Friday Beakful of Knowledge

Yesterday, I promised to explain the astronomical significance of the Tropics.

The Tropic of Cancer and the Tropic of Capricorn mark the latitudes where the sun is directly overhead on the June and December solstices, respectively. Between those two latitudes, at some point during the year, the sun is directly overhead. North of the Tropic of Cancer and south of the Tropic of Capricorn, the sun is never directly overhead; its highest point at noon is something less than 90 degrees from the horizon. In winter, the angle of noon is lower, and in summer, the angle of noon is higher.

The word "tropic" comes from the Greek word "tropikos," meaning "to turn." Because the sun appeared to turn around on the solstices--on the winter solstice, the angle of noon stopped descending and headed back up in the sky, vice versa on the summer solstice--the word also came to be synonymous with "solstice." Hence, the latitudes of the solstices became the Tropics.

The reason they are called the Tropic of Cancer and the Tropic of Capricorn brings us back to precession of the equinoxes. The two Tropics were named about 2000 years ago, when the sun appeared to be in the constellation Cancer on the June solstice and in the constellation Capricorn on the December solstice. Because of precession, the sun is now in the constellation Sagittarius on the December solstice and in Gemini or Taurus on the June solstice. The June solstice position crossed the official astronomical boundary into Taurus about 25 years ago, but the official and traditional boundaries differ; according to traditional boundaries, the June solstice is still in Gemini.