Seasons

Resources

Seasons on Earth are found only in the temperate zones, which extend from 23.5° north (and south) latitude to 66.5° north (and south) latitude. In these regions of Earth nature exhibits four seasons: spring, summer, autumn (or fall) and winter. Each season is characterized by differences in temperature, amounts of precipitation, and the length of daylight. Spring comes from an Old English word meaning to rise. Summer originated as a Sanskrit word meaning half year or season. Autumn comes originally from a Etruscan word for maturing. Winter comes from an Old English word meaning wet or water. The equatorial regions or torrid zones have no appreciable seasonal changes and here one generally finds only a wet season and a dry season. Polar regions have only a light season and a dark season.

In the Northern Hemisphere, astronomers assign an arbitrary starting date for each season. Spring begins around March 21 and summer begins around June 21. Autumn begins around September 23 and winter around December 21. Because every fourth year is a leap year and February then has 29 days, the dates of these seasonal starting points change slightly. In the Southern Hemisphere the seasons are reversed with spring beginning in September, summer in December, fall in March, and winter in June. Seasons in the Southern Hemisphere are generally milder due to the moderating presence of larger amounts of ocean surface as compared to the Northern Hemisphere.

Changes in the seasons are caused by Earth’s movement around the sun. Because Earth orbits the sun at varying distances, many people think that the seasons result from the changes in Earth-sun distance. This belief is incorrect. In fact, Earth is actually closer to the sun in January compared to June by approximately three million miles.

Earth makes one complete revolution about the sun each year. The reason for the seasons is that the axis of Earth’s rotation is tilted with respect to the plane of its orbit. This tilt, called the obliquity of Earth’s axis, is 23.5 degrees from a line drawn perpendicular to the plane of Earth’s orbit. As Earth orbits the sun, there are times of the year when the North Pole is alternately tilted toward the sun (during northern hemispheric summer) or tilted away from the sun (during northern hemispheric winter). At other times the axis is generally parallel to the incoming sun’s rays. During summer, two effects contribute to produce warmer weather. First, the sun’s rays fall more directly on Earth’s surface and this results in a stronger heating effect. The second reason for the seasonal temperature differences results from the differences in the amount of daylight hours versus nighttime hours. The sun’s rays warm Earth during daylight hours and Earth cools at night by re-radiating heat back into space. This is the major reason for the warmer days of summer and cooler days of winter. The orientation of Earth’s axis during summer results in longer periods of daylight and shorter periods of darkness at this time of year. At the mid-northerly latitudes summer days have about 16 hours of warming daylight and only eight hours of cooling nights. During mid-winter the pattern is reversed. To demonstrate that it is the daylight versus darkness ratio that produces climates that make growing seasons possible, one should note that even in regions only 30° from the poles one finds plants such as wheat, corn, and potatoes growing. In these regions the Sun is never very high in the sky but because of the orientation of Earth’s axis, the sun remains above the horizon for periods for over 20 hours a day from late spring to late summer.

Astronomers have assigned names to the dates at which the official seasons begin. When the axis of Earth is perpendicular to the incoming sun’s rays in spring the sun stands directly over the equator at noon. As a result, daylight hours equal night time hours everywhere on Earth. This gives rise to the name given to this date, the vernal equinox. Vernal refers to spring and the word equinox means equal night. On the first day of fall, the autumnal equinox also produces 12 hours of daylight and 12 hours of darkness everywhere on Earth.

The name given for the first day of summer results from the observation that as the days get longer during the spring, the sun’s height over its noon horizon increases until it reaches June 21. Then on successive days it dips lower in the sky as Earth moves toward the autumn and winter seasons. This gives rise to the name for that date, the Summer Solstice, because it is as though the sun stands still in its noon height above

KEY TERMS

Autumnal equinox —The date in the fall of the year when Earth experiences 12 hours of daylight and 12 hours of darkness, usually about September 23rd.

Obliquity —The amount of tilt of Earth’s axis. This tilt is equal to 23.5 degrees drawn from a line perpendicular to the orbit of Earth.

Summer solstice —The date on which the sun is highest in the sky at noon, usually about June 21st.

Temperate zones —The two regions on Earth bounded by the 23.5 degree latitude and the 66.5 degree latitude.

Torrid zone —A zone on Earth bounded by 23.5 degrees North and South Latitude.

Vernal Equinox —The intersection of the celestial equator and ecliptic which the sun appears to reach on or about March 21.

Winter solstice —The date on which the sun’s noontime height is at its lowest, usually on December 21st.

the horizon. The Winter Solstice is likewise named because on December 21 the sun reaches the lowest noon time height and appears to stand still on that date as well.

