Updated Info on Autumnal Equinox
At the autumnal equinox , the Sun appears to cross the celestial equator, from north to south; this marks the beginning of autumn in the Northern Hemisphere.
The autumnal equinox is at ecliptic longitude 180° and at right ascension 12h.
These two great circles meet at two points, one of which is the vernal equinox and the other the autumnal equinox.
In the southern hemisphere, the autumnal equinox corresponds to the center of the Sun crossing the celestial equator moving northward and occurs on the date of the northern vernal equinox.
The Autumnal Equinox signals the end of the summer months and the beginning of winter.
In the language of science, an equinox is either of two points on the celestial sphere where the ecliptic and the celestial equator intersect.
An equinox is the moment in time when the centre of the Sun can be observed to be directly above the Earth ’s equator, occurring around March 20 and September 22 each year.
One of the two places in the sky where the ecliptic crosses the celestial equator; or one of the two times of the year when the Sun crosses these points.
When the Sun passes this point, on about 23 September each year, nights begin to grow longer than days, and continue to do so until the Winter Solstice in December.
The picture on the left shows the view from the solar system , and from on the surface of the earth.
In fact, the Sun is at the equator, so both halves of the Earth are getting about the same amount of sunlight.
For the rest of us, it’s one of two times a year when the Sun crosses the equator, and the day and night are of approximately equal length.
The upper panel shows that on an equinox , neither half of the Earth points directly towards the Sun.
The dates of maximum tilt of the Earth’s equator correspond to the summer solstice and winter solstice, and the dates of zero tilt to the vernal equinox and autumnal equinox.
The date when night and day are nearly of the same length and Sun crosses the celestial equator moving southward.
South of the equator, its name is less appropriate, since it corresponds to the beginning of summer, though for historical reasons the name autumnal tends to be used for the September equinox in both hemispheres.
Either of two moments in the year when the Sun is exactly above the Equator and day and night are of equal length all over Earth; also, either of two points in the sky where the ecliptic and the celestial equator intersect.
The changes in day length result from the changes in the tilt of the earth with respect to the sun, or to what is known as the apparent movement of the sun, although it is, of course, the earth which moves.
On a day which has an equinox, the centre of the Sun will spend a nearly equal amount of time above and below the horizon at every location on Earth and night and day will be of nearly the same length.
Neither hemisphere gets more sunlight than the other, so both have similar seasons.
