Jimmy Westlake: Equinox   Autumn officially arrives for the
northern hemisphere at 10:03 p.m. Mountain Standard Time on
Friday. How do astronomers determine the precise moment that
marks the beginning of autumn?  Autumn begins the instant the
sun crosses the equator on its way south. Thanks to the 23
½-degreee tilt of the Earth's axis of rotation, the sun spends
half the year shining straight down on the northern hemisphere
and the other half of the year shining straight down on the
southern hemisphere. Separating these two extremes are two days
called the equinoxes - six months apart - when the sun shines
straight down on the Earth's equator.  Equinox is a word that
means "equal nights," and is used to describe these two special
days when every location on Earth experiences exactly 12 hours
of sunlight and 12 hours of darkness. These also are the only
two days of the year when the sun rises exactly in the east and
sets exactly in the west.  You've probably noticed recently that
the sun is rising later and se