Find your way around the night sky! Below is a free sky map for SEPTEMBER 2015 as well as a printable version, courtesy of astronomer Jeff DeTray.
Sky Map for SEPTEMBER 2015
Each month, Jeff DeTray’s Sky Maps provides a sky map which highlights beautiful events in the evening sky—stars, constellations, planets, conjunctions with the Moon, meteor showers, and other amazing celestial objects. Follow more of Jeff’s sky adventures at AstronomyBoy.com.
Click-and-Print Sky Map
Just click here or on the image below to open the printable map—then bring outside!
Sky Map Highlights: September 2015
The Winged Horse and the Dome of the Sky
When you look to the east on September evenings, it’s difficult to miss the Great Square. This large asterism (unofficial star pattern) is the body of Pegasus, the Winged Horse. At the date and time of our Sky Map, the Great Square appears balanced on one corner, looking like a huge diamond in the sky. Pegasus is mostly upside down in this view, but you should be able to follow the graceful curve of his neck, ending with the star Enif, or “nose” in Arabic.
It can be difficult to get your bearings when looking at the sky, and one of the main problems is coming to terms with the scale of the sky. In other words, how do we describe the size of something like the Great Square of Pegasus? It makes no sense to say that the Great Square is so many inches or feet or miles wide.
For millennia, astronomers have solved this problem by measuring apparent sizes and distances in the sky using degrees. It works like this.
When you stand under the sky, it’s as if you are looking up at the inside of a giant dome or bowl, with the stars, planets, galaxies, etc., stuck to the inside surface of the dome. Ancient peoples thought this literally to be true. We now know that the Dome of the Sky is not an actual, physical dome, but sometimes it is helpful to pretend it is.
If you draw an imaginary arc starting at the horizon, passing directly overhead, then down to the opposite horizon, you’ve drawn a half-circle on the Dome of the Sky. Basic geometry tells us that a full circle is 360 degrees, so the half-circle is 180 degrees. Now we are getting somewhere! If we know that the Dome of the Sky measures 180 degrees from horizon to horizon, then we can describe objects in the sky as being a specific number of degrees in size.
Let’s put this knowledge to practical use. Stretch out your arm to its full length, make a fist, and hold your fist up against the sky. In that position, your fist covers a 10-degree width of sky. The Great Square of Pegasus measures about 15 degrees on each of its four sides. So your fist held at arm’s length will just fit inside the Great Square. This is helpful in confirming that the star pattern you are observing really is the Great Square and not some random assortment of stars. A fist is superimposed on the Great Square on the Sky Map.
By the way, the Great Square also stands out because the area inside the square is nearly empty of stars. It’s a big black box bounded by four bright stars at the corners. Those four bright stars have Arabic names corresponding to various parts of the horse’s anatomy as noted on the Sky Map. From the star Scheat (Arabic for “upper arm”; see “Horses Have Arms?”), the two front legs of Pegasus extend up and to the right.
To the right of the Great Square is another asterism, the so-called “Circlet” of Pisces. It just so happens that the Circlet is about 5 degrees wide, the same as the first three fingers of your hand (like the Boy Scout salute) held at arm’s length. As with the Great Square, you can use your own fingers to measure the size of the asterism to help confirm that you have found the Circlet.
To the left and above the Great Square are Cassiopeia and Cepheus, the Queen and King. They are one and a half and two “fist widths” wide, respectively, or 15 and 20 degrees.
Horses Have Arms?
According to sources I have found, the name of the star Scheat derives from the Arabic Al Sa’id, which means “The Upper Arm” or from Sa’d, meaning “fortunate” or “lucky.” In astronomy texts, “The Upper Arm” is the customary translation. I have never seen it translated in any other way, despite that fact that horses do not have arms.
Most names of bright stars originated thousands of years ago, and it is nigh impossible to be sure of the linguistic subtleties brought to bear on the naming. Modern astronomers rarely use the ancient names, and in most cases aren’t even aware of them. Scheat is known to most astronomers as Beta Pegasi or 53 Pegasi.
“Beta” indicates it was the second star (the first being “Alpha”) in Pegasus to be entered into the Uranometria catalog of bright stars published in 1603 by Johann Bayer. 53 Pegasi is its catalog number in John Flamsteed’s 1712 Historia Coelestis Britannica catalog.
There are two or three more star catalogs in wide use today, so many stars have five modern designations in addition to whatever names were given to them by the ancients.
Sky map produced using Chris Marriott’s Skymap Pro
See our Sky Watch page for more highlights of the monthly sky, courtesy of The Old Farmer’s Almanac.