Get to know 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.

In the language of science, an equinox is either of two points on the celestial sphere where the ecliptic and the celestial equator intersect.

The picture on the left shows the view from the solar system , and from on the surface of the earth.

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.

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 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 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.

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.

At noontime in the Northern Hemisphere the sun appears high in the sky during summertime and low in the sky during winter.

Neither hemisphere gets more sunlight than the other, so both have similar seasons.

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.

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.

Some believe that one can only balance an egg within a few hours before or after the exact time of the equinox.

The source of this material is Windows to the Universe, at http://www.windows.ucar.edu/ at the University Corporation for Atmospheric Research.

Because the earth is rotating like a top or gyroscope, it points in a fixed direction continuously towards a point in space near the North Star.

The vernal equinox, also known as “the first point of Aries,” is the point at which the Sun appears to cross the celestial equator from south to north.

It is highest at the summer solstice and lowest at the winter solstice.

The direction of this tilt is effectively constant, relative to the stars, so that the Earth’s north pole always points towards Polaris, the Pole Star, and the south pole always points at the constellation of Octans.

The bottom panel shows how this looks to someone standing on the ground in Topeka.

Note that the times below were calculated using AutumnalEquinox in the Mathematica application package Scientific Astronomer, which is accurate to within only an hour or so, and in practice gives times that differ by up to 15 minutes from those computed by the U.S. Naval Observatory.

Many people believe that since the equinox is a time of balance where the daylight hours and nighttime hours are equal, that by some mystical force one can balance eggs on their end on these days.

Each year, the Earth completes a circuit of the Sun, and for its poles to remain fixed against the stars, their direction must rotate relative to the Sun.

The Astronomical Almanac for the Year 1999.

Mankind has followed the cycles of the Sun for millennia.

Equinoxes occur because the Earth’s axis of rotation isn’t aligned with the plane of its orbit around the Sun: it tilts over by about 23½°.

The equinoxes are not fixed points on the celestial sphere but move westward along the ecliptic, passing through all the constellations of the zodiac in 26,000 years.