Autumnal Equinox 2011: The First Day of Fall is Here

The first day of fall is here. The autumnal equinox 2011 occurred today at around 5 a.m., signaling the start of the autumn season.

The four seasons, winter, spring, fall, and summer, aren’t just an arbitrary segments of the year. Each season coincides with a cosmic occurrence. Judith Young, a professor of astronomy at the University of Massachusetts in Amherst, says that each season has a start and an end that is clearly defined by the position of the sun.

Young said:

“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. It doesn’t matter where you are on Earth—that’s true.”

The equinoxes fall roughly halfway between the two solstices and signal the start of spring and fall. Here’s a video from National Geographic explaining the seasons.

According to Space.com, the word equinox means equal day and night, but that’s a little over simplified. Today, people will see slightly more daylight than darkness. The true 50/50 split between light and dark won’t occur for a few days.

Space.com also points out that the Autumnal Equinox isn’t a day, but a specific moment in time. The equinox occurs when the sun crosses the celestial equator, which occurred this morning at about 5 a.m. EST.

The Autumnal Equinox occurs around the same time each year, but because of the Gregorian Calendar the equinox doesn’t always occur on the same day. The Gregorian calendar defines a year as 365 days, while it actually takes the earth 365.25 days to orbit the sun.

Are you doing anything to celebrate the first day of fall?