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Submitted by George Elliott on Tue, 03/20/2012 - 10:07am.
One can define the seasons in any one of a number of ways. For example, the meteorological spring season runs from March 1st through May 31st. In parts of Europe as well, this period is defined as the official spring season. In our part of the world, the “official” seasons are predicated on the times and dates when the Sun passes over certain latitudes of Earth, in an apparent north and south motion as the Earth revolves around the Sun.
The start of spring (for the Northern Hemisphere) is defined as the moment in March when the Sun passes over Earth's equator heading north - an event called the vernal equinox. This moment can come at any time of day or night.
The Sun appears to move north and south in our sky during the year because Earth's axis is tilted with respect to our orbit around the Sun . For skywatchers at mid-northern latitudes, the effect is to make the Sun appear highest in the sky in June. At that time the Northern Hemisphere is tipped sunward and gets heated by more direct solar rays, making summer. Six months later, when we're on the opposite side of our orbit, the Northern Hemisphere is tipped away from the Sun; the solar rays come slanting in to our part of the world and heat the air and ground less, making winter.
An equinox happens when the Sun is halfway through its journey from one of these solstices to the other. Several other noteworthy things happen on the equinox date:
First, day and night are approximately the same length; the word "equinox" comes from the Latin for "equal night." (A look in your almanac will reveal that day and night are not exactly 12 hours long at the equinox, for two reasons. Number one, sunrise and sunset are defined as when the Sun's upper edge - not its center - crosses the horizon. Second, whenever the Sun is very near the horizon, refraction by Earth's atmosphere shifts its position upward slightly.)
The Sun rises due east and sets due west everywhere on Earth on the two equinoxes (vernal and autumnal). The spring and fall equinoxes are the only times of the year when this happens.
If you were standing on the equator, the Sun would pass exactly overhead in the middle of the day. If you were at the North Pole, the Sun would be skimming the horizon, just beginning the six-month polar day.
In the Southern Hemisphere, the March equinox marks the start of autumn, and the September equinox marks the start of spring. Summer in the Southern Hemisphere begins in December, winter in June.
And finally, eggs do not balance on end more easily at the equinox than at other times!
By: George Elliott