So... what is this equinox thing?

Earth at Equinox

Twice a year the earth experiences equinoxes, that point in the earth's rotation around the sun when the axis of the earth (see the "poles" marking Earth's axis in the diagram to the left) is tilted neither *toward* nor *away from* the sun. The axial tilt of the earth is what causes the seasonal changes.

 This occurs in March (~ March 20) and September (~ Sept. 22) each year.





The March equinox is the beginning of the spring season in the northern hemisphere as the tilt of Earth is angled toward the sun (longer days in North America, Europe, and Asia) and the fall season in the southern hemisphere as Earth's tilt angles the southern hemisphere away from the sun (shorter days in most of South America, the southern half of Africa, and Australia); the September equinox is when the sun's rays, due to Earth's axial tilt, will start to fall more strongly on the southern than the northern hemisphere, spring in the southern hemisphere starts and fall in the northern hemisphere.


Due to the effect of light refraction (basically the bending of light around *edges*) and the way that sunrise and sunset are calculated (the visible edge of the sun on the horizon rather than the center of the sun crossing the horizon), day and night lengths are close to, but not to, equal on the equinoxes in the northern  and southern hemispheres. 


Here's what both of the equinoxes (left) and both of the solstices (right) look like from space:

...the next post should be much easier, on *both* of us (writer AND reader, lol)... writing when sick is ridiculous! ;)