The Seasons: Spring, Summer, Fall and Winter

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 amount of rain (precipitation) received defines the seasons, 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, changes in the amount of sunlight, which can cause plants to become dormant and animals to either migrate or hibernate, mark the seasons. These regions recognize four seasons: 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. Typically recognized as March 21 in the Northern Hemisphere, it 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. Typically recognized as June 21 in the Northern Hemisphere, it 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. The Northern Hemisphere typically recognizes September 22 as the first day of autumn, while the Southern Hemisphere typically recognizes September 23 as the first day of spring.
• 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. Typically recognized as December 21 in the Northern Hemisphere, it marks the first day of summer in the Southern Hemisphere.

In 2010, the equinoxes and solstices take place at:

In 2011, the equinoxes and solstices take place at:

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