Tonight – April 4, 2017 – the waxing gibbous moon can guide you to the location of the constellation Cancer the Crab on the sky’s dome. Cancer is faint, so you won’t see it well in the moonlight. But, if you note Cancer’s location with respect to other stars, and come back when the moon has moved away, you will glimpse a beautiful star cluster within Cancer.
You’ve probably heard of the constellation Cancer, but there’s a good chance you’ve never seen it. As constellations go, Cancer the Crab is probably the least seen among the most famous constellations. Its primary competitors in the famous-but-not-recognizable category are probably Aries the Ram and Aquarius the Water Bearer.
Why is Cancer so elusive? Simply because it’s faint. Cancer’s brightest star is magnitude 3.5, meaning that none of these stars is visible from light-polluted cities or suburbs.
That’s why tonight might be your night. The waxing gibbous moon shines right in front of this famous constellation this evening, so when you gaze in the moon’s direction you are gazing toward Cancer.
Alas, the moon is very bright, and Cancer is faint. Even if you have a clear sky, the bright light of the moon will wash out this constellation’s stars from view.
Starting around mid-April 2017, the moon will have dropped out of the early evening sky, and you’ll be able to identify Cancer. Use bright the bright Gemini stars, Castor and Pollux, plus Regulus, the brightest star in the constellation Leo the Lion, to find it. Cancer faithfully glimmers between Regulus and the two bright Gemini stars, Castor and Pollux.
The moon will move away eventually, but remember those stars! They’ll always be near Cancer.
And, if you see Cancer on a moonless night, you’ll have a wonderful surprise in store, especially if you’re looking in a dark sky away from city lights:
In a dark sky, if your eyesight is good, you can make out a patch of haze in the midst of Cancer’s stars. If you have binoculars, you’ll more clearly see a cluster of stars within Cancer, called Praesepe – also known as the open star cluster Messier 44 – most frequently called the Beehive.
Cancer is well known not because it’s bright, but because the sun in its yearly journey passes directly in front of it from about July 20 to August 9.
So by definition, faint as it is, Cancer is a constellation of the zodiac.
Bottom line: Use the moon to locate the constellation Cancer on April 4, 2017. Cancer is faint so you won’t see it well until the moon moves away.