In just two days, the 2013 autumnal equinox arrives.
At 4:44 p.m. ET, on Sunday, September 22, 2013, the days and nights will be of equal length.
The event marks the beginning of the fall season in the northern hemisphere and the start of spring in the southern hemisphere.
The word “equinox” actually comes from Latin meaning “equal night” and refers to the 12-hour-long day and night that occurs only twice each year.
If you’ve been observing the mid-day position of the sun over the course of the summer, you’ll now notice that it’s been slowly sinking closer to the southern horizon and creating ever longer shadows.
In the winter, the Earth is actually closer to the sun than it is in the summer, but the angle of the sun makes temperatures colder here in the northern hemisphere.
It’s only on the spring and autumnal equinox that the sun rises due east and sets due west.
Astronomically speaking, the autumnal equinox marks one of the four critical points in the changing of the seasons.
The Earth spins on its axis, which is tilted at 23.5 degrees with respect to its orbital plane.
On these days, however, the Earth’s axis is neither tilted away nor towards the sun, but has both northern and southern hemispheres experiencing equal amounts of sunshine.
Learn more in the video below.