GeoTutor: Seasons – The Solstices and Equinoxes
Why does sunlight feel less intense in the winter than in summer? And why do nights seem so much longer in winter, too?
Seasonal variations can be felt in the changes in daylength (the time span between sunrise and sunset) and in the intensity of insolation, or incoming solar radiation. These variations are caused by Earth’s rotation, revolution, unchanging axial tilt, and sphericity.
These factors influence the angle at which the Sun’s rays hit Earth’s surface during the year. The Sun’s rays are most intense when they are directly overhead, or at the subsolar point. The latitude at which the Sun is directly overhead is known as the Sun’s declination. The shift of the Sun’s declination from the Tropic of Cancer to the Tropic of Capricorn and back marks the progression of the seasons over a year.
The progression of the seasons can be seen in Figure 1. The varying daylengths at different latitudes can be seen with the help of the circle of illumination—the line marking Earth’s lighted and dark areas. The circle of illumination halves the Equator throughout the year, resulting in near-equal days and nights at all times of the year, but it varies at other latitudes according to the season. The Arctic and Antarctic Circles can have constant daylight or constant twilight depending on whether they are tilted toward or away from the Sun, respectively. Note in Figure 1 how the circle of illumination covers more or less of Earth at a given latitude during the year, which indicates how long or short daylight will be.
Figure 1. The Progression of the Seasons
Part A – The June and December solstices
On the June and December solstices, the Sun’s declination reaches its northernmost and southernmost extremes. Latitudes beyond the tropics never receive the direct rays of the Sun.
The following conditions occur during the June solstice, which takes place annually on June 20 or 21:
- The North Pole is tilted towards the Sun.
- The Sun is directly overhead at the Tropic of Cancer, its northernmost declination.
- The Northern Hemisphere experiences summer, and daylength is at its longest. The Sun never sets at latitudes within the Arctic Circle, even at night, leading to the phenomenon of the “Midnight Sun,” or constant light.
- The reverse occurs in the Southern Hemisphere, where daylength is at its shortest. South of the Antarctic Circle, however, there is constant darkness.
- Daylength at the Equator remains approximately equal to the length of night.
The reverse conditions occur during the December solstice, which falls on December 21 or 22:
- The South Pole is tilted towards the Sun.
- The Sun’s direct rays fall on the Tropic of Capricorn, the southernmost point of declination.
- The Southern Hemisphere experiences summer and its longest day of the year, and the “Midnight Sun” phenomenon occurs at latitudes within the Antarctic Circle.
- The Northern Hemisphere experiences reverse conditions of a winter season, and the longest night of the year. Constant darkness occurs north of the Arctic Circle.
- Day and night still remain approximately equal at the Equator.
Drag the appropriate labels to their respective targets. Place the blue solstice characteristic labels in the blue targets and the pink solstice name labels in the pink targets. The yellow lines, of course, represent the Sun’s rays.
sun’s declination, summer, Southern Hemisphere
sun’s declination, summer, Northern Hemisphere
longer nights, shorter days
equal days, nights
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Part B – The March and September equinoxes
At the two equinoxes, Earth’s tilt is neither toward nor away from the Sun. As a result, the Sun’s declination is at the Equator, meaning its rays fall directly there. The circle of illumination passes through the poles, resulting in near-equal days and nights everywhere on Earth, including the North and South Poles, as Figure 2 shows.
Figure 2. Sunlight at the Equinoxes
On the March equinox, which occurs on March 20 or 21, the Northern Hemisphere experiences spring and the Southern Hemisphere has autumn. On the September equinox, which occurs on September 22 or 23, the reverse is true.
After the spring equinox, in either hemisphere, days grow longer until reaching their maximum at the summer solstice. Then, they begin to shorten again until the autumnal equinox when daylength once again is equal to night. Days continue to shorten after this event until they are at their shortest at the winter solstice.
Which of the following are true of equinoxes?
Select the three that apply.
Select the three that apply.
Daylength decreases after autumnal equinoxSolar declination at the EquatorSolar declination at the Tropic of Cancer12-hour daylength at all latitudesMidnight Sun or constant light at the North Pole during the March equinox