Happy Autumnal Equinox! September 23, 2010Posted by jcconwell in Astronomy.
Even though the temperature was in the 90’s, today is the beginning of astronomical autumn. The autumnal equinox occurred at 3:09 am UTC today. An equinox occurs twice a year, when the Earth’s axis is perpendicular to the Sun. The name “equinox” is derived from the Latin aequus (equal) and nox (night), because around the equinox, the night and day are approximately equally long.
The exact time is determined when the Sun passes through a special point in the sky, as shown below.
Relative to the distance stars the Sun follows a path in the sky, due to the orbit of the Earth called the ecliptic. There is also a line in the sky called the celestial equator, which is the projection of the Earth’s equator into the sky. These two paths that circles in the sky intersect at two points, the vernal and autumnal equinoxes. When the sun travels through these points we have the official beginning of spring and autumn.
Tomorrow: Jupiter and the Observatory Open House