Today is the Autumnal Equinox. It signals the technical beginning of Fall or Autumn. It is the point where there is exactly 12 hours of daylight and 12 hours of darkness at the equator. If you live anywhere else, however, you will see a little bit more or a little bit less than 12 hours of daylight. The daylight hours are dwindling and will continue to do so until we reach the Winter Solstice, the shortest day of the year and the start of winter.
The ecliptic, celestial poles and celestial equator
During the course of a year the Earth completes one orbit around the Sun. To us on Earth we see this as the Sun moving against the background of stars through the year, along an imaginary line which we call the ecliptic. This defines the plane in which the Earth and most of the other planets orbit around the Sun. The directions to the north and south ecliptic poles are at right angles to this. The Zodiac is the band of constellations running along the ecliptic.
The Earth's orbit around the Sun takes 365.25 days. The Earth's axis of rotation, tilted at 23.5 degrees to the line of the poles of the ecliptic, gives us the directions to the north and south celestial poles. The bright star Polaris is currently showing us the direction of the north celestial pole. Like a spinning top this axis is precessing around the ecliptic pole with a period of 26,000 years.
The celestial equator is the projection of the Earth's equator onto the sky. As the Sun moves in its apparent track along the ecliptic it is for half the year seen to be above the equator (northern summer) and half the year below the equator (northern winter). The Sun will therefore appear to cross the equator twice in a year.
At the times when the Sun is crossing the celestial equator day and night are of nearly equal length at all latitudes and so we call these dates the equinoxes ('equal night'). In March, as the Sun is moving northwards along the ecliptic, this is called the vernal equinox and in September as the Sun is moving southwards we refer to it as the autumnal equinox. The equinoxes are also the points on the celestial sphere where the ecliptic and equator cross and the vernal equinox is used as the zero point in measuring star co-ordinates.
Why do the equinoxes not always occur on the same days each year?
The Earth takes approximately 365.25 days to go around the Sun. This is the reason we have a leap year every 4 years, to add another day to our calendar so that there is not a gradual drift of date through the seasons. For the same reason the precise time of the equinoxes are not the same each year, and generally will occur about 6 hours later each year, with a jump of a day (backwards) on leap years. In fact it is only after a complete leap-year cycle of four centuries that these dates will be repeated.
In ancient times, the Autumn Equinox was cause for a variety of pagan festivals, among them the celebration of the birth of Mabon, the son of Mordon, the Goddess of the earth.
It is also a time to celebrate with a variety of Fall and Harvest Festivals. People enjoy fall festivals as they sense the closure of a great summer season and the coming of a long winter. The fall festivals are the last of the outdoor events until spring.
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