Saturday, September 22, 2012

The Autumnal Equinox

The word equinox comes from the Latin words for "equal night." The fall and spring equinoxes are the only days of the year in which the Sun crosses the celestial equator.

An equinox occurs twice a year (around 20 March and 22 September), when the tilt of the Earth's axis is inclined neither away from nor towards the Sun, the center of the Sun being in the same plane as the Earth's equator. The term equinox can also be used in a broader sense, meaning the date when such a passage happens. The name "equinox" is derived from the Latin aequus (equal) and nox (night), because around the equinox, the night and day have approximately equal length.

It is the date at which sunset and sunrise are exactly 12 hours apart (known as the equilux.) Because times of sunset and sunrise vary with an observer's geographic location (longitude and latitude), the equilux likewise depends on location and does not exist for locations sufficiently close to the Equator. The equinox, however, is a precise moment in time which is common to all observers on Earth.