Seasonal Cycle

The seasonal cycles are generally thought of as spring, summer, autumn, and winter. The four seasons are based on the amount of sunlight reaching a hemisphere due to the Earth’s angle relative to the sun. The elliptical orbit of the Earth prevents it from traveling at a constant speed. Furthermore, the main cause of seasonal effects as we know them is due to the angle of the Earth’s rotation axis (23.5 degrees) with respect to its elliptical path around the sun. The Earth’s orbit results in a hemisphere either being more or less titled toward the sun, which results in the seasonal cycles. For example, when a hemisphere is leaning toward the sun, the nights are shorter and the days are longer. In contrast, when a hemisphere is leaning away from the sun, the nights are longer and the days are shorter.

Some believe that the distance of the Earth relative to the sun results in warmer versus cooler temperatures. However, in the Northern Hemisphere, the Earth is closest to the sun in early January (91.6 million miles) and furthest away (94.8 million mi., or 152.5 million km) in early July. Even though it is 3.2 million mi. (5.14 million km) farther from the sun, the Earth averages about 4 degrees F (2.2 degrees C) warmer in July than January, because the Northern Hemisphere is 40 percent land and the Southern Hemisphere is 20 percent land. Both areas receive the same amount of sunlight, but land area heats up faster than water bodies. Thus, the amount of the land area is more of a predictor of temperatures than its closeness to the sun.

Seasons

There are two different views of the seasons: the astrological and the meteorological. Astrological seasons coincide with the Earth’s transit over the equator (equinoxes) and the tropics of Cancer and Capricorn (solstices). For example, as the Earth rotates around the sun, the sun is positioned directly over the equator (0 degrees) around March 21 (vernal equinox), the tropic of Cancer (23.5 degrees north) around June 21 (summer solstice), the equator (0 degrees) around September 21 (autumnal equinox), and the tropic of Capricorn (23.5 degrees south) around December 21 (winter solstice). In Latin, the word equinox means “equal night.” On the vernal equinox (around March 21) and autumnal equinox (around September 21), the sun’s energy is balanced between the Northern and Southern Hemispheres. On the equinox, day and night are approximately equal in length. The March equinox marks the start of spring in the Northern Hemisphere and the start of autumn in the Southern Hemisphere.

The term solstice means “stand still” in Latin. During the summer solstice in the Northern Hemisphere, the North Pole (location 90 degrees north) is tilted toward the sun (around June 21) and directly over the Tropic of Cancer. During the winter solstice, the North Pole is tilted away from the sun (around December 21) and directly over the Tropic of Capricorn. During the summer solstice, the Northern Hemisphere is positioned to receive extended periods of sunlight due to its position relative to the sun. The extended periods of sunlight coupled with shorter periods of darkness at night results in increased temperatures in the Earth’s Northern Hemisphere. All locations in the Northern Hemisphere experience longer days, and locations north of the Arctic Circle (66.5 degrees north) experience a 24-hour period of sunlight on the summer solstice. In contrast, during the winter solstice in the Northern Hemisphere, the Earth is leaning away from the sun and the Earth receives a shorter period of sunlight due to its position relative to the sun. For example, all locations north of the Arctic Circle experience a 24-hour period of darkness on the winter solstice (December 21).

The seasons in the Southern Hemisphere are opposite to the seasons in the Northern Hemisphere. For example, spring in the Southern Hemisphere is equivalent to autumn in the Northern Hemisphere. The months of June, July, and August are generally the hottest months in the Northern Hemisphere, while December, January, and February are the hottest months in the Southern Hemisphere.

Each of the astrological seasons (spring, summer, autumn, and winter) are not equal in length. This is due to Earth’s elliptical orbit around the sun. For example, the vernal equinox transit lasts 92.8 days; the summer solstice, 93.6 days; the autumnal equinox, 89.8 days; and the winter solstice period, 89 days. In the Southern Hemisphere, autumn and winter seasons are generally longer, and occur at the same time as the spring and summer seasons in the Northern Hemisphere. Differences in the length of seasons are due primarily to the Earth’s elliptical orbit around the sun, where Earth tends to move faster when closer to the sun and slower when further away from the sun.

Meteorological Seasons

The meteorological seasons are different from the astrological seasons in the following ways: (1) seasons last 90 days each, (2) seasons begin about three weeks before the astrological seasons, and (3) seasons are based on temperature and heat lag and not the Earth’s position. The four meteorological seasons in the Northern Hemisphere are: spring (begins March 1), summer (begins June 1), autumn (begins September 1), and winter (begins December 1). In the Southern Hemisphere, the four seasons are the opposite, with spring starting on September 1, summer on December 1, autumn on March 1, and winter on June 1.