Happy Autumnal Equinox!

Today's the fall, or autumnal, equinox. The Sun will rise due East, spend 12 hours above the horizon, and set due West. After today, the Sun heads south for the winter! A lovely image of the Sun's paths through the sky at the extremes of our orbit is given in today's Astronomy Picture of the Day.

Follow up:

The reason is that the Earth's poles are tilted, by 23.5°, to the plane of Earth's orbit around the Sun. Thus, the Sun isn't lined up directly with Earth's equator - except for two days each year, on the spring and fall equinoxes. On the equinoxes, the Sun appears to follow a path in the sky directly over Earth's equator. The rest of the time, the Sun's either north of that path (in the Northern Hemisphere's spring and summer months) or south of it (in the fall and winter). It reaches its farthest position north on the summer solstice, and its most southerly position on the winter solstice.

The image above shows how those paths might be oriented in the sky at the latitude of Columbus, Georgia. Note that as the Sun gets lower in the sky, sunlight's coming in at more of an angle. That means that as we move toward the winter solstice, the sunlight is more spread out, and won't heat the ground as efficiently. As a result, the northern hemisphere gets colder. Only when we're well past the winter solstice, and the Sun starts getting higher in the sky, will it start to warm up again under the influence of more direct sunlight.

Traditionally, the equinoxes and solstices mark the beginning of each season. Thus, the fall equinox also marks the first day of autumn. Those of us in Georgia may be inclined to celebrate the upcoming cooler days...