Autumnal Equinox

Today is the autumnal equinox, otherwise known as the first day of Autumn (the first day of Spring in the southern hemisphere). Discounting twilight, and the refraction of light through the atmosphere, and a couple of other minor discrepancies, the length of the day equals the length of the night.

The changing seasons are due to the 23 degree tilt of the earth relative to its orbital plane around the sun. Today, the sun's path is directly over the equator.

Also, today the sun rises exactly in the east and sets exactly in the west. Mark the spots with gigantic stone blocks and you'll always know exactly where east and west are.

Or you could use a compass. Or a GPS. We sure do have things a lot easier than our ancient ancestors did...