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Seasons

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Objectives

Vocabulary

  • equinox : When the Sun is directly above the Equator.
  • solstice : When the Sun is closest to one of the poles.
  • summer solstice : When the Sun is directly above the Tropic of Cancer. This is the longest day of the year in the Northern Hemisphere and the shortest in the Southern Hemisphere.
  • winter solstice : When the Sun is directly above the Tropic of Capricorn. This is the longest day of the year in the Southern Hemisphere and the shortest in the Northern Hemisphere.

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

A common misconception is that the Sun is closer to Earth in the summer and farther away from it during the winter. Instead, the seasons are caused by the 23.5 o  tilt of Earth’s axis of rotation relative to its plane of orbit around the Sun ( Figure   below ).

Earth's axis is tilted

The Earth’s tilt on its axis leads to one hemisphere facing the Sun more than the other hemisphere and gives rise to seasons.

During the summer, areas north of the Equator experience longer days and shorter nights. In the Southern Hemisphere, the Sun is as low in the sky as it will be and so it is their winter. Locations will have longer nights and shorter days. The opposite occurs on winter solstice, which begins on December 21. Depending on whether it is winter or summer 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

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

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.

An animation of the seasons from the University of Illinois is seen here:  http://projects.astro.illinois.edu/data/Seasons/seasons.html . Notice the area of solar radiation, or insolation, in the lower right of the screen.

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.

Practice

Use the resource below to answer the questions that follow.

  1. What causes Earth's seasons?
  2. What is the longest day of the year in the Northern Hemisphere?
  3. What occurs at the equinoxes?
  4. What happens during the winter solstice?
  5. When is it summer in the Southern Hemisphere?

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?

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