Autumnal Equinox 2014: Facts About the First Day of Fall

Pack ice forms along a barrier island outside Kaktovik, in the Arctic region of Alaska.

PHOTOGRAPH BY SYLVAIN CORDIER, CORBIS

John Roach

for National Geographic

PUBLISHED SEPTEMBER 22, 2014

Say goodbye to summer: The Northern Hemisphere's autumnal equinox—the first day of fall—occurs Monday, September 22.

With its brilliant colors and crisper days, autumn is a favorite season for many. But what actually happens to make the seasons change? (See National Geographic's pictures of fall.)

The answer is the "clearly definable" position of thesun on the summer and winter solstices, the lateJudith Young, a professor of astronomy at the University of Massachusetts in Amherst, told National Geographic News in 2011.

"The solstices are very accurately measured as the northernmost point that the sun rises along the horizon in June and the southernmost point along the horizon in December," she said. (Seepictures of the sun's path across the sky—an entire year in a single frame.)

In modern times, the solstice points became the astronomical definitions of when the summer and winter seasons begin. In the Northern Hemisphere, June features the summer solstice, while in the Southern Hemisphere, June marks the first day of winter. (See "Summer Solstice Pictures: From Stonehenge to Carhenge.")

Since the equinoxes fall roughly halfway between the solstices, they got pegged as the starts of the other two seasons, fall and spring, Young said.