Hot Info: Autumnal Equinox

The Autumnal Equinox signals the end of the summer months and the beginning of winter.

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

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 is at ecliptic longitude 180° and at right ascension 12h.

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.

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 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.

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.

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 date when night and day are nearly of the same length and Sun crosses the celestial equator moving southward.

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.

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

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.

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

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.

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.

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

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

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

As seen by an observer on Earth, it may appear to the casual observer that the Sun revolves around the Earth once a year.

In sunrise /sunset tables, the assumed semidiameter of the sun is 16 minutes of arc and the atmospheric refraction is assumed to be 34 minutes of arc.

Also at that moment both the north pole and south pole of the Earth are just on the terminator, and day and night are divided equally between the hemispheres.

How high the Sun gets in your sky, and how long it is above the horizon during the day, depend not only on the season, but also on your latitude.

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.

The Earth’s seasons are caused by the rotation axis of the Earth not being perpendicular to its orbital plane.