Constellation of the Month: Cancer, the Crab
Cancer, the Crab, hangs high in our western sky during evenings in May. This constellation consists of four fourth-magnitude stars that form a faint "Y" in the sky. If you are in a dark area, you will notice a fu--y patch near the intersection of the "Y." This is the bright open cluster M44, known as the Beehive or Praesepe.
This constellation is related to Hercules, the great hero. Zeus had been up to his old tricks, and the result was Hercules. Zeus's wife, Hera, was furious, and swore to kill Hercules. None of her attempts succeeded, and so she caused Hercules to go mad and kill his wife, Megara, as well as their children. To atone for this act, he had to accomplish 13 tasks to be forgiven.
One of these famous Labors of Hercules was to dispatch the Lernaen Hydra. While Hercules was fighting the Hydra, Hera thought she could distract Hercules by sending a giant crab to pinch his toe during the battle. Hercules did not miss a beat: With his mighty foot, he stepped on the crab, smashing its shell and killing it. To honor the crab's service, Hera put the crab in the sky forevermore as the constellation Cancer.
As if this were not enough mythology, the two stars closest to M44 (the Beehive) are called Assellus Borealis (the Northern Ass, 43 Gamma Cancri) and Assellus Australis (the Southern Ass, 47 Delta Cancri). According to Eratosthenes, these were the asses that the gods Dionysus and Silenus rode to do battle with the Titans. The braying of these two animals so frightened the Titans that the gods won the war and, as a reward, the two animals were put in the sky. To provide them food, a manager (Praesepe in Latin) was put there with them.
The Greek poet Aratos was the first to record this open cluster as "a little mist" in 260 BC. Galileo was the first to resolve it into 40 faint stars with his telescope in 1609. These 40 stars (and more than 60 additional ones) occupy an area some 58 trillion miles across, at a distance of 3,400 trillion miles. In our sky, this area is 1.6 degrees across, with a total magnitude of 3.1.
The Praesepe is similar in composition, age and motion to the Hyades cluster in Taurus. They probably both came from a common gas cloud some 400 million years ago. They are now 60 degrees apart in our sky.
The Planets for May 2006
Mars is a tiny 4.6 seconds-of-arc across this month. It will be in Gemini all month, shining at magnitude 1.6. It crosses Castor's legs and Pollux's waist as it traverses this constellation, adding an extra red "star" to the Twins. Mars is about halfway up in the west as it gets dark, and sets around midnight.
Saturn shines at magnitude zero in our constellation of the month, Cancer. Hanging high in the west at sunset, Saturn sets around 1 a.m. Saturn will be passing 0.8 degrees south of the Praesepe next month on June 5. For this month, Saturn's ball is 17.6 seconds-of-arc across, and the rings are 39.9 seconds-of-arc across with the southern face showing. They are tilted up 19.7 degrees to our line-of-sight.
Jupiter begins the month near the third-magnitude star Zubenelgenubi in the constellation Libra. It will drift slowly eastward away from Zubenelgenubi, but will remain in Libra all month long. The Giant Planet is low in the east after sunset, and sets around 5:30 in the morning. Jupiter is in opposition on May 4, when it is exactly on the opposite side of the Earth from the Sun. At midmonth, Jupiter will be at magnitude -2.5, stretching 44.5 seconds-of-arc across.
The brightest object in the morning sky besides the Sun and Moon is the planet Venus. The Goddess of Love rises around 4:30 a.m., and remains visible until just before sunrise. Dimming slightly, Venus is at magnitude -4.0 and is in the gibbous phase at 71 percent illuminated. Venus is slowly shrinking and is 15.5 seconds-of-arc across. So when you see this glittering jewel in the early morning sky, make a wish on the Morning Star and "keep watching the sky"!
An amateur astronomer for more than 35 years, Bert Stevens