Among the 88 constellations, Cancer is certainly one of the least conspicuous.
Yet it’s a name familiar to most everyone, since Cancer is part of the zodiac—the band of sky through which the sun, moon, and planets move. This constellation is difficult to spot, however, because of its faint stars; only one is barely brighter than fourth magnitude. In light-polluted city skies, Cancer is outright invisible: It looks like a hole in the sky between Regulus in the constellation Leo and Castor and Pollux in Gemini. At nightfall in April, these easy-to-spot stars shine in the southwest sky, about fifty degrees above the horizon. Procyon, the brightest star in the constellation Canis Minor (“Lesser Dog”), sits just below Cancer.
Making heads or tails of the animals, objects and characters that the constellations are supposed to represent is no easy feat. Cancer is a prime example, with its main stars forming an upside-down Y in the sky. You’ll need tons of imagination to make out a crab or crayfish! The stars at the top of the inverted Y and the left branch form the animal’s claws, and the middle area its eyes.
This constellation was first drawn up in antiquity. The Babylonians imagined a turtle in this area of the sky, and the Egyptians a beetle. It was only in Ancient Greece that the constellation became associated with a clawed creature. According to ancient mythology, the crab was sent by the goddess Hera to torment Hercules during his battle with the Hydra, but the demigod crushed the crab with his foot, after which Hera placed it among the stars as the constellation Cancer.
Ready for another interesting fact? Between 900 and 500 B.C., the sun reached its most northerly position in this constellation on the summer solstice, which is why the tropic in the Northern Hemisphere is called the “Tropic of Cancer.” (This position now occurs in the constellation Gemini due to the precession of the equinoxes.)
While the constellation Cancer may appear unremarkable at first, deep in its centre is the wondrous star cluster called M44—the 44th entry in 18th-century astronomer Charles Messier’s catalogue of celestial objects—commonly known as the Beehive Cluster because of its resemblance to a swarm of busy bees. In very dark skies, M44 looks like a hazy patch of light to the naked eye, but binoculars or a small telescope will reveal its stunning glory. Located 600 light-years away from us, it contains nearly 1,000 stars born around 750 million years ago.
On the evening of April 13, the gibbous moon will sit immediately to the left of M44. It’ll be a wonderful opportunity to spot this star cluster more easily, but you’ll need binoculars to distinguish it through the dazzling light cast by our moon.
If you have access to a telescope, you’ll also be able to see yet another star cluster known as M67, located close to the left tip of the upside-down Y. At over four billion years old, it is one of the oldest known open clusters.
The planets in April
Mars remains the only planet visible during the first part of the night. You can look for it above the western horizon once the sky darkens, about 45 minutes after sunset. Its orange hue will make it easy to spot. But don’t confuse Mars with a slightly brighter star located nearby: Aldebaran, the alpha star in Taurus, is very similar in colour to the red planet.
Around 8:30 on the evening of April 8, Mars will be 30 degrees above the western horizon, with Aldebaran to its left and the crescent moon below. The next day, the moon will hang a few degrees above Aldebaran. As the evenings progress, you’ll see that Mars moves as well, along a path that gradually takes it a dozen degrees above the brightest star in Taurus.
Jupiter steals the show in the second half of the night. The brilliant gas giant rises earlier and earlier above the southeastern horizon; it appears around 1:30 a.m. in early April and around 11:30 p.m. at the end of the month. Saturn emerges in the same direction as Jupiter, but approximately two hours later, although the famous ringed planet shines much less brightly than Jupiter. The moon will join the planetary duo late at night from April 23 to 25. At around 4:30 a.m. on the 23rd, the gibbous moon will sit just 2 degrees to the right of Jupiter due south, while Saturn will shine to the left of the two celestial bodies. The moon will appear between the pair on the morning of April 24, and less than 4 degrees to the right of the ringed planet on the 25th.
Early risers will still have a chance to catch a glimpse of Venus. In April, the dazzling Morning Star rises in the east barely an hour before the sun and will be visible only in the light of dawn, very low on the horizon. On April 1, the thin crescent moon will hang to the right of Venus.