On Sunday, September 22, the earth's Northern Hemisphere will move into fall and the Southern Hemisphere will move into spring. The celestial shift that prompts this transition is called an equinox, which happens biannually, around March 21 and September 21.
While rotating on its polar axis every 24 hours, the earth sees night and day - as well as the annual cycle of seasons caused by its 365.25 orbit around the sun. The equinox (latin for equal night) occurs when the earth's rotation intersects with its orbit. Because Earth is so huge, its mass creates an extremely powerful gyroscopic effect, causing the poles to mostly point in the same direction (though major earthquakes can cause tiny axis wobbling). The direction the north and south poles are pointing has very important consequences for the change of seasons.
The poles are tilted 23.4 degrees relative to the earth's plane of orbit. This tilt always points toward the celestial pole in the sky, making it appear that the sun is moving across the sky at an angle to the celestial equator, from the vantage of Earthlings. Twice a year, the sun crosses the celestial equator, changing the direction the rays of the sun fall on Earth.
On Sunday, the sun will move from the Northern Hemisphere to the Southern Hemisphere, passing along the earth's equator, rising exactly in the east and setting exactly in the west. Day and night will be of roughly equal length, hence the equinox name. On Monday, the sun will begin shining more on the southern half of Earth, and less in the north, where autumn will begin. For the next 3 months, the sun will continue to "move" southward, marking the solstice on December 21. The days in the north will gradually become shorter and colder. Though, after the solstice, more sunshine will begin coming northward again.
In related news, one can still see the tail end of the Harvest Moon (or Full Corn Moon, as Algonquian tribes called it) in the night's sky. Up until the solstice in December, one also has a chance to check out a “Hunter’s Moon”, a “Blood Moon”, a “Sanguine Moon”, a “Beaver Moon”, a “Frosty Moon”, an “Oak Moon”, a “Cold Moon” and a “Long Nights Moon”, depending on which lunar nomenclature one prefers.
Image courtesy of Wikimedia Commons.