Hot Info: Autumnal Equinox
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.
The Autumnal Equinox signals the end of the summer months and the beginning of winter.
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.
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.
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.
The exact date and time of the fall equinox, when the sun moves into the astrological sign of Libra, varies from year to year.
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 upper panel shows that on an equinox , neither half of the Earth points directly towards the Sun.
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.
At noontime in the Northern Hemisphere the sun appears high in the sky during summertime and low in the sky during winter.
The date when night and day are nearly of the same length and Sun crosses the celestial equator moving southward.
Neither hemisphere gets more sunlight than the other, so both have similar seasons.
When a pole is angled towards the Sun, its hemisphere receives more hours of sunlight, and when a pole is turned away from the Sun, its hemisphere experiences long cold nights.
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.
