Sky Map (Star Chart): September 2018

Watch for Algol: The Demon Star

By Jeff DeTray from
September 1, 2018
SkyMap September

See this printable sky map for September to navigate the skies and look for Algol, the “Demon Star”—plus, find helpful stargazing tips.

Click-and-Print Sky Map

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

September printable sky map


Algol: The Demon Star

There are approximately 9,000 stars bright enough to be seen with your unaided eyes—that is, without using binoculars or a telescope. The vast majority of these “naked eye” stars glow with a steady light. Night after night, century after century, millennium after millennium, their brightness remains the same. But there are exceptions.

About two dozen of the 9,000 naked eye stars change in brightness. These are the variable stars. They may change in brightness slowly or rapidly, randomly or on a regular schedule. You can see one of the most famous of all variable stars in action on several occasions during the month of September.

The star in question is Algol, in the constellation Perseus, the Greek hero. Algol is known as the Demon Star and has been associated with bad luck and calamity since ancient times. Algol’s name derives from an Arabic phrase meaning “the head of the ogre” (or “… demon”). The reasons behind these dire connections are not entirely clear. Had the ancients even noticed Algol’s variability? Even if they did, why would they assign demonic intentions to the star? There are several theories about this, but we just don’t know for certain.

What we do know is that like clockwork, every 2.86 days, Algol dims dramatically, only to return to its original brightness a few hours later. This amazing event is easy to see with nothing more than your own eyes.

When to Look

The key to observing this variability is knowing ahead of time when to expect the minimum brightness. In September 2018, these (Eastern) times will be on the 1st, at 4:52 a.m.; the 5th, 1:40 a.m.; the 7th, 10:29 p.m.; the 25th, 3:21 a.m.; and the 28th, 12:09 a.m. (There are other minima during the month, but they do not occur during darkness in the Northern Hemisphere.

Subtract 1, 2, or 3 hours if you are in the Central, Mountain, or Pacific time zones, respectively. For other parts of the world and other times of the year, you’ll need to consult an online calculator. Use your favorite search engine to look for “minima of Algol.”

What Causes Algol’s Brightness to Change?

Algol’s changes in brightness occur because it is actually composed of two stars in very close proximity that orbit around one another—nearly touching—every 2.86 days. As shown in the inset on the sky map, the Algol system is oriented so that from our Earthly point of view, the dimmer of the two stars passes in front of (eclipses) the brighter star, partially hiding it from us. When the brighter star is partly hidden, Algol looks dimmer than usual.

The whole process of dimming and returning to normal brightness takes about 10 hours, but the changes are mainly noticeable for only 2 hours on either side of the times listed above. One observing strategy is to take a good look at Algol at least 2 hours before minimum, carefully noting its brightness. Then wait 2 hours and check it again at minimum brightness. The difference is dramatic! Two hours later, Algol will again be bright. To better observe the gradual dimming, check Algol every 30 minutes during the 2 hours leading up to the minimum.

This part of the September sky is also notable for some very distinctive asterisms (unofficial star patterns). These include the Great Square in Pegasus, the Big “W” of Cassiopeia, the House shape of Cepheus, and the Circlet and Vee in Pisces. Check them out while waiting for the Demon Star to dim and brighten.

September 2018 Sky Map

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

September 2018 Sky Map
Sky map produced using Chris Marriott’s Skymap Pro

Note: How to Read the Sky Map

Our sky map does not show the entire sky. Instead, the monthly map focuses on a particular region of the sky where something interesting is happening that month. 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 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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