Why does Earth have seasons? Itâ€™s natural to think our worldâ€™s seasons result from Earthâ€™s changing distance from the sun. But you can easily understand thatâ€™s not the case, when you realize that Earth is farther from the sun in July (northern summer) and closer in January (northern winter). The fact that Earthâ€™s Northern and Southern Hemispheres have their summers and winters at opposite times of the year provides a clue to the real reason for seasons: that reason is Earthâ€™s 23-and-a-half-degree tilt on its axis. The photos and video on this page â€“ from NASA â€“ show Earthâ€™s solstices and equinoxes from space.
They can help you visualize why our seasons unfold as they do, continuously, throughout each year.
EUMETSATâ€˜s Meteosat-9 (a weather satellite) captured the four views above of Earth from geosynchronous orbit a few years ago. A satellite in geosynchronous orbit stays over the same point on Earth all the time. The images above show how sunlight fell on the Earth on December 21, 2010 (upper left), March 20 (upper right), June 21 (lower left), and September 20, 2011 (lower right). Each image was taken at 6:12 a.m. local time.
Around 6 a.m. local time each day, the sun, Earth, and any geosynchronous satellite form a right angle, affording straight-down view of Earthâ€™s terminator line, that is, the line between our worldâ€™s day and night sides. The shape of this line between night and day varies with the seasons, which means different lengths of days and differing amounts of warming sunshine.
The line is actually a curve because the Earth is round, but satellite images show it in two dimensions only.
On March 20 and September 20, the terminator is a straight north-south line, and the sun is said to sit directly above the equator. On December 21, the sun resides directly over the Tropic of Capricorn when viewed from the ground, and sunlight spreads over more of the Southern Hemisphere. On June 21, the sun sits above the Tropic of Cancer, spreading more sunlight in the north.
Whatâ€™s causing all this change? Itâ€™s tempting to image itâ€™s the sun moving north or south through the seasons. But thatâ€™s not it. Instead, the change in the orientation and angles between the Earth and the sun result from Earthâ€™s never-ending motion in orbit around the sun.
The axis of the Earth is tilted 23.5 degrees relative to the sun and the ecliptic plane. The axis is tilted away from the sun at the December solstice and toward the sun at the June solstice, spreading more and less light on each hemisphere. At the equinoxes, the tilt is at a right angle to the sun and the light is spread evenly.
Bottom line: A video from NASA shows how sunlight falls on Earthâ€™s surface during the solstices and equinoxes, as seen by the weather satellite Meteosat-9.