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.