Tonight – June 26, 2017 – the waxing crescent moon beams in front of the constellation Cancer, the faintest constellation of the zodiac. We draw in the stick figure of Cancer for you on the star chart above. In the real sky, you’re likely not to pick out any stars in the moon’s vicinity, because this constellation is faint … and obscured by evening twilight. But Cancer’s stars will be there, behind the moon’s glare, all the same.
Zodiacal constellations like Cancer distinguish themselves from other constellations in that the sun, moon and planets travel within their borders.
The sun travels in front of the constellation Cancer from about July 21 to August 10 annually. That may seem to be at odds with the horoscope page in the newspaper, which probably gives the sun’s passage through Cancer for around June 21 to July 21.
Why the discrepancy? It’s because a zodiac sign is different from a zodiac constellation, though people often confuse the two.
Here’s the definition of a tropical zodiac sign. By definition, the sun is always at the first point of the sign Cancer on the June solstice, irrespective of which group of stars backdrops the sun at this time. Look at the globe to note the Tropic of Cancer, which marks the northernmost extent of the sun’s travels on the June solstice.
Here’s the definition of a zodiac constellation. Any constellation can be thought of as a section of the starry sky, seen and wondered about for millennia by stargazers. In our time, constellations have official borders, which were agreed upon by the International Astronomical Union in the 1930s.
Over two thousand years ago, the sun actually shone in front of the constellation Cancer on the June solstice. Today, the June solstice sun shines in front of the constellation Taurus. Therefore, the first point of the sign Cancer presently resides in the constellation Taurus.
Because the Earth’s axis continually changes its tilt relative to the backdrop stars, the June solstice sun swings westward through all the constellations of the zodiac in a period of about 26,000 years. All the while, the June solstice sun always marks the first point of the sign Cancer.
Bottom line: The moon is in front of the constellation Cancer the Crab on June 26, 2017.
Bruce McClure has served as lead writer for EarthSky's popular Tonight pages since 2004. 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 writes and hosts public astronomy programs and planetarium programs in and around his home in upstate New York.