What Causes Summer in the Northern Hemisphere?
As the last days of spring slip away, people in the Northern Hemisphere await the return of summer's long days and warmer weather. These characteristics of a typical summer day are the result of Earth's rotation around the sun. The complex interactions of Earth and sun are key to understanding what causes the changes of the seasons.
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Definition
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In the Northern Hemisphere, meteorologists define the beginning and end of summer on precise days. Summer begins each year with the summer solstice on June 21. This day, often called "midsummer," is the longest day of the year. The summer season lasts several months, ending with the autumnal equinox on September 23.
Earth's Tilt
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Changes in the seasons occur because of the relative position of the Earth and sun. The Earth has an axial tilt of 23.5 degrees. To understand the Earth's tilt, imagine a line drawn through the center of the planet from the North Pole to the South Pole. As the planet turns, it does not rotate around the line bisecting the North and South Poles. Rather, it spins around a different axis at an angle 23.5 degrees from the polar axis. This rotational axis causes the Earth to tilt, putting it in a constantly changing position relative to the sun.
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Relationship to the Sun
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During the summer, the tilt of the Earth causes the Northern Hemisphere to be inclined toward the sun. As a result, the sun shines longer each day and the temperature becomes hotter. June 21, the summer solstice, represents the day on which the inclination of the Earth toward the sun is largest. This causes sunlight to shine on the Northern Hemisphere for a longer period of time than any other day of the year. The autumnal equinox represents the time at which the Earth's axis is neither inclined toward or away from the sun, ending summer in the Northern Hemisphere.
Misconceptions
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Many people mistakenly believe that summer occurs because the Earth tilts toward the sun during that season. However, the Earth tilts at the same angle at all times. Its tilt remains constant throughout the year as it orbits the sun, although the Northern Hemisphere differs in its inclination toward or away from the sun. Summer occurs because of changes in the relative location of the Northern Hemisphere to the sun, not because the Earth changes its tilt.
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References
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