One of two days during the year when the declination of the Sun is at the equator. The autumnal equinox denotes the first day of the fall season. For the Northern Hemisphere, the date of autumnal equi [..]

The equinox at which the sun approaches the Southern Hemisphere, marking the start of astronomical autumn in the Northern Hemisphere. The time of this occurrence is approximately September 22. On that [..]

The equinox at which the sun approaches the Southern Hemisphere and passes directly over the equator. Occurs around September 23.

autumn day, usually around September 22, when day and night are of generally equal length.

The equinox (approximately 22 September) at which the sun crosses the celestial equator from the Northern Hemisphere to the Southern Hemisphere.

The point where the Sun crosses the celestial equator from north to south, around September 22. The beginning of Fall.

first day of autumn. (September 20 or 21) average speed -

Point at which on the celestial sphere that the equator and the Ecliptic intersect. Generally the autumnal equinox occurs on or about September 23 in the northern hemisphere; this also signifies sprin [..]

the equinox

Equaling or approximating a mean.

The equinox at which the sun approaches the Southern Hemisphere, marking the start of astronomical autumn in the Northern Hemisphere. The time of this occurrence is approximately September 22. On that day, daylight is everywhere 12 hours. Compare with vernal equinox, offset by six months.

is the time that signals the end of the summer months and the beginning of winter. It is when the Sun passes the equator.

The equinox that occurs in September. Compare vernal equinox. definition courtesy of: NWS Space Weather Prediction Center

Taking place in the Northern Hemispheric autumn, 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 Septemb [..]

The equinox at which the sun approaches the Southern Hemisphere, marking the start of astronomical autumn in the Northern Hemisphere. The time of this occurrence is approximately September 22. On that [..]