Night Sky Map for September 2020: Pegasus & Measuring the Sky

Free September Sky Map (Star Chart)

By Jeff DeTray from AstronomyBoy.com
September 1, 2020
September Night Sky Map
Jeff DeTray

Welcome to the Night Sky Map for September 2020! This month, watch for Pegasus, the Winged Horse, and learn about measuring the sky!

Just click here or on the image below to open the printable map—then bring outside!

September printable sky map

Pegasus, the Winged Horse

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. On September evenings, The Great Square appears balanced on one corner, looking like a huge, slightly lopsided, diamond shape in the sky. Pegasus is upside down at this time of year, but you should be able to follow the graceful curve of his neck, ending with the star Enid, or “nose” in Arabic. And from the star Scheat (Arabic for “upper arm”) the two front legs of Pegasus extend up and to the right.

Measuring the Sky

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 on the sky in terms of degrees. Here’s how it works.

Under a Starry Dome

When you stand under the sky, it’s as if you are looking up at the inside of a giant dome or an upturned 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 isn’t an actual, physical dome, but sometimes it is helpful to pretend it is.

If you draw an imaginary arc on the Dome, 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. Eighth grade geometry tells us that a full circle is 360 degrees, so the half-circle we’ve just drawn measures 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. On this month’s map, a gray fist is superimposed on the Great Square.

By the way, the Great Square also stands out because the area inside the square is nearly devoid of stars. It’s a large 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. 

The Circlet

To the right of the Great Square is another asterism, the so-called “Circlet” of Pisces. The five-sided Circlet is about 5 degrees across, which just happens to be the span of the first three fingers of your hand held at arm’s length. As with the Great Square, you can use your own three 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 from end to end.

Click here or on map below to enlarge (PDF).

September Night Sky Map
Sky map produced using Chris Marriott’s Skymap Pro

Note: How to Read the Sky Map

Our monthly sky map does not show the entire sky which would be almost impossible. Instead, the map focuses on a particular region of the sky each month where something interesting is happening. The legend on the map always tells you which direction you should facing, based on midnight viewing. For example, if the map legend says “Looking Southeast,” you should face southeast when using the map.

The map is accurate for any location at a so-called “mid northern” latitude. That includes anywhere in the 48 U.S. states, southern Canada, central and southern Europe, central Asia, and Japan. If you are located substantially north of these areas, objects on our map will appear lower in your sky, and some objects near the horizon will not be visible at all. If you are substantially south of these areas, everything on our map will appear higher in your sky.

The items labeled in green on the sky map are known as asterisms. These are distinctive star patterns that lie within constellations. When getting your bearings under the stars, it’s often easiest to spot an asterism and use it as a guide to finding the parent constellation.

The numbers along the white “Your Horizon” curve at the bottom of the map are compass points, shown on degrees. As you turn your head from side to side, you will be looking in the compass direction indicated by those numbers. The horizon line is curved in order to preserve the geometry of objects in the sky. If we made the horizon line straight, the geometry of objects in the sky would be distorted.

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