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.
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 farther 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, and it’s also a variable star. That means it varies in brightness at a regular rate, every few days, which 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. Plus, 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.
The Double Cluster in Perseus
Last, but not least, 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.
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.
Bruce McClure served as lead writer for EarthSky's popular Tonight pages from 2004 to 2021, when he opted for a much-deserved retirement. He's a sundial aficionado, whose love for the heavens has taken him to Lake Titicaca in Bolivia and sailing in the North Atlantic, where he earned his celestial navigation certificate through the School of Ocean Sailing and Navigation. He also wrote and hosted public astronomy programs and planetarium programs in and around his home in upstate New York.
Our Editor-in-Chief Deborah Byrd works to keep all the astronomy balls in the air between EarthSky's website, YouTube page and social media platforms. She's the primary editor of our popular daily newsletter and a frequent host of EarthSky livestreams. Deborah created the EarthSky radio series in 1991 and founded EarthSky.org in 1994. Prior to that, she had worked for the University of Texas McDonald Observatory since 1976, and created and produced their Star Date radio series. She has won a galaxy of awards from the broadcasting and science communities, including having an asteroid named 3505 Byrd in her honor. In 2020, she won the Education Prize from the American Astronomical Society, the largest organization of professional astronomers in North America. A science communicator and educator since 1976, Byrd believes in science as a force for good in the world and a vital tool for the 21st century. "Being an EarthSky editor is like hosting a big global party for cool nature-lovers," she says.
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