It is rare when we highlight a celestial event (or events) that involves the zodiacal constellation of Cancer the Crab. In Greek mythology, Cancer was summoned to distract Hercules when he was battling the multiheaded Hydra the Serpent. The crab was crushed by Hercules’ foot, but to reward his effort Hera placed it among the stars.
One of my astronomy mentors, the late Dr. Ken Franklin of New York’s Hayden Planetarium, used to refer to Cancer as “the empty space” in the sky. Indeed, it’s the least conspicuous and second smallest of the 12 zodiacal constellations and quite frankly, aside from being in the zodiac, it’s probably noteworthy only because it contains one of the biggest and brightest open star clusters in the sky. More on that in a moment.
Appearing more than halfway up in the west-southwest sky at nightfall in early May, Cancer occupies the seemingly vacant area between two of the sky’s showpieces: the stars marking the heads of Gemini the Twins, Pollux and Castor (to its lower right), and the famous Sickle or backwards question mark pattern of stars marking the head and mane of Leo the Lion (to its upper left).
An M&M night (Moon and Mars)
On Saturday evening (May 3), as the sky darkens, check out the rather wide (45-percent illuminated) waxing crescent moon, which will happen to be located in the middle of Cancer. You probably will not notice any of its associated stars, but what you will notice is a bright yellow-orange object positioned closely below the moon. That’s not a star, however, but a planet: Mars.
If you haven’t looked at it for a few months, you might be surprised, since Mars appears much dimmer now compared to back in mid-January, when it was so much closer to Earth. On Jan. 12, Mars was at its closest point to the Earth for 2025, 59.7 million miles (96.1 million km) away. At that time, Mars was shining practically as bright as Sirius, the brightest star in the sky. But now Mars has receded to a distance of 135 million miles (217 million km) from Earth, and consequently it now appears only one-ninth as bright. On Saturday evening, you’ll see Mars sitting a little over one degree below the moon’s southern limb.
And if you have binoculars, you’ll also notice a splattering of tiny stars just off to the left of Mars, marking the heart of the crab and the star cluster that we had mentioned earlier.
Little Mist … Big Star Cluster
Cancer’s big star cluster is known formally as Messier 44 (M44), but is more popularly…
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