Fall begins in the Northern Hemisphere. How does the equinox allow you to find the exact location of East and West? And can you balance an egg on the equinox?
The autumnal equinox occurs on September 23, 2007, at 5:51 am EDT. This is the beginning of autumn in the Northern Hemisphere according to the astronomical world. The Southern Hemisphere ends its winter and begins spring.
If you can remember that the Northern and Southern Hemispheres are always in opposite seasons, you may be able to remember why we have the seasons. It is NOT because Earth's orbit is slightly eliptical and off-center, causing the Earth to be a bit farther from the sun at one point of the year than the other. If that were the reason for the seasons, the North and South Hemisphere would share the same seasons, but they do not. While Northern Hemisphere residents call September 22/23 the autumnal equinox, for those below the equator it is the vernal (spring) equinox.
So it is the tilt of the Earth's axis that makes the difference. During the Northern Hemisphere's summer, the North Pole is tilting toward the sun receiving more direct rays and the South Pole is tilted away, thus the reason for winter in the Southern Hemisphere. During the Northern Hemisphere's winter, the North Pole is tilting away from the sun and the South Pole is tilted toward the sun, thus their summer. Earth being at perihelion and aphelion (closest and farthest point from the sun in its orbit) is inconsequential to the seasons.
If you want to find the exact direction of East and West without a compass, the equinox is the best time to do it. On the equinoxes, the sun rises directly in the east and sets directly in the west. You may have already noticed the sunset moving farther south. It will continue to slip past directly west and head to the southwest until the winter solstice, when it will rest at its southernmost point before heading north again.
The equinox is marked by the geometric center of the sun's disk crossing the equator of Earth. Many people believe that on the equinox day and night are equal, but this is not exactly so. For those in the Northern Hemisphere, day will remain about seven to 10 minutes longer than night on the fall equinox, and day and night will become equal a couple days later. The reason the equality in day and night does not occur on the equinox has to do with the sunrise and sunset being measured by the edges of the sun and not the geometric center and the diffraction of light as it passes through Earth's atmosphere. For an excellent description of this, read the U.S. Naval Observatory's Equinox Page.
Can you stand an egg on end on the equinoxes of September 23 and March 20? Sure you can. You can also stand them on end November 23 and April 20 and July 8 for that matter. While it has been a tradition in many schools to show how the Earth is in "balance" on the equinox by having children try to balance eggs on their ends, the balanced egg has nothing to do with the seasons and everything to do with the contents of the egg and the steadiness of the child's hands.