In the past, early humans celebrated the changes in the seasons on some of these cardinal dates. The vernal equinox was a day of celebration for the early Celtic tribes in ancient Britain, France, and Ireland. Other northern European tribes also marked the return of warmer weather on this date. Even the winter solstice was a time to celebrate, as it marked the lengthening days that would lead to spring. The ancient Romans celebrated the Feast of Saturnalia on the winter solstice. And even though there are no historical records to support the choice of a late December date for the birth of Christ, Christians in the fourth century AD chose to celebrate his birth on the winter solstice. In the Julian calendar system in use at that time this date fell on December 25.

See also Global climate.

Resources

BOOKS

Norton, A.J., ed. Observing the Universe: A Guide to Observational Astronomy and Planetary Science. Cambridge, United Kingdom: Cambridge University Press, 2004.

Tarbuck, E.J., F.K. Lutgens, and D. Tasa. Earth Science 11th ed. Upper Saddle River, New Jersey: Prentice Hall, 2005.

Darrel B. Hoff

Seasons

Seasons, which generally coincide with annual changes in weather patterns, are most pronounced in temperate zones. These zones extend from 23.5° north (and south) latitude to 66.5° north (and south) latitude. Within these latitudes, nature generally exhibits four seasons; spring, summer, autumn (or fall) and winter. Each season is characterized by differences in temperature , amounts of precipitation , and the length of daylight.

Seasonal observations have been noted in the earliest known written records of history. In fact, seasonal changes have affected the course of history in the outcomes of battles or movements of peoples in search of longer growing seasons has often been greatly influenced by seasonal changes. Spring comes from an Old English word meaning to rise; summer originated as a Sanskrit word meaning half year or season. Autumn comes originally from an Etruscan word for maturing. Winter comes from an Old English word meaning wet or water . The equatorial regions or torrid zones have no noticeable seasonal changes and one generally finds only a wet season and a dry season in these zones. In the polar regions the seasons are closely related to the amount of sunlight received, resulting in a light season and a dark season.

Seasons are tied to the apparent movements of the Sun and stars across the celestial sphere. In the Northern Hemisphere, spring begins at the vernal equinox (around March 21) when sunlight is directly incident on the equator with equal distribution of light to the Northern and Southern Hemispheres. Summer begins at the summer solstice (approximately June 21) when the Sun is at its apparent maximum declination. Autumn begins at the autumnal equinox around September 23, and winter at the winter solstice (minimum declination in the Northern Hemisphere) that occurs approximately December 21. Because every fourth year is a leap year and February then has 29 days, the dates of these seasonal starting points change slightly. In the Southern Hemisphere, the seasons are reversed with spring beginning in September, summer in December, fall in March, and winter in June. Seasons in the Southern Hemisphere are generally milder due to the moderating presence of larger amounts of ocean surface as compared to the Northern Hemisphere.

Changes in the seasons are caused by Earth's movement around the Sun. Because Earth orbits the Sun at varying distances, many people assume that the seasons result from the changes in the Earth-Sun distance. This belief is incorrect. In fact, Earth is actually closer to the Sun in January compared to June by approximately three million miles.

Earth makes one complete revolution about the Sun each year. The major reason that the seasons occur is that the axis of Earth's rotation is tilted with respect to the plane of its orbit. This tilt, called the obliquity of Earth's axis, is 23.5 degrees from a line drawn perpendicular to the plane of Earth's orbit. As Earth orbits the Sun, there are times of the year when the North Pole is alternately tilted toward the Sun (during northern hemispheric summer) or tilted away from the Sun (during northern hemispheric winter). At other times, the axis is generally perpendicular to the incoming Sun's rays. During summer, two effects contribute to produce warmer weather. First, the Sun's rays fall more directly on Earth's surface and this results in a stronger heating effect. The second reason for the seasonal temperature differences results from the differences in the amount of daylight hours versus nighttime hours. The Sun's rays warm Earth during daylight hours; Earth cools at night by re-radiating heat back into space . This is the major reason for the warmer days of summer and cooler days of winter. The orientation of Earth's axis during summer results in longer periods of daylight and shorter periods of darkness at this time of year. At the mid-northerly latitudes, summer days have about 16 hours of warming daylight and only eight hours of cooling nights. During mid-winter the pattern is reversed, resulting in longer nights and shorter days. To demonstrate that it is the daylight versus darkness ratio that produces climates that make growing seasons possible, one should note that even in regions only 30° from the poles one finds plants such as wheat, corn, and potatoes growing. In these regions the Sun is never very high in the sky but because of the orientation of Earth's axis, the Sun remains above the horizon for periods for over 20 hours a day from late spring to late summer.

