The autumnal equinox is the official start of autumn. At the exact time of the equinox, the sun crosses the celestial equator from the northern part of the sky to the southern part of the sky.
“Equinox” means “equal night” — the hours of day and night are supposedly equal. The hours of day and night are close but not exactly equal at the time of the spring equinox around March 21 and at the time of the autumnal equinox around Sept. 21.
For Tucson’s latitude of 32 degrees north, the times of equal daylight (from sunrise to sunset) and night (from sunset to sunrise) are 5-6 days prior to March 21 and 5-6 days after Sept. 21. Around the times of the vernal and autumnal equinox, the sun rises due east and sets due west.
Check that out on Sunday when the sun rises at 6:12 a.m. due east and sets at 6:20 p.m. due west.