The Legacy of the Celestial Crab
Located between Gemini and Leo lies Cancer, the faintest of the twelve zodiac constellations. Cancer represents the crab that, in Greek mythology, bit the toes of Heracles during his battle with the multi-headed Hydra. Angered, Heracles (the Greek name for Hercules) crushed the crab with his foot. The hapless creature was banished to the heavens where it is one of the most inconspicuous of the classical constellations.
Despite its faintness, the constellation of Cancer is of some significance. A little over 2,000 years ago the Sun, in its annual track through the zodiac, would reach the Summer solstice point near the stars of Cancer. In the northern hemisphere, Summer solstice occurs when the Sun is at its northernmost distance from the celestial equator, and hence the occasion of the longest day in the northern hemisphere. The celestial cartographers of the time placed the crab there because the solstice also marks a reversal in the Sun's north-south motion, similar to how a living crab will hesitate then change the direction it moves. Due to precession (a wobble in the Earth's axis that takes 26,000 years to complete one cycle) the Summer solstice point currently occupies a position in Taurus near the border with Gemini. However, the Tropic of Cancer provides an ongoing legacy to its namesake, for it is the northern latitude where the Sun lies directly overhead at noon on Summer solstice.
The constellation Cancer contains no stars brighter than 4th magnitude which makes it very difficult to see in light-polluted skies. It does however contain a wonderful star cluster that is commonly called the Beehive (M44 on the map). Before the invention of the telescope this object was only visible as a small fuzzy patch or cloud in the night sky. To some ancient astronomers it was also known by the name Praesepe which means the "Manger." The two nearby stars on either side are associated with an ancient myth and are called "the donkeys", presumably feeding on the hay from the manger.
It wasn't until 1610 when Galileo pointed his telescope to Praesepe that the glow was revealed for the first time to be a multitude of faint stars, more like a celestial beehive than a cloud. Only the slightest optical aid (binoculars or a small telescope) is required to resolve the widely scattered stars in M44. Can you see the beehive?
Clear skies till next month!