Astronomers have assigned names to the dates at which the official seasons begin. When the axis of Earth is perpendicular to the incoming Sun's rays in spring the Sun stands directly over the equator at noon. As a result, daylight hours equal nighttime hours everywhere on Earth. This gives rise to the name given to this date, the vernal equinox. Vernal refers to spring and the word equinox means equal night. On the first day of fall, the autumnal equinox also produces 12 hours of daylight and 12 hours of darkness everywhere on Earth.

The name given for the first day of summer results from the observation that as the days get longer during the spring, the Sun's height over its noon horizon increases until it reaches June 21. Then on successive days, it dips lower in the sky as Earth moves toward the autumn and winter seasons. This gives rise to the name for that date, the summer solstice, because it is as though the Sun "stands still" in its noon height above the horizon. The winter solstice is likewise named because on December 21 the Sun reaches the lowest noon time height and appears to "stand still" on that date as well.

In the past, early humans celebrated the changes in the seasons on some of these cardinal dates. The vernal equinox was a day of celebration for the early Celtic tribes in ancient Britain, France, and Ireland. Other northern European tribes also marked the return of warmer weather on this date. Even the winter solstice was a time to celebrate, as it marked the lengthening days that would lead to spring. The ancient Romans celebrated the Feast of Saturnalia on the winter solstice. And even though there are no historical records to support the choice of a late December date for the birth of Christ, Christians in the fourth centurya.d.chose to celebrate Christmas near the winter solstice.

See also Atmospheric circulation; Celestial sphere: The apparent movements of the Sun, Moon, planets, and stars; Latitude and longitude; Seasonal winds; Solar illumination: Seasonal and diurnal patterns

Seasons

Seasons on Earth are found only in the temperate zones. These zones extend from 23.5° north (and south) latitude to 66.5° north (and south) latitude. In these regions of Earth nature exhibits four seasons; spring, summer, autumn (or fall) and winter. Each season is characterized by differences in temperature , amounts of precipitation , and the length of daylight. Spring comes from an Old English word meaning to rise. Summer originated as a Sanskrit word meaning half year or season. Autumn comes originally from a Etruscan word for maturing. Winter comes from an Old English word meaning wet or water. The equatorial regions or torrid zones have no appreciable seasonal changes and here one generally finds only a wet season and a dry season. In the polar regions we have only a light season and a dark season.

In the Northern Hemisphere, astronomers assign an arbitrary starting date for each season. Spring begins around March 21 and summer begins around June 21. Autumn begins around September 23 and winter around December 21. Because every fourth year is a leap year and February then has 29 days, the dates of these seasonal starting points change slightly. In the Southern Hemisphere the seasons are reversed with spring beginning in September, summer in December, fall in March, and winter in June. Seasons in the Southern Hemisphere are generally milder due to the moderating presence of larger amounts of ocean surface as compared to the Northern Hemisphere.

Changes in the seasons are caused by Earth's movement around the Sun . Because Earth orbits the Sun at varying distances, many people think that the seasons result from the changes in the Earth-Sun distance. This belief is incorrect. In fact, Earth is actually closer to the Sun in January compared to June by approximately three million miles.

Earth makes one complete revolution about the Sun each year. The reason for the seasons is that the axis of Earth's rotation is tilted with respect to the plane of its orbit . This tilt, called the obliquity of Earth's axis, is 23.5degrees from a line drawn perpendicular to the plane of Earth's orbit. As Earth orbits the Sun, there are times of the year when the North Pole is alternately tilted toward the Sun (during northern hemispheric summer) or tilted away from the Sun (during northern hemispheric winter). At other times the axis is generally parallel to the incoming Sun's rays. During summer, two effects contribute to produce warmer weather . First, the Sun's rays fall more directly on Earth's surface and this results in a stronger heating effect. The second reason for the seasonal temperature differences results from the differences in the amount of daylight hours versus nighttime hours. The Sun's rays warm Earth during daylight hours and Earth cools at night by re-radiating heat back into space . This is the major reason for the warmer days of summer and cooler days of winter. The orientation of Earth's axis during summer results in longer periods of daylight and shorter periods of darkness at this time of year. At the mid-northerly latitudes summer days have about 16 hours of warming daylight and only eight hours of cooling nights. During mid-winter the pattern is reversed and we have longer nights and shorter days. To demonstrate that it is the daylight versus darkness ratio that produces climates that make growing seasons possible, one should note that even in regions only 30° from the poles one finds plants such as wheat , corn, and potatoes growing. In these regions the Sun is never very high in the sky but because of the orientation of Earth's axis, the Sun remains above the horizon for periods for over 20 hours a day from late spring to late summer.

