The summer triangle formed by Altair, Vega and Deneb still dominates the south western sky but is slowing sinking towards the evening horizon now that we are officially into autumn. (The Equinox was on September 22, at 20:44 UTC.)
By mid month the constellation Cygnus (The Swan) will be high over head and a little to the west. The popular name for this constellation is the “Northern Cross” which is a more descriptive name making it easier to find. From a dark site the dark lanes of the Milky Way can be see passing through this constellation.
A little to the east, Pegasus (The Winged Horse) is high in the sky with Andromeda (The Chained Maiden) attached to upper left corner of the large square that forms the most identifiable feature of Pegasus. Therefore, the most noteworthy object in the evening sky this month is the Andromeda Galaxy (M31).
The Andromeda Galaxy is 2.5mLy away, which puts it in our own back yard (The Local Group). M31 has the distinction of being the furthest object that can be seen without a telescope or binoculars and was the galaxy the Hubble used to establish that these objects were “island universes” and not part of the Milk Way. Even from a moderately light polluted sky it can be seen unaided as a distinct elongated smudge. With binoculars the edges become more defined and in a small telescope (80mm) the spiral arms and central bulge start to appear.
M31 is also referred to as our sister galaxy because it’s similar in size and shape to our own Milky Way. So looking at Andromeda, is like seeing the Milky Way as it looked 2.5 million years ago.
The chart below provides some tips on how to find it. Start with the star Alpheratz in the top left corner of Pegasus. Count three stars to the left to Mirach (with Alpheratz being star No 1). Then count three stars up (Mirach as No 1). The second set of 3 stars are faint so may not be obvious at first. M31 will be 4° to the right and elongated as shown.
On the other side of Pegasus, towards the meridian and 12° above the celestial equator, is the Pegasus Cluster (M1). It’s a small globular cluster that shows up in binoculars as a “non-stellar” object – meaning it looks like a fuzzy star. It is mag 7.5 and 31′ in diameter. In a 100 to 150mm scope it’s obvious that it’s a globular cluster. In 200mm+ scope many individual stars can be resolved with nice colour.
A bit lower sitting a half a degree below the celestial equator is M2. It is also mag 7.5 but smaller at 23′ in diameter. This means that it will appear brighter than M15 since magnitude is a measure of total brightness. A larger object means less surface brighteness since the available light is spread over a larger area.
A more challenging object for binoculars is the Helix Nebula (NGC 7293). Just 22° above the horizon (at the time for the chart), it requires a clear dark sky to see. In binoculars the mag 7.6, 13′ object will appear as a slightly brighter, diffuse spot. In large scopes the outer ring and central star become visible.
More towards the north between Cassiopeia and Perseus is the Double Cluster (NGC 869 and NGC 884 or Caldwell 14). These two bright open clusters can be seen in the same field of view of a 100mm scope. Unaided they show up as a distinct fuzzy patch. Individual stars can be resolved in binoculars.
Venus is by far the brightest object in the early evening western sky and sets about 8pm by mid month.
Saturn now sets about 7pm so is pretty much gone for the year.
Uranus and Neptune are in a good position but hard to find and require a medium to large scope (100mm to 200mm). Neptune is a relatively bright magnitude 5.7 so can actually be seen in a small scope. But without any good reference stars to locate it, it can be a challenge. It does appear as blue-green to provide a clue that you’ve found it. In a large scope, the disk can just be resolved. Neptune is magnitude 7.8 but a little easier to find using the background stars in Aquarius as a reference. Still, it’s best seen in a 200mm+ scope.
By mid-month Jupiter is rising at 11pm so is actually an accessibly object. It’s at opposition January 5th, when it will be due south at midnight.
Lastly, comet Ison is making the news. Still, it’s going to be a challenge. In early November when it’s going to be at its brightest, it’s also going to be close to the sun. So i am not planning any casual observing of this object just yet.