Earth & Space Science
Students learn how the angle, duration, and intensity at which sunlight strikes Earth (due to its tilted axis and orbit around the sun) determine the seasons.
After completing this tutorial, you will be able to complete the following:
Earth is generally thought to experience four major seasons: spring, summer, autumn (fall), and winter, with certain variations, depending on relative position. Each season has characteristic lengths of day and intensity of sunlight. For instance, summer has longer days and more direct sunlight, while winter has shorter days and less intense sunlight. Spring falls between winter and summer, while autumn falls between summer and winter.
During the year, the seasons change as Earth orbits the sun, according to its position around the sun. This is due to the tilted axis upon which Earth rotates, which is an imaginary line between the North Pole and South Pole. The axial tilt is approximately 23.5? from perpendicular to the plane of orbit. Due to the tilt of Earth's axis (which is relatively fixed) combined with its orbit, different parts of the planet are oriented toward the sun at different times during the year. This means that different regions receive sunlight for different lengths of time, and that the angle of sunlight striking Earth at any specific location changes throughout the year, as Earth orbits the sun. The greater the angle that sunlight strikes the planet, the wider the area receiving the sunlight, compared to when the sunlight strikes Earth perpendicularly, or most directly.
There are two solstices during the year. These are the days when the sunlight striking Earth perpendicularly is the farthest north or south of the equator. For instance, the Northern Hemisphere is tilted toward the sun and experiences its longest day and most direct sunlight on June 21, its summer solstice, where sunlight strikes Earth perpendicularly at noon on the Tropic of Cancer, 23.5? north latitude. At the same time, it is winter in the Southern Hemisphere, which is tilted away from the sun. The Southern Hemisphere experiences its shortest day and least direct sunlight on this day. Similarly, December 21 is the Northern Hemisphere's winter solstice, where sunlight strikes Earth perpendicularly at the Tropic of Capricorn, 23.5? south latitude. This is the shortest day of the year in the Northern Hemisphere where the sunlight is least direct. At the same time, the Southern Hemisphere is experiencing summer solstice, the longest day of the year.
There are also two annual equinoxes when sunlight strikes perpendicular to Earth's surface at the equator. On these days, day and night are of equal duration regardless of the location on Earth, except at the poles. March 20 and September 23 mark the beginnings of spring and fall in the Northern Hemisphere. Similarly, in the Southern Hemisphere, they represent the beginnings of fall and spring. At Earth's poles, above or below the Arctic Circle and Antarctic Circle, the equinoxes mark the change from 24 hours of darkness to 24 hours of daylight. Because of the extreme locations of the poles, the sun moves around the horizon, rather than setting and rising, between the spring and fall equinoxes, and then falls below the horizon and remains there until the following spring equinox. Therefore, the poles experience either 24 hours of daylight, or darkness.
In summary, the seasons change as Earth moves through its orbit around the sun due to the planet's axial tilt, which affects the duration and intensity of sunlight striking the different hemispheres. The longer the days, and the more direct the sunlight, the warmer the season is. At the same time, the opposing hemisphere experiences shorter days and less direct sunlight. In short, opposing hemispheres experience approximately opposite seasons. On the other hand, the region between the Tropics of Cancer and Capricorn does not experience marked seasons due to its relatively stable orientation toward the sun.
|Approximate Time||40 Minutes|
|Pre-requisite Concepts||Students should be familiar with these concepts: axis, Earth, orbit, perpendicular, season, and sun.|
|Course||Earth & Space Science|
|Type of Tutorial||Concept Development|
|Key Vocabulary||axis, Earth, equinox|