Astronomers have assigned names to the dates at which the official seasons begin. When the axis of Earth is perfectly parallel to the incoming Sun's rays in spring the Sun stands directly over the equator at noon. As a result, daylight hours equal night time hours everywhere on Earth. This gives rise to the name given to this date, the vernal equinox . Vernal refers to spring and the word equinox means equal night. On the first day of fall, the autumnal equinox also produces 12 hours of daylight and 12 hours of darkness everywhere on Earth.

The name given for the first day of summer results from the observation that as the days get longer during the spring, the Sun's height over its noon horizon increases until it reaches June 21. Then on successive days it dips lower in the sky as Earth moves toward the autumn and winter seasons. This gives rise to the name for that date, the Summer Solstice , because it is as though the Sun "stands still" in its noon height above the horizon. The Winter Solstice is likewise named because on December 21 the sun reaches the lowest noon time height and appears to "stand still" on that date as well.

In the past, early humans celebrated the changes in the seasons on some of these cardinal dates. The vernal equinox was a day of celebration for the early Celtic tribes in ancient Britain, France, and Ireland. Other northern European tribes also marked the return of warmer weather on this date. Even the winter solstice was a time to celebrate, as it marked the lengthening days that would lead to spring. The ancient Romans celebrated the Feast of Saturnalia on the winter solstice. And even though there are no historical records to support the choice of a late December date for the birth of Christ, Christians in the fourth century a.d. chose to celebrate his birth on the winter solstice. In the Julian calendar system in use at that time this date fell on December 25.

See also Global climate.

Resources

books

Abell, George, David Morrison, and Sydney Wolff. Exploration of the Universe. 6th ed. Philadelphia: Saunders, 1993.

Bacon, Dennis Henry, and Percy Seymour. A Mechanical History of the Universe. London: Philip Wilson Publishing, Ltd., 2003.

Hartman, William. The Cosmic Voyage. Belmont, CA: Wadsworth, 1992.

Pasachoff, Jay. Astronomy: From the Earth to the Universe. 4th ed. Philadelphia: Saunders, 1991.

Darrel B. Hoff

KEY TERMS

Autumnal equinox

—The date in the fall of the year when Earth experiences 12 hours of daylight and 12 hours of darkness, usually about September 23rd.

Obliquity

—The amount of tilt of Earth's axis. This tilt is equal to 23.5 degrees drawn from a line perpendicular to the orbit of Earth.

Summer solstice

—The date on which the Sun is highest in the sky at noon, usually about June 21st.

Temperate zones

—The two regions on Earth bounded by the 23.5 degree latitude and the 66.5 degree latitude.

Torrid zone

—A zone on Earth bounded by 23.5 degrees North and South Latitude.

Vernal Equinox

—The intersection of the celestial equator and ecliptic which the Sun appears to reach on or about March 21.

Winter solstice

—The date on which the Sun's noontime height is at its lowest, usually on December 21st.

Seasons

Seasons on Earth are characterized by differences in temperature and the length of daylight. The four distinct seasons—spring, summer, autumn (or fall), and winter—are found only in the temperate zones. These zones extend from 23.5 degrees North (and South) latitude to 66.5 degrees North (and South) latitude. The equatorial regions or torrid zones have no noticeable seasonal changes, only a wet season and a dry season. Polar regions experience only a light season and a dark season.

Spring comes from an Old English word meaning "to rise." Summer originated as a Sanskrit word meaning "half year" or "season." Autumn comes originally from a Etruscan word for "maturing." Winter comes from an Old English word meaning "wet" or "water."

In the Northern Hemisphere, astronomers assign an arbitrary starting date for each season. Spring begins around March 21, summer around June 22, autumn around September 23, and winter around December 22. In the Southern Hemisphere, the seasons are reversed with spring beginning in September, summer in December, fall in March, and winter in

June. Seasons in the Southern Hemisphere are generally milder because of the larger amounts of ocean surface in that hemisphere. Since oceans heat up and cool down much slower than landmasses, they exert a moderating force on temperatures.

Reason for the seasons

Earth makes one complete revolution about the Sun each year. Changes in the seasons are caused not by the varying distance between Earth and the Sun, but by the tilt of Earth on its axis during that revolution. (Earth's axis of rotation is tilted 23.5 degrees to the plane of its orbit.) As Earth orbits the Sun, there are times of the year when the North Pole is alternately tilted toward the Sun (during Northern Hemispheric summer) or tilted away from the Sun (during Northern Hemispheric winter). At other times the axis is generally parallel to the incoming Sun's rays.

