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Solar Radiation and the Earth
On average, the Earth receives around 1,366 watts per square meter of solar radiation each day. However, a number of factors determine how much solar radiation the Earth is actually receiving at any given moment. The shape of its orbit around the sun resembles an oval, rather than a circle. This means that throughout the year, the Earth̵7;s distance from the sun is constantly changing. At perihelion, the Earth is closest to the sun. At aphelion, the sun is the furthest from the sun. At perihelion, the Earth is about 5,986,760 kilometers (3,720,000 miles) closer to the sun than during aphelion. The Earth, as a whole, receives the most solar radiation during perihelion, and it receives the least amount during aphelion.
Solar Radiation and High Latitudes
During the Northern Hemisphere̵7;s summer, the North Pole receives the most solar radiation on Earth. During January and December, however, the Southern Hemisphere experiences summer, and the opposite occurs. During the peaks of summer and winter, the North Pole receives 1,264 watts and 0 watts per square meter of solar energy per day, respectively. During the winter solstice and summer solstice at the South Pole, on the other hand, solar radiation amounts to 0 watts and 1,351 watts per day, respectively. During the equinoxes, no solar radiation whatsoever reaches either pole. At the poles, the sun only crosses the horizon twice a year. During the Northern Hemisphere spring equinox, the South Pole winter begins and the sun disappears from the sky entirely. It does not return until the Northern Hemisphere fall equinox. The opposite occurs at the North Pole.
Solar Radiation and Equator
During the equinoxes, the sun is directly over the equator, and the equator is receiving the most solar radiation. Still, the equator never receives as much solar radiation as the poles do during their respective summers. In fact, solar radiation at the equator is nearly constant throughout the year. In the sky, the sun̵7;s angle to the ground only varies by about 47 degrees throughout the year. During the Northern Hemisphere̵7;s summer, the angle of the sun in the sky to the ground is just 23.5 degrees to the north. During the Northern Hemisphere's summer solstice, the equator receives approximately 920 watts per square meter per day. During the equinoxes, the equator receives, on average, 1,038 watts. During the Northern Hemisphere's winter solstice, however, the equator receives about 984 watts. Each day of the year, the equator receives 12 hours of sunlight.
Solar Radiation at Middle Latitudes
The sun is directly overhead at the Tropic of Cancer and the Tropic of Capricorn during the Northern Hemisphere̵7;s summer and the Southern Hemisphere̵7;s summer, respectively. The Tropic of Cancer is located at 23.5 degrees north, while the Tropic of Capricorn is at 23.5 degrees south. During the Southern Hemisphere̵7;s summer, this angle is 23.5 degrees as well, but the sun is in the southern sky. The middle latitudes experience a similar pattern as their hemisphere̵7;s poles, but not nearly to the same extreme. During the summer solstice, when the sun is at the highest point in the sky, the middle latitudes are experiencing the most solar radiation. However, during the winter solstice, the middle latitudes are receiving the least amount of radiation. Sun exposure ranges from a low of 9.2 hours during winter solstice in the Northern Hemisphere to a day length of 14.8 hours during the summer solstices of both hemispheres. At 40 degrees north latitude, the sun provides 1,165 watts per square meter during the Northern Hemisphere summer solstice, 795 watts during the equinoxes, and 376 watts during the Northern Hemisphere summer solstice. During the Northern Hemisphere summer solstice, the -40 degrees latitude receives 352 watts. It receives 795 watts during summer equinoxes and 1,245 watts during the Northern Hemisphere winter equinox.