What’s Up with the Tropics?

Latitude and longitude coordinates are seen everywhere; they’re immensely useful for marking geographic location and helping with navigation. The system is simple enough: latitude is distance north or south from the equator and longitude is distance east or west from the Prime Meridian. But what are the Tropics of Cancer and of Capricorn that are referenced on the map? What is the significance of these specific latitudes?

The region between the Tropic of Cancer and the Tropic of Capricorn encompasses all locations on Earth where the Sun reaches the zenith at local noon, which happens twice a year. The Tropic of Cancer is the northernmost latitude where this occurs, and the Tropic of Capricorn is the farthest south. The sun reaches the zenith at the Tropic of Cancer at the June solstice, marking summer for the northern hemisphere. Likewise, the sun reaches the zenith at the Tropic of Capricorn at the December solstice, marking summer for the southern hemisphere.

But why the references to the constellations of Cancer and Capricorn? Thousands of years ago, when the Tropic of Cancer was named, the June solstice occurred when the Sun was located in Cancer. However, due to the precession of the Earth’s rotational axis, the solstice no longer occurs when the Sun is in Cancer. The likewise is true with the December solstice and Capricorn. Although the timing of the solstices in relation to the stars has changes, it is still much easier to keep the same names rather than constantly renaming these special latitudes.