I am Joel Block.

And I am Deborah Byrd for Earth and Sky -- Friday, September 22. Every year, on or near today's date, the sun passes directly overhead at earth's equator. This is the September equinox. It's the moment each year when the sun crosses from the northern half of the celestial sphere to the southern half.

This year's September equinox happens at precisely 4: 03 Universal Time tomorrow. At this same moment in Indonesia, it's high noon Saturday, with the sun shining straight overhead at the equator. Here in the U. S. it's 11: 03 Central Daylight Time tonight.

The September equinox marks the unofficial beginning of autumn in the northern hemisphere and spring in the southern hemisphere. On the day of the equinox, everyone receives approximately 12 hours of daylight and 12 hours of darkness. That will change by the month's end, however, as the sun's path continues to shift toward south.

At the earth's north and south poles, there is no east or west. From the exact north pole, for example, every direction is south. On the day of the equinox, as seen from the poles, the sun is like a horse on a merry-go-round, trotting along the horizon all day long. In reality, slowly but surely the sun is rising at the south pole while setting at the north pole.

For sky charts and more information, come to . We are Block and Byrd for Earth and Sky.