On July 20, 2017, if you could see stars at daytime, you’d see the sun leaving the constellation Gemini and entering the constellation Cancer the Crab. The sun will remain within Cancer’s borders until August 10, 2017, at which time the sun will move into the constellation Leo the Lion.
This doesn’t jibe with your horoscope? Right. It’s because we’re talking about actual constellations in the sky, not signs of the zodiac.
Chart of constellation Cancer via IAU
As seen from Earth, the sun appears to travel full circle (360o) in front of the 13 constellations of the zodiac in one-year’s time. Of course, this apparent eastward motion of the sun through the background stars is really a reflection of our planet Earth circling the sun.
Previously, we stated that the Earth goes full circle around the sun in one year. We should amend that statement to specify that Earth goes full circle around the sun, relative to the stars of the zodiac, in one sidereal year. So one sidereal year later, on July 20, 2018, the sun will again be at the border of the constellations Gemini and Cancer.
Some of you may know that the sun enters the sign Cancer every year on or near June 20, or precisely at the instant of the June solstice. That’s in spite of the fact that the sun enters the constellation Cancer about one month later, on or near July 20.
Have a globe? Look for the tropic of Cancer at about 23.5o north latitude. To this day, we still say the sun at its northernmost point on the June solstice resides at zenith (straight overhead) over the tropic of Cancer.
However, if we had a time machine, and could transport ourselves back to the year 12 B.C., we’d actually find the sun entering the constellation Cancer on the June solstice. The sun would also be entering the sign Cancer as well, when the constellation Cancer and the sign Cancer were in alignment on the sky’s dome over two thousand years ago.
The seasonal or tropical year, as measured by successive returns to the June solstice, is about 20 minutes shorter than the sidereal year – the year as measured by the backdrop stars. For that reason, the June solstice point has shifted about 30o westward relative to the constellations of the zodiac since the year 12 B.C. That means the sun on the June solstice now shines at the border of the constellations Gemini and Taurus, rather than at the border of the constellations Cancer and Gemini.
Signs of the tropical zodiac remain fixed relative to the equinox and solstice points. Therefore, the sun always enters the sign Cancer on the June solstice, irrespective of which constellation backdrops the sun at this time.
In our day and age, the sun enters the constellation Cancer each year on or near July 20, and then enters the sign Leo approximately two days thereafter, on or near July 22.
Bottom line: You may know that the sun enters the sign Cancer every year at the June solstice. The sun enters the constellation Cancer a month later, on or near July 20.
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.