Equinox
The point at which the ecliptic intersects the celestial equator. Days and nights are most nearly equal in duration. In the Northern Hemisphere, the vernal equinox falls on or about March 20 and the autumnal equinox on or about September 22.
Ecliptic
The sun’s apparent path across the sky that tracks a circle through the celestial sphere.
Celestial Equator
The projection of the plane of the geographical equator upon the celestial sphere.
Day
Considered a basic unit of time as defined by the earth’s motion. It represents the time needed for one complete revolution of the earth about its own axis. Also know as a sidereal day, it is approximately equal to 23 hours, 56 minutes, and 4.09 seconds.
Night
The period of the day between dusk and dawn.
Vernal Equinox
Taking place in the Northern Hemispheric spring, it is the point at which the ecliptic intersects the celestial equator. Days and nights are most nearly equal in duration. It falls on or about March 20 and is considered the beginning of spring in the Northern Hemisphere and autumn in the Southern Hemisphere. It is the astronomical opposite of the autumnal equinox.
Autumn
The season of the year which occurs as the sun approaches the winter solstice, and characterized by decreasing temperatures in the mid-latitudes. Customarily, this refers to the months of September, October, and November in the North Hemisphere and the months of March, April, and May in the Southern Hemisphere. Astronomically, this is the period between the autumnal equinox and the winter solstice.