Autumnal Equinox 2012: First Day Of Fall Explained By Shifting Sunlight, Shadow (VIDEOS)

Summer's warm days have ended, and the first day of fall approches.

Technically speaking, the first day of fall is known as the autumnal equinox, and it is expected to arrive Saturday, Sept. 22, at 6:49 a.m. EDT (14:49 Universal Time). In the Northern Hemisphere, the sun rises later and the nights come sooner. In the Southern Hemisphere, spring begins. On the equinox, the day and night are of approximately equal length.

EarthSky describes the equinox, which occurs twice per year, as "when the tilt of the Earth’s axis and Earth’s orbit around the sun combine in such a way that the axis is inclined neither away from nor toward the sun."

This change occurs due to the shifting position of the Earth as it rotates around the sun annually, according to NASA. Seasons change as the North Pole points towards and away from the sun. When it is pointed towards the sun, the sun's rays hit it more directly and there is summer. At this time, the South Pole is pointed away from the sun, making it winter.

Signs of the autumnal equinox can be seen in nature starting on the first day of fall, according to EarthSky. The arc of the sun across the sky changes, birds and butterflies migrate south, trees and plants are ending their growth cycles, the weather gets cooler and the Autumn Star "Fomalhaut" can be seen traveling the sky each night.

Mid-latitudes of the Northern Hemisphere lose the most light during the autumnal equinox, according to Washington Post Capital Weather Gang reporter Justin Grieser. Washington, D.C. loses 1 hour and 16 minutes of daylight in September, while a more northern city like Fairbanks, Alaska, loses 3 hours and 20 minutes.

There are some myths regarding the equinoxes. One is that the entire arctic region experiences six months of daylight and six months of darkness. This belief treats "night" as if twilight does not exist, according to's Joe Rao.

"So even at the North Pole, while the sun disappears from view for six months beginning on Sept. 25, to state that 'total darkness' immediately sets in is hardly the case! In fact, civil [bright] twilight does not end there until Oct. 8," he writes. Twenty-four hour darkness lasts in the North Pole for 11 weeks, not six months.

The autumnal equinox might bring cold weather, but it also increases the odds of seeing Northern Lights. From now until the end of October, the chances of seeing the glow of aurora are greater, CBS News reports. Auroras peak in frequency around this time and around the vernal equinox, because it is at these times when geomagnetic storms are strongest.

Each year on the autumnal equinox, celebrants gather at the pyramid at El Castillo at Mexico's ancient Mayan city of Chichen Itza, according to National Geographic. Sunlight hits the pyramid at such an angle that it casts a shadow on the side of a staircase each equinox. Viewers watch as the shadow joins with the stone head and forms a huge, glowing "serpent."