Autumn Arrives Today

Greg Wilson/Anderson Observer

Autumn arrives this morning for all of us in the Northern Hemisphere, while those in the Southern Hemisphere are gearing up for warmer spring weather. 

Today at 8:44 a.m. Eastern Daylight Time, the sun will cross the celestial equator, or an imaginary line that projects Earth's equator into space. At this exact moment, the Northern and Southern hemispheres will receive an equal amount of sunshine, and the length of day and night will be approximately equal around the world — hence the term "equinox," which is derived from the Latin phrase meaning "equal night."

Most years, this happens on either Sept. 22 or 23. However, every once in a while, the autumn equinox can occur on Sept. 21 or 24. This happens because the length of a calendar year (365 days) is not equal to the time it takes for Earth to travel around the sun (365.25 days). To make up for this inconsistency, people have observed "leap years" for the last two millennia. By adding a "leap day" (Feb. 29) to the calendar every four years, we have managed to keep our seasons more or less consistent from year to year. 

However, leap years don't ensure that equinoxes always fall on the same date. "Because of leap years, the dates of the equinoxes and solstices can shift by a day or two over time, causing the start dates of the seasons to shift, too," according to The Old Farmer's Almanac.