheus to the pole star, Polaris, in Ursa Minor, and continue in a straight line to the star Alioth in the handle of the Plough, part of Ursa Major. Just as before, follow the arc of the handle down till you find the bright star Arcturus in the constellation Bootes then follow round eastwards to spot the Keystone of Hercules not missing out the small but attractive constellation, Corona Borealis, on the way. One more step eastwards and you are back at the Summer Triangle comprising Vega in Lyra, Deneb in Cygnus and Altair in Aquila from where we found the Great Square of Pegasus at the start. Now give yourself a pat on the back because you have gone round half the visible night sky and identified twelve constellations.
Erratum: In last month’s blog, ’Looking to the Skies August 2020’, the line pointing to the variable star delta Cep should have gone past the first star to the top right hand one of the three stars in the bottom left hand corner of Cepheus. Sorry for the ambiguity.
Something to look out for
This is the month of the autumn equinox when as the Earth revolves around the Sun, the Sun’s apparent path round the ecliptic crosses the celestial equator and the Sun is overhead at the equator and everywhere on earth has almost equal amounts of daylight and darkness. It will happen on the 22nd of September but of course it happens at a specific time which is about a quarter of a day later each year until it is corrected for with the extra day of a leap year and so the date of the autumn equinox can vary by a day or two but it is usually on the 22nd or 23rd. Another feature of an equinox is that the sun rises due east and sets due west so note some landmark on the horizon in line with the sun at sunrise and sunset and you have your east and west directions. As far as astronomers are concerned it means we are into Autumn with more starry evenings.
There are exciting times ahead because the planet Mars is going to be a major attraction in the night sky in the coming months. Try to catch it early in the month because it rises in the east with the Moon on the 5th September at 9.30pm BST. I say catch it early in the month because there is a little project you might enjoy doing- observing the retrograde motion of Mars. By the 10th of September it stops its apparent eastward motion against the background stars of Pisces and then appears to move westwards until November. Use the stars nPsc, mPsc and ePsc on the lower branch of Pisces as your guide. It should pass between the first two of these stars about the 30th September and it will be getting brighter throughout the month. I hope the diagram clarifies the situation. The orange line is the path which Mars will follow during its retrograde motion from 1st September to the first week in November.
Finally there will be a conjunction, I use the term loosely to mean a coming together rather than the more technical definition, of the Moon, Jupiter and Saturn on the 25th September, best viewed to the south just after 8.00pm BST.