Timing of Breeding in birds

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Equinox: The two days of the year when the hours of day equal the hours of night. This occurs at the same time, March and September, in both the northern and southern hemispheres. During this time, all points along the Earth's axis are facing the Sun equally.

Hemisphere: one half of a planet. Earth is divided into the northern and southern hemispheres, which are separated by the equator.

Solstice: The shortest or longest day of the year. Solstices occur because the Earth's axis is at an angle to the Sun. If the northern hemisphere is at its farthest tilt towards the Sun, it is the longest day of the year (summer solstice) there, but it is the shortest day of the year (winter solstice) in the southern hemisphere, which is at its farthest tilt away from the sun.

The Seasons

The seasons change over the year as the Earth circles the Sun. Many people think that the seasons are caused by how far the Earth is from the Sun. The thought is that when it’s cold, the Earth is far away from the Sun and when it’s hot, the Earth is close to the Sun. If this were true, the seasons would be the same in the northern and southern hemispheres of the Earth. People in Australia would have summer time when people in the United States also have summer time, but this isn’t true. When it’s summer time in the United States, it’s winter time in Australia.

The seasons of Earth

What is behind the change of seasons? No, it's not the distance from the Sun to the Earth. The answer lies in the tilt of the Earth. Click to see an animated version.

Earth axis wobble

The Earth is not only tilted it also changes its tilt. This wobble shifts the tilt between 22.1 and 24.5 degrees. It is not a rapid change like a toy spinning top. It takes about 41,000 years to complete a full cycle.

The seasons are caused by the way that the Earth sits, also described as the tilt of its axis, which is like a line through the center of the Earth. The tilt of the axis changes the angle between the Sun’s rays (solar radiation) and the surface of the Earth. When the northern hemisphere is tilted towards the Sun, the days are longer and lots of direct solar radiation lands on its surface, warming the air and land.

At the same time, on the southern hemisphere, the days are shorter and less light lands on the surface of the Earth, causing it to cool. As the Earth circles around the Sun, the northern and southern hemispheres reverse their positions. As each hemisphere faces towards or away from the Sun in turn, we experience the changing seasons. Learn how the changing seasons affect life on Earth.


Additional images via Wikimedia Commons. Leaves turning colors by Bengt Nyman.

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Bibliographic details:

  • Article: The Seasons
  • Author(s): Dr. Biology
  • Publisher: Arizona State University School of Life Sciences Ask A Biologist
  • Site name: ASU - Ask A Biologist
  • Date published: June 9, 2012
  • Date accessed: July 16, 2018
  • Link: https://askabiologist.asu.edu/seasons

APA Style

Dr. Biology. (2012, June 09). The Seasons. ASU - Ask A Biologist. Retrieved July 16, 2018 from https://askabiologist.asu.edu/seasons

American Psychological Association. For more info, see http://owl.english.purdue.edu/owl/resource/560/10/

Chicago Manual of Style

Dr. Biology. "The Seasons". ASU - Ask A Biologist. 09 June, 2012. https://askabiologist.asu.edu/seasons

MLA 2017 Style

Dr. Biology. "The Seasons". ASU - Ask A Biologist. 09 Jun 2012. ASU - Ask A Biologist, Web. 16 Jul 2018. https://askabiologist.asu.edu/seasons

Modern Language Association, 7th Ed. For more info, see http://owl.english.purdue.edu/owl/resource/747/08/
Fall leaves

Signals of change. In some areas of the world, changing leaf color is a sign of cooler weather from a change in the season.

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