The 2013 September equinox came at 3:44 p.m. Sunday, so autumn is officially here. The sun is rising later now and nightfall comes sooner. This is our autumn equinox, when the days are getting shorter in the Northern Hemisphere.
At this equinox, day and night are approximately equal in length. An equinox is an astronomical event, caused by Earth’s tilt on its axis and ceaseless orbit around the sun. The Earth doesn't orbit upright, but instead is tilted on its axis by 23-and-a-half degrees, Earth’s Northern and Southern Hemispheres trade places in receiving the sun’s light and warmth most directly.
We have an equinox twice a year – spring and fall – 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.
So as a refresher, seasons are the result of the tilt of the Earth's axis. As the Earth orbits around the sun, the direct angle changes and so do seasons.