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Seasons

Do you like the seasons?

Do you live in a place with well-defined seasons? Do you appreciate the change of the seasons, from cold and dark to hot and bright, over the months? 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 ). Solstice refers to the position of the Sun when it is closest to one of the poles. At summer solstice, June 21 or 22, Earth’s axis points toward the Sun and so the Sun is directly overhead at its furthest north point of the year, the Tropic of Cancer (23.5 o N).

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 far away 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. More about seasons can be found in the Atmospheric Processes chapter.

Solar Radiation on Earth

Different parts of the Earth receive different amounts of solar radiation. Which part of the planet receives the most solar radiation? The Sun's rays strike the surface most directly at the Equator.

Different areas also receive different amounts of sunlight in different seasons. What causes the seasons? The seasons are caused by the direction Earth's axis is pointing relative to the Sun.

The Earth revolves around the Sun once each year and spins on its axis of rotation once each day. This axis of rotation is tilted 23.5 o relative to its plane of orbit around the Sun. The axis of rotation is pointed toward Polaris, the North Star. As the Earth orbits the Sun, the tilt of Earth's axis stays lined up with the North Star.

Northern Hemisphere Summer

The North Pole is tilted towards the Sun and the Sun's rays strike the Northern Hemisphere more directly in summer ( Figure below ). At the summer solstice, June 21 or 22, the Sun's rays hit the Earth most directly along the Tropic of Cancer (23.5 o N); that is, the angle of incidence of the Sun’s rays there is zero (the angle of incidence is the deviation in the angle of an incoming ray from straight on). When it is summer solstice in the Northern Hemisphere, it 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 tilt of Earth's axis points away from the Sun ( Figure below ). Light from the Sun is spread out over a larger area, so that area isn't heated as much. 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

In Southern Hemisphere summer, the Sun’s rays directly strike the Tropic of Capricorn (23.5 o 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

Halfway between the two solstices, the Sun's rays shine most directly at the Equator, called an equinox ( Figure below ). The daylight and nighttime hours are exactly equal on an equinox. The autumnal equinox happens on September 22 or 23 and 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

  • In the Northern Hemisphere, at summer solstice the Sun is closest to the north pole (around June 22) and at winter solstice, the Sun is closest to the south pole (around December 22). In the Southern Hemisphere, the names are changed.
  • Over the course of a year, the amount of solar energy received by the Equator is greater than the amount received elsewhere.
  • At equinox the Sun is directly over the Equator; autumnal equinox is around September 22 and spring equinox is around March 22 in the Northern Hemisphere.

Practice

Use these resources 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 summer in the Southern Hemisphere?

http://www.learner.org/jnorth/tm/mclass/eclipticsimulator.swf

6. Place the observer on the Tropic of Capricorn and run the animation. When does the observer get the most direct sunlight (90 degree angle)?

7. Place the observer in the Arctic Circle and run the simulation. Explain what occurs for the observer over the year.

8. Place the observer close to your present latitude and run the simulation. Explain what the observer experiences over a year.

Review

1. At summer solstice in the Northern Hemisphere, what is the date and where is the Sun? What is happening in the Southern Hemisphere at that time?

2. Since the Sun is up for months during the summer at the north pole, why is it that the Equator actually gets the most solar radiation over the course of a year?

3. What are equinoxes and when do they come?

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