When Do the Seasons Change?

Temperate and sub polar regions of the world are generally characterized by four distinct seasons during a calender year – spring, summer, winter and fall. The different seasons vary by temperature, weather, length of day/night and ecological patterns. As one of the more visible signs of Earth's changeability, humans have long been fascinated by the why and the when of seasonal change.

In 2010, (in the U.S,  which is in the Northern Hemisphere):

• Spring begins on March 20th at 1:32pm EDT
• Summer begins on June 21st at 7:26am EDT
• Fall begins on September 22nd at 11:09pm EDT
• Winter begins on December 21st at 6:38pm EDT

There are very definite ways scientists measure and decide on the moment that marks the change of a season. The results of these measurements are the determination of four distinct days which begin the seasons. These days are called the autumnal and vernal equinox and the summer and winter solstice. They do not come on the same date every year; but, they don't vary more than a few days each year.

Fall and Spring Seasons

There is a precise day when the length of day and the length of night are equal everywhere on the earth. Both parts of the day are exactly 12 hours long. This is the day that marks the beginning of spring, or fall, as the case may be. This day is called the autumnal equinox to start the fall season and the vernal equinox to start the spring season.

Furthermore, on the first day of fall and spring, the sun rises exactly due east, and sets exactly due west.

In most parts of the world, the number of hours of daylight will lengthen through the spring season and then shorten through the fall season. However, if you live precisely on the equator (like in Brazil, Gabon, Kenya, or Indonesia), your days and nights always have an equal number of hours and so sunrise and sunset hours do not fluctuate with the time of the year.

Winter and Summer Seasons

The winter and summer seasons are marked by solstices. The solstice is the day that the sun is directly overhead at noon in the tropic of Cancer, which is the northernmost path of the sun or the tropic of Capricorn, which is the southernmost path of the sun.

In other words, summer starts on the day the sun is furthest to the north (or that the earth's axial tilt is closest to the sun), while winter begins the day the sun is furthest to the south (or the axial tilt is farthest away from the sun).

Although you would expect it to be so, the hottest part of the day is not at noon when the sun is directly overhead, but several hours later due to a time lag for the waters and ground to heat up.

Longest and Shortest Days

In the Northern Hemisphere, the longest day on earth happens during the summer solstice. To the north of the Arctic circle in Alaska, there is a continuous 24-hour day with the sun never setting.

The shortest day on earth occurs during the winter solstice.

As is easily observed, the hemispheres are effected oppositely by the earth's movements (as they are on opposite sides of the earth). The first day of summer in the Northern Hemisphere is the first day of winter in the Southern Hemisphere.

Why the Seasons Change

Before more modern equipment and astrological practices, the Greeks came up with a fascinating myth to explain the seasons. As the story goes, Demeter, the goddess of the harvest, had her daughter Persephone abducted by Hades, the god of the underworld, for marriage. Riddled with despair, she caused the earth to go barren. As a result, Zeus forced Hades to return Persephone, but a deal was made where Persephone would return to her husband for one-fourth of the year. It is during the period of the year that Demeter grieves for the loss of her daughter that the land goes barren (winter).

Of course now, science has explained the seasons to be a result of the tilt of the earth.

By understanding the autumnal and the vernal equinox as well as the summer and winter solstices it is easier to see why the seasonal change seems gradual and the seasons appear to flow into each other.