The Zodiac is the narrow band of stars circling through the heavens, marking the sun’s annual path or moon’s monthly path in front of the backdrop stars. Tonight’s moon – May 23, 2015 – is meeting up with the brilliant planet Jupiter in front of one of the faintest constellations of the Zodiac, the constellation Cancer the Crab. Although Cancer is faint, you can use the moon and Jupiter – the brightest and third-brightest celestial bodies of nighttime, respectively – to locate this hard-to-see constellation this evening.
Venus, the second-brightest celestial object in the evening sky, after the moon, appears in the western sky at nightfall. The sky chart below shows a greater area of the May 23 evening sky, to include the planet Venus below the moon and Jupiter, and the star Regulus above the moon and Jupiter.
Why does the moon pass this constellation so often? The moon orbits nearly on the ecliptic, or plane of Earth’s orbit around the sun. So as the moon makes its monthly rounds – orbiting again and again around Earth – it moves in front of the constellations of the Zodiac, passing the same stars and constellations again and again.
We draw in the stick figure of Cancer the Crab in the sky chart below. You may have difficulty seeing it, however, because all of Cancer’s stars are pretty dim.
Take a look at the above chart to see the boundaries of Cancer. The sun passes in front of this constellation from about July 20 to August 10 every year. Wait, you say, that doesn’t match astrological dates? That’s because the constellation of Cancer as we see it in the real sky – and the astrological sign of Cancer – are not the same thing.
Cancer, though one of the dimmest constellations of the Zodiac, is still very significant. In ancient times, this constellation won much fanfare because the sun shone in front of Cancer during the Northern Hemisphere’s summer solstice. Some 2,800 years ago, the star Asellus Australis actually marked the summer solstice point in the sky. Nowadays, the sun has its annual conjunction with Asellus Australis on or near August 1.
Also in our day, the sun shines in front of the constellation Taurus the Bull on the summer solstice. Because the sun reaches its northernmost point from the Earth’s equator on the summer solstice, you’d think the northernmost extent of the sun’s travels would be called the tropic of Taurus. But no. Look on the globe and you’ll find the sun’s northernmost latitude labeled as the tropic of Cancer, in deference to this faint yet celebrated constellation.
Bottom line: On May 23, 2015, the moon is in front of the faint constellation Cancer. The moon’s glare will make Cancer tough to spot. Good night to use your imagination!