What Exactly is the Equinox?

Why do we have summer and winter? And why is the southern half of the world always at the opposite time of the seasonal year?

We have to imagine ourselves in space (which of course we are) and that our north pole is always pointed up into space towards Polaris, the north star. The sun, in this example, has to be fixed in space, it's "top" pointing north, too. The earth, then, would be the only one of the three moving.

The earth's axis (the imaginary pole that runs through its center from pole to pole) is slightly angled, except during the two days of the year that we designate as the spring and fall equinoxes. At those moments, the equator is directly out to the side of the sun. Days and nights are an equal twelve hours. If our imagination places the earth directly off to the left of the sun's center, with north still straight up from the north pole, the daily full turn of the earth will be half day and half night. As the days continue, and the earth begins its movement towards the "top" of the sun, the north pole still points straight up, but the angle the sun strikes the lower hemisphere is exposed to it longer during the twenty-four hours cycle of the earth's rotation. It is then summer in Australia, while North American is experiencing winter.

Half a year later, the earth has moved around to the right side of our imagined universe, its north pole still pointed up. Another equinox occurs, the days and nights being the same length of time. As the earth then moves towards the "bottom" of the sun, the sun shines longer on the "top" half of the earth. It is then winter in Australia and summer in the United States.