The 16th Solar term: the Autumnal Equinox

You may know that the solstices and equinoxes signal the changing of the seasons on Earth, but do you remember which is which? Are they just different names for the same thing? A solstice and an equinox are sort of opposites.

The seasons on Earth change because the planet is slightly tilted on its axis as it travels around the Sun. This means different points on Earth receive more or less sunlight at different times of year. If Earth were not tilted, the Sun would always appear to be directly above the Equator, the amount of light a given location receives would be fixed, and there would be no seasons. There also would be no need to mark equinoxes or solstices.

Because the Earth is tilted on its axis concerning the plane of its orbit around the sun, different parts of its surface are in direct (overhead) sunlight at other times of the year.

The solstices happen in June (20 or 21) and December (21 or 22). These are the days when the Sun's path in the sky is the farthest north or south from the Equator. A hemisphere's winter solstice is the shortest day of the year and its summer solstice is the year's longest. In the Northern Hemisphere, the June solstice marks the start of summer: the North Pole is tilted closest to the Sun, and the Sun's rays are directly overhead at the Tropic of Cancer. The December solstice marks the start of winter: at this point, the South Pole is tilted closest to the Sun, and the Sun's rays are directly overhead at the Tropic of Capricorn. (In the Southern Hemisphere the seasons are reversed.)