Fall officially arrives in the northern hemisphere this Saturday at 7:49 A.M. Pacific Daylight Time.
The event is know as the autumnal equinox and it marks the time of year when day and night are about equal due to the position of the Earth and the Sun.
The amount of daylight and darkness aren't exactly equal due to refraction, or bending, of light as it passes through the Earth's atmosphere.
Technically and autumnal (and vernal or spring) equinox are points in the Earth's orbit around the Sun when the center of the Sun and the Earth's equator are on the same plane.
In other words, if you were standing on the equator at exactly the time of the equinox the center of the Sun would be directly overhead at a 90-degree angle.
The same phenomenon occurs in spring during the vernal equinox but the Earth has moved half-way around the Sun in it's orbit.
But before we get there we have to go through the winter solstice on December 21.
That's when the northern hemisphere is tilted farthest away from the sun and it appears low in the winter sky.