We have reached the Autumnal Equinox. Unless, of course, you live below the equator. Then, you've reached the Spring Equinox.
Technically speaking, this occurs when the sun crosses the celestial equator and moves southward in the Northern Hemisphere during the September equinox.
The location on the Earth where the sun is directly overhead at solar noon is known as the subsolar point. The subsolar point occurs on the equator during the September and March equinoxes. At that time, the Earth’s axis of rotation is perpendicular to the line connecting the centers of the Earth and the sun.
This is the time when many people believe that the Earth experiences 12 hours of day and night. However, that's not precisely true. During the equinox, the length of night and day across the world is nearly, but not entirely, equal.
This is because the day is slightly longer in places that are further away from the equator, and because the sun takes longer to rise and set in these locations. Furthermore, the sun takes longer to rise and set farther from the equator because it does not set straight down -- it moves in a horizontal direction. Moreover, there is an atmospheric refraction that causes the sun's disk to appear higher in the sky than it would if earth had no atmosphere.
If you don't care for all that technical stuff, just remember this. Summer officially is over.
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