Regardless of where you live, you almost certainly enjoy the benefits of a greater length of daylight in the summer months. If you live in the Northern Hemisphere, summer runs from late June to late September; in the Southern Hemisphere, summer occupies the same period winter does north of the equator, late December through late March. This increase in summer sunlight results from a combination of increasingly earlier sunrises and ever-later sunsets.
Why does the planet experience more sunlight in the summer and correspondingly less in the winter? The answer combines basic geometry with basic astronomy, though not in the way you may think.
Astronomical Factors Determining Length of Daylight
Earth, on average, is about 93 million miles (150 million kilometers) from the sun. The shape of the orbit is not a circle but an ellipse, so the Earth comes as close to around 91 million miles to the sun in January and strays as far as about 95 million miles in July.
Clearly, however, it is not this variation that makes the summer months warmer and better-lit than the winter months. Instead, the seasons in their entirety result from the Earth being tilted by 23.5 degrees from a line perpendicular to its orbital path around the sun. This tilt always "points" in the same direction with respect to the sun, whereas the Earth completes a circuit around it over the course of a year. This means that, instead of every part of the planet getting 12 hours of sun and 12 hours of darkness every throughout the year, as would occur if the Earth's rotation were