What is the Autumnal Equinox?

Seasons are major divisions of the year, generally based on periodic changes in weather. They occur due to the tilt of the Earth's axis (axial tilt) in relation to the plane of its orbit around the Sun. In most of the planet's tropical and subtropical regions, the seasons are generally defined by the amount of rain (precipitation) they receive, given that this can vary more dramatically than temperatures. Consequently, those regions typically have a dry season and a wet (monsoon) season. In the planet's temperate and polar regions, the seasons are marked by changes in the amount of sunlight, which can cause plants to become dormant and animals to either migrate or hibernate. Four seasons are generally recognized in these regions: spring, summer, fall (autumn) and winter.

The days on which the seasons change coincide with the days on which the planet's axial tilt is either closest or furthest from the sun (solstices), and the days when the sun is directly over the Earth's equator (equinoxes). Seasons in the Northern Hemisphere are opposite of those in the Southern Hemisphere, meaning when one experiences spring, the other experiences autumn. Likewise, when one hemisphere experiences summer, the other experiences winter:

The spring equinox marks the first day of the spring season. On this day, the Sun is directly over the earth's equator, and daylight lasts 12 hours in the Northern Hemisphere and increasing. This day is typically recognized as March 21 in the Northern Hemisphere, and marks the first day of autumn in the Southern Hemisphere.

The summer solstice marks the first day of the summer season. On this day, the northern half of the Earth is tilted closest toward the Sun, and the Northern Hemisphere experiences the longest day (or most hours of daylight) of the year. This day is typically recognized as June 21 in the Northern Hemisphere, and marks the first day of winter in the Southern Hemisphere.

The autumnal equinox marks the first day of the fall season. On this day, the Sun is again directly over the earth's equator, and daylight lasts 12 hours in the Northern Hemisphere and decreasing. This day is typically recognized as September 22 in the Northern Hemisphere. In the Southern Hemisphere, the first day of spring is recognized on September 23.

The winter solstice marks the first day of the winter season. On this day, the northern half of the Earth is tilted furthest from the Sun, and the Northern Hemisphere experiences the longest night (or most hours of darkness) of the year. This day is typically recognized as December 21 in the Northern Hemisphere, and marks the first day of summer in the Southern Hemisphere.

In 2010, the equinoxes and solstices take place at:

First Day of Spring (Vernal Equinox) 1:32 p.m. ET March 20

First Day of Summer (Summer Solstice) 7:28 a.m. ET June 21

First Day of Autumn (Autumnal Equinox) 11:09 p.m. ET September 22

First Day of Winter (Winter Solstice) 6:38 p.m. ET December 21

Additional information on the seasons and how we experience them based on the Earth's rotation on its axis as it travels around the Sun is available from the National Aeronautics and Space Administration (NASA). NASA also has images of observed reflected solar radiation during winter and summer solstices.

Additional Resources:

Equinoxes and Solstices, 2008-2012

Time Exhibits

2012 and the Maya Calendar

Standards: Daylight Saving Time (DST)

Standards: Leap Year

Standards: Official Time