Cassiopeia and Perseus on October evenings

Star chart shows Cassiopeia and Perseus, above and below, with stars Mirfak and Algol.
Find Perseus with the help of the constellation Cassiopeia the Queen. Cassiopeia’s distinctive M or W shape is ascending in the northeast on Northern Hemisphere autumn evenings, with Perseus following. Cassiopeia and Perseus travel together in a great arc around the northern sky. Be sure to look for the star Algol, sometimes called the Ghoul Star or Demon Star … a perfect star for Halloween!

If you know how to locate the easy-to-spot constellation Cassiopeia the Queen, with its distinctive M or W shape, then you’re ready to find Perseus. Perseus the Hero follows Cassiopeia across the sky as seen from the Northern Hemisphere on autumn and winter evenings. Cassiopeia and Perseus are tokens of autumn in the Northern Hemisphere.

Cassiopeia’s bright stars and easy-to-recognize shape makes it easy to identify and, therefore, one of the most famous constellations in the sky. Perseus’s stars are fainter than Cassiopeia’s, and its shape is less pronounced. Because it appears at its best during the evening on cold months, I think of its brightest stars as forming an icicle dangling down from Cassiopeia, with the star Algol off to the side. If you have access to a dark sky, it will be that much easier to spot Perseus.

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Antique etching of Perseus the Hero in Greek warrior garb, holding a sword and the snaky head of Medusa.
In skylore and mythology, Perseus beheaded Medusa, a witch who had snakes for hair. This image is from Urania’s Mirror, a set of constellation cards published in London around 1825. Image via Wikimedia Commons.

Where to find Cassiopeia and Perseus

You’ll see both Cassiopeia and Perseus in the northeast on October evenings. The pair will be higher up in the evening sky in late autumn and winter. Or, as night passes, you’ll see them both ascending in the northeast, then arcing high in the north, then descending in the northwest, with Perseus following Cassiopeia all the while. Both Perseus and Cassiopeia are circumpolar from about 40 degrees north latitude to further north. In other words, as seen from northern parts of the U.S. and Canada, they never set below the horizon, but instead circle endlessly around Polaris, the North Star.

The brightest star in Cassiopeia is Schedar. However, all the stars shine similarly bright, which is part of what makes the constellation so easy to pick out.

The brightest star in Perseus is Mirfak. But its second brightest star, Algol, gets more attention. Algol is the Demon Star. It marks the head of the demon, Medusa, while it’s also an inconstant star. It varies in brightness at a regular rate, every few days, that observers can watch without the need for optical aid.

The radiant for the Perseids

Although these constellations are best seen in the evening in autumn and winter, you can also see Cassiopeia and Perseus in the latter part of Northern Hemisphere summer, from late night until dawn. The annual Perseid meteor shower peaks around August 12 or 13, and if you trace the paths of these Perseid meteors backward, they’ll appear to originate between the two constellations.

Star chart with small cross showing meteor radiant point between Cassiopeia and Perseus, with labels.
The radiant point of August’s famous Perseid meteor shower is between Cassiopeia and Perseus. Image via
Circular photo of entire sky with stars and Milky Way and short bright streak above center.
View at EarthSky Community Photos. | Tom Wildoner of the Dark Side Observatory in Weatherly, Pennsylvania, has an automatic camera set up for meteor observing. He caught this one on August 11, 2019, and wrote: “You can see this bright meteor streak above center near the constellation Cassiopeia (sideways W) and pointing in the direction of Perseus. The brighter stars have been enhanced in this image to help orient your view. North is marked on the image.” Thanks, Tom!

The Double Cluster in Perseus

After you’ve found Cassiopeia and Perseus, be sure to scan between them with your binoculars. Assuming your sky is dark, you’ll easily spot the magnificent Double Cluster in Perseus. This pair of open clusters makes for an easy target through a telescope and will wow your friends.

Star field with two separate tight groupings of multiple stars.
View larger.| The Double Cluster in Perseus, via Tom Wildoner at the Dark Side Observatory in Weatherly, Pennsylvania.

Bottom line: Cassiopeia and Perseus are neighbors in the fall sky. Use Cassiopeia’s distinctive W or M shape to locate the dimmer Perseus on autumn and winter evenings.

October 28, 2021

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Bruce McClure

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