Fall officially starts next Thursday, Sept. 22 at 7:21 a.m. Mountain Standard Time (MST). That’s when the sun shines directly on the equator as it moves south from the northern part of the sky.
As the Earth travels on its orbit around the sun, the sun appears to move against the starry background. The tilt of the Earth’s axis, with respect to its orbital plane around the sun, gives the appearance of the sun being in the northern part of the sky from the vernal (spring) equinox around March 21 until the autumnal equinox around Sept. 21.
We cannot actually see the starry background, because the sun is too bright. However, ancient observers learned to look at the stars along the horizon just before sunrise and just after sunset, giving them a good idea as to the sun’s actual position in the sky.
