The Spring and Fall Equinox are very interesting things. First let’s go over some astronomical background for anyone who doesn’t know. The Earth Rotates on an axis, completing a 360 rotation every 24 hours. It also orbits the Sun, making a complete trip around the sun approximately every 365 days, although it is actually closer to 365.25 days, which is why every fourth year is a leap year. So the Earth spins on an axis, and also goes around the sun.
One important thing you need to know is that the axis on which the Earth rotates is tilted. Tilted 23 degrees to be exact. That is why we have seasons, because as the Earth goes around the sun, the Northern and Southern Hemispheres get different intensities of light. When the Northern hemisphere is tilted slightly away from the sun, the light it recieves is much less direct than the light recieved by the Southern Hemisphere, which gives us in the Norhtern Hemisphere our Winter, and the Southern Hemisphere its Summer. As the Earth travels around the sun, the tilt, remains the same, and points in the same direction, towards Polaris, which is the North Star. So as it travels around the Sun, the Earth tilts the Northern Hemisphere either slightly towards or slightly away from the Sun. From the Northern Hemisphere’s Winter where it is tilted away, it shifts to being tilted slightly towards.
So here’s a simple bit, if your half of the Earth(North or South) is tilted towards the Sun, it’s Summer, tilted away is Winter, and between those two lie Spring and Fall. The exact day when the shift from titled towards to away is called an equinox. Equinox means “Equal Day and Night” because on these days the daylight and night hours are almost exactly the same. It is also on the Equinoxs when the Northern and Southern Hemisphere get the same amount and intensity of sunlight. So halfway between the two Solstices (Summer meaning day of most sunlight for the Northern Hemisphere, and Winter, meaning the day of least sunlight for the Northern Hemisphere) lie the two Equinoxs.
The Spring Equinox, which is generally on March 21st marks the first day of Spring. The Fall Equinox, which is usually September 22nd or 23rd marks the beginning of Fall. While on these days generally, it varies every year. The 2009 Spring Equinox will fall on March 20th at 11:47 am. The 2009 Fall Equinox will occur on September 22nd at 9:29 pm.
There is much more to know, about Equinoxs and Solstices and Seasons and Rotation and much more, but for the average everyday person, you now know alot of info about the start of Sprin and Fall.