By Peter Forrester | March 9, 2019
Next in our series of Zodiac Constellations is Cancer, the crab. In case you’re wondering, cancer is Latin for crab, and the disease is named after the crab, rather than the other way around.
Cancer is just to the left of Gemini. It is visible in the early evening in the eastern sky. However, it is the second dimmest of the 12 zodiac constellations. Its brightest star, Beta Cancri, only has an apparent magnitude of 3.5, and there is only one other star that is brighter than 4th magnitude. So you will need a clear moonless sky and be removed from city lights in order to see it. This is the best month to see it, around 9 pm. The Sun is located in Cancer between July 20 and August 9, although astrologers will use other dates for their sign of the zodiac called Cancer.
Just to the left of Cancer is another bright constellation, Leo the Lion. So the best way to find Cancer is to look in between Leo and Gemini. Look for a shape somewhat like an upside-down Y. The points are often identified as the pincers of a crab, but it has been described as various other creatures by ancient societies.
Ten stars in Cancer are known to have planets. One of them, called 55 Cancri, has 5 known planets – one a bit bigger than Earth, and the other 4 being gas giants. One of these gas giants is in the “habitable” zone where, based on its distance from the star, liquid water could exist and therefore life similar to what we have on Earth.
Another prominent object in Cancer is an “open cluster” of stars called Praesepe (Latin for manger), also known as the Beehive Cluster. It is located about 600 light-years from Earth, but is still one of the closest open clusters, defined as “a group of up to a few thousands stars that were formed from the same giant molecular cloud and have roughly the same age”.
Praesepe has an apparent magnitude of 3.7, very similar to Beta Cancri. The name comes from the ancient Greeks and Romans, who saw it as a manger out of which two donkeys were eating. This cluster was one of the first things Galileo Galilei looked at with his telescope in 1609. He saw 40 stars in Praesepe, which just looks like a nebula or cloud to the naked eye (best time to observe it is from February to May). Three planets are now known to exist orbiting two Sun-like stars in the cluster.
Coming back to Earth, the Tropic of Cancer is named for the constellation. It is a line you will see on a globe, the northern edge of a band around the middle of the planet known as the Tropics. It is about 23.5 degrees north of the Equator, and the line is the furthest north where the Sun can be directly overhead. Hawaii is located just south of this line (except for the many uninhabited islands at the northwest end of the island chain). The line is called after the constellation Cancer because the time the Sun is overhead is during the summer solstice (June 20), during which the Sun used to be located in Cancer (it is now in Taurus, because of a process called “precession of the equinoxes” or “axial precession”). The southern edge of the Tropics is the Tropic of Capricorn, which is the same distance south of the Equator.
I wish you all the best in your continued effort to admire these amazing objects in the sky above you. And soon it will be warm enough that you actually want to go out there at night. By the way, don’t forget to turn your clocks ahead tonight or in the morning for our annual tradition called Daylight Savings Time (at least for most of the United States).
Previous in series: Zodiac Constellation #1: Gemini
Wikipedia. (2019, January 11). Axial precession. Retrieved from https://en.wikipedia.org/wiki/Axial_precession.
Wikipedia. (2019, February 14). Beehive Cluster. Retrieved from https://en.wikipedia.org/wiki/Beehive_Cluster.
Wikipedia. (2019, February 14). Cancer (constellation). Retrieved from https://en.wikipedia.org/wiki/Cancer_(constellation).
Wikipedia. (2019, March 6). Open cluster. Retrieved from https://en.wikipedia.org/wiki/Open_cluster.
Wikipedia. (2019, March 5). Tropic of Cancer. Retrieved from https://en.wikipedia.org/wiki/Tropic_of_Cancer.