Today is the autumnal equinox.

The autumnal equinox is the day when the sun crosses the equator from the Northern to the Southern Hemisphere, and when day and night are the same length. The autumnal equinox also marks the point when days begin getting shorter than nights in the Northern Hemisphere and is considered the beginning of the fall season. September 23 is the autumnal equinox in 2006, as measured in universal time at Greenwich, England. Because of time zone differences, the autumnal equinox in 2006 actually falls on September 22 in most of the United States and on the 23rd in the Eastern U.S., Europe and Asia.

Many cultures celebrate the changing of the seasons. The autumnal equinox is a national holiday in Japan, where they traditionally visit the graves of deceased family members. In China, however, the Mid-Autumn or Moon Festival is celebrated on a fixed calendar date—the 15th day of the 8th lunar month, which falls on October 6 in 2006.

Xpeditions
Students become "seasons sleuths" in A Reason for the Season (K–12), an Xpeditions Activity. Students learn about the changing of the seasons and test their knowledge with a cosmic map. Activities for students and parents relate to keeping discussions about the universe and the people in it alive over the course of the calendar year.

In Recording the Changing Seasons in Ancient Times (6–8), students learn about Stonehenge and read ancient myths and stories explaining the seasons. Acting as historians, the students use Web sites to find information about how ancient cultures kept track of and celebrated the seasons.

In The Sun and the Earth (3–5), students explore the relationship between the Earth and the Sun and how this relationship affects observable phenomena on Earth, such as the change of seasons.

Science NetLinks
The purpose of the lesson The Four Seasons (6–8) is to dispel common misconceptions and help students understand that the tilt of the earth's axis causes seasons. After reading about the topic, students observe a demonstration of tilt and the way the earth moves in relation to the sun.