Words to Know

Autumnal equinox: Date in the fall of the year when Earth experiences 12 hours of daylight and 12 hours of darkness, usually around September 23.

Summer solstice: Date on which the Sun is highest in the sky at noon in the Northern Hemisphere, usually around June 22.

Temperate zones: Two regions on Earth bounded by 23.5 degrees latitude and the 66.5 degrees latitude.

Torrid zone: Zone on Earth bounded by 23.5 degrees north and south latitude.

Vernal equinox: Date in the spring of the year when Earth experiences 12 hours of daylight and 12 hours of darkness, usually around March 21.

Winter solstice: Date on which the Sun's noontime height is at its lowest in the Northern Hemisphere, usually on December 22.

During summer, two effects contribute to produce warmer weather. First, the Sun's rays fall more directly on Earth's surface, producing a stronger heating effect. Second, daylight hours outnumber nighttime hours. The Sun's rays warm Earth during daylight hours and Earth cools at night by reradiating heat back into space. Since there are longer periods of daylight and shorter periods of darkness during the summer, Earth receives more solar heat then it releases back into space. Thus, areas experiencing summer stay warmer.

The equinox

When the axis of Earth is perfectly parallel to the incoming rays of the Sun in spring—around March 21—the Sun rises in a direction that is due east everywhere on Earth and stands directly over the equator at noon. As a result, daylight hours equal nighttime hours everywhere on Earth. This effect gives rise to the name given to this date, the vernal equinox. Vernal comes from the Latin word for "spring," while equinox is formed from the Latin word for "equal night." The corresponding date in the fall when 12 hours of daylight and 12 hours of darkness occur everywhere on Earth—around September 23—is known as the autumnal equinox.

The solstice

After the vernal equinox, the Sun continues to move in a northward direction and rise a little farther north of east each day until around June 22. On this day, the Sun has reached its extreme northward position and seems to stand still in its noon height above the horizon. For this reason, the date is known as the summer solstice, from the Latin words meaning "sun stands still." The summer solstice, the longest day and shortest night of the year in the Northern Hemisphere, marks the beginning of summer in the Northern Hemisphere. Afterward, the Sun begins to move southward. It crosses the celestial equator (the autumnal equinox) and continues to move southward, rising a little farther south of east each day until it reaches its most extreme southward position around December 22—the winter solstice (the shortest day and longest night in the Northern Hemisphere). Afterward, the Sun begins its northward movement back to the vernal equinox.

Celebrating the seasons

Early societies celebrated the changes in the seasons on some of these cardinal dates. The vernal equinox was a day of celebration for the early Celtic tribes in ancient England, France, and Ireland. Other northern European tribes also marked the return of warmer weather on this date. Even the winter solstice was a time to celebrate, as it marked the lengthening days that would lead to spring. The ancient Romans celebrated the Feast of Saturnalia on the winter solstice. And even though there are no historical records to support the choice of a late December date for the birth of Jesus of Nazareth, Christians in the fourth century a.d. chose to celebrate Jesus' birth on the winter solstice. In the Julian calendar system in use at that time, this date fell on December 25.

[See also Calendar; Global climate ]

sea·son / ˈsēzən/ • n. each of the four divisions of the year (spring, summer, autumn, and winter) marked by particular weather patterns and daylight hours, resulting from the earth's changing position with regard to the sun. ∎  a period of the year characterized by a particular climatic feature or marked by a particular activity, event, or festivity: the rainy season the season for gathering pine needles. ∎  a fixed time in the year when a particular sport is played: basketball season is over. ∎  the time of year when a particular fruit, vegetable, or other food is plentiful and in good condition: the pies are made with fruit that is in season lobster season. ∎  an indefinite or unspecified period of time; a while: this most beautiful soul, who walked with me for a season in this world. ∎ archaic a proper or suitable time: to everything there is a season.• v. [tr.] 1. add salt, herbs, pepper, or other spices to (food): season the soup to taste with salt and pepper | [as adj.] (seasoned) seasoned flour. ∎  add a quality or feature to (something), esp. so as to make it more lively or exciting: his conversation is seasoned liberally with exclamation points and punch lines.2. make (wood) suitable for use as timber by adjusting its moisture content to that of the environment in which it will be used: [as adj.] (seasoned) it was made from seasoned, untreated oak. ∎  [as adj.] (seasoned) accustomed to particular conditions; experienced: she is a seasoned traveler.PHRASES: for all seasons suitable in or appropriate for every kind of weather: a coat for all seasons. ∎  adaptable to any circumstance: a singer for all seasons.season's greetings used as an expression of goodwill at Christmas or the New Year.