October 21, 2006
This animation shows how a gnomon, that part of a sundial that casts the shadow, traces a straight line on the equinoxes -- here on the dial plate of a 2 1/2 story tall sundial, located at the Gallivan Center Plaza in downtown Salt Lake City, Utah. The longitude is 111.8 west, and the time of the animation is between 12:30 p.m. and 2:00 p.m. (local time). Astrometric equinox occurs when the Sun reaches an ecliptic longitude of 180 degrees -- a point located in the constellation Virgo, near the star beta Virgo. This position of the Sun marks the beginning of the fall season. When the Sun reaches this ecliptic position, the length of day and night are the same at all locations on the Earth, and the shadow of the Sun on a sundial's gnomon traces a straight line.
At the instant of this year's astrometric solstice (September 23, 2006, at 4:03 UT), the Sun was directly over the equator, not far from Indonesia. Each geographic location on the Earth experienced its own local autumnal equinox at local "apparent" noon over the next 24 hours. The angular size of this light gnomon is 1 degree.