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The Tropic of Capricorn is the most southerly latitude at which the Sun can appear directly overhead. It only happens once a year at the December solstice. It is one of the five major circles of latitude along with the Tropic of Cancer, the Arctic and Antarctic Circles, and the equator. All of these points, except the equator, are dictated by the tilt of the Earth’s axis of rotation relative to the plane of orbit. It currently lies 23°26?16? south of the equator. It is currently drifting north at the rate of almost half a second of latitude, which is about 15 meters per year.
In the northern hemisphere the equivalent of the Tropic of Capricorn is the Tropic of Cancer. The position of the Tropic of Capricorn is not fixed, but varies over time. It received its name about 2,000 years ago when the sun was entering the Capricornus constellation at the December solstice. Currently, due to the precession of the equinox, the sun appears in the Sagittarius constellation.
The Earth’s axial tilt varies between 22.1° and 24.5° over a 42,000 year period, and the tilt is decreasing. In addition to this steady decrease there are much smaller short term variations, known as nutation, mainly due to the changing plane of the moon’s orbit. This can shift the Earth’s axial tilt by plus or minus 0.005 degree. This is part of the cause of precession of the equinox.
We have written many articles about the Tropic of Capricorn for Universe Today. Here’s a Bad Astronomy and Universe Today forum discussion about the Tropic of Capricorn, and here’s an article about the Tropic of Cancer.
We’ve also recorded an episode of Astronomy Cast all about planet Earth. Listen here, Episode 51: Earth.