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3.7: Seasons

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    5345
  • Do you like the seasons?

    Do you live in a place with well-defined seasons? Do you appreciate the change of the seasons? In other words, are you happy that Earth's axis is tilted?

    Earth’s Seasons

    Some people think that Earth is closer to the Sun in the summer and farther away from the Sun in the the winter. But that's not true! Why can't that be true? Because when it's summer in one hemisphere, it's winter in the other. So what does cause the seasons? The seasons are caused by the 23.5° tilt of Earth’s axis. One hemisphere points more directly toward the Sun than the other hemisphere. As Earth orbits the Sun, the tilt of Earth's axis stays lined up with the North Star.

    Solstice refers to the position of the Sun when it is closest to one of the poles. At equinox, the Sun is directly over the Equator.

    Northern Hemisphere Summer/Southern Hemisphere Winter

    During summer in the Northern Hemisphere, the North Pole is tilted toward the Sun. The Sun's rays strike the Northern Hemisphere more directly (Figure below). The region gets a lot of sunlight. Summer solstice is June 21 or 22. At that time, the Sun's rays hit directly at the Tropic of Cancer (23.5°N). This is the farthest north that the Sun will be directly overhead. Summer solstice in the Northern Hemisphere is winter solstice in the Southern Hemisphere.

    Summer solstice in the Northern Hemisphere

    Summer solstice in the Northern Hemisphere.

    Northern Hemisphere Winter/Southern Hemisphere Summer

    Winter solstice for the Northern Hemisphere happens on December 21 or 22. The North Pole of Earth's axis points away from the Sun (Figure below). Light from the Sun is spread out over a larger area. With fewer daylight hours in winter, there is also less time for the Sun to warm the area. When it is winter in the Northern Hemisphere, it is summer in the Southern Hemisphere.

    Summer solstice in the Southern Hemisphere

    During summer in the Southern Hemisphere, the Sun’s rays directly strike the Tropic of Capricorn (23.5°S). Sunlight is spread across a large area near the South Pole. No sunlight reaches the North Pole.

    Equinox

    Equinox comes halfway between the two solstices. At equinoxes, the Sun's rays shine most directly at the Equator (Figure below). The daylight and nighttime hours are exactly equal on an equinox. The autumnal, or fall, equinox happens on September 22 or 23. The vernal, or spring, equinox happens March 21 or 22 in the Northern Hemisphere.

    Pictures of Earth during the spring equinox, summer solstice, vernal equinox, and winter solstice

    Where sunlight reaches on spring equinox, summer solstice, vernal equinox, and winter solstice. The time is 9:00 p.m. Universal Time, at Greenwich, England.

    Summary

    • Earth has seasons because of the (23.5°) tilt of its axis of rotation.
    • In the Northern Hemisphere, at summer solstice the Sun is closest to the North Pole (around June 22). At winter solstice, the Sun is closest to the South Pole (around December 22). In the Southern Hemisphere, the names are changed.
    • At equinox, the Sun is directly over the Equator. Autumnal equinox is around September 22. Spring equinox is around March 22.

    Review

    1. Imagine that it is summer solstice in the Northern Hemisphere. What is the date, and where is the Sun? What is happening in the Southern Hemisphere?
    2. Describe why Earth has seasons.
    3. What are equinoxes? When do they come?