Autumn arrived in the world on Sept. 22, 2010, at 11:13 p.m. EST. And for the first time since 1991, the harvest moon was full in the early hours of Sept. 23. A full harvest moon so close to the autumnal equinox is the “traditional definition” of a harvest moon. And as National Geographic reports, the world is celebrating.

An autumnal equinox occurs when the sun sets due west and rises due east. It marks the change of the season and also when day and night fall into balance, lasting for roughly the same equal lengths. After the autumnal equinox, the days become notably shorter. The harvest moon refers to the autumnal moon at its brightest, when farmers worked to bring in their crops by its light. This year, these two events coincided in the wee hours of Sept. 23 in the northern hemisphere. This event will not occur again until 2029.

The autumnal equinox happens when the sun’s disk crosses the celestial equator, an imaginary line that projects out from the Earth’s equator. If a person is standing directly at the Earth’s equator, he or she can see the sun pass directly overhead on this day. But don’t expect an equinox to yield a perfectly mathematical day, as time is peculiar. In 1582, Pope Gregory XIII established the Gregorian calendar, which we use today to account for these irregularities in time.

Judith Young is a professor of astronomy at the University of Massachusetts in Amherst. As she explained to National Geographic, most people will never see the full 12 hours of sunup and sundown on the autumnal equinox. The sun sets and rises at an angle for people who don’t live at the equator, and most people have hills and foliage obstructing their view of a sunrise or sunset. Further, the Earth’s slightly elliptical orbit alters the length of days. According to Young, "We arrive at the September equinox a day late, because we were going a little bit slower [around the sun] in July, and we arrive at the March equinox a day earlier."

Nonetheless, the world is celebrating the harvest moon and autumnal equinox as it has for centuries. The Mayans built the pyramid of Kukulkan in Chichin Itza to honor the occasion. As the autumnal equinox occurs, a play of light creates the illusion of the serpent god Kukulkan slithering its way down the pyramid. China continues to celebrate the harvest moon with festivals. They commemorate the moon goddess, whose origins are linked to this time, as well as the family.

The September harvest moon and equinox is not the only special cosmic event this month. Jupiter is the closest it’s been to the Earth since 1963, and the largest planet in our solar system was aligned with Uranus on Sept. 17.

For further reading:

World celebrates autumnal equinox 2010, brightest in over a decade
For the first time since 1991, a full harvest moon shines down on the fall equinox.