Day 45: All About The Tropics

Today, we crossed over the Tropic of Capricorn, heading south towards New Zealand. As I write this, we are heading 166 degrees south, at a latitude of 25 degrees, 28 minutes, and 35 seconds south. That got me wondering, what are the tropics, why are they named Cancer and Capricorn, and what's the reasoning behind it all? The quick Wikipedia definition is "The tropics are the regions of Earth surrounding the Equator. They are defined in latitude by the Tropic of Cancer in the Northern Hemisphere at 23°26′10.2″ (or 23.43615°) N and the Tropic of Capricorn in the Southern Hemisphere at 23°26′10.2″ (or 23.43615°) S." That is a lot of math and geometry. 

Put in layman's terms, if you're in either of the Tropics, the sun will be directly overhead at least once at noon. On the tropical line at 23.5 degrees, you'll only see that at summer solstice. The closer you are to the equator, the more often that occurs. So that's an easier explanation - but why 23.5 degrees?

So the earth rotates around the sun, but it does so at an angle, or what scientists call an axial tilt. And that offset is .... 23.5 degrees. The diagram to the right illustrates that. Actually, as scientific nerd trivia, the axial tilt varies over a 41,000-year cycle from 22.1 to 24.5 degrees. 

My final question was, "Why Cancer and Capricorn?" It obviously had something to do with the stars and constellations. Over 2,000 years ago, when the line of latitude was named, the Sun was in the constellation of Cancer at the June solstice. No longer. Because of the precession of the equinoxes, the sun is now in Gemini. But don't worry - in about 23,000 years, it will be back in Cancer. Likewise, for Capricorn - which was in Capricornus back then but is now in Sagittarius. 

So now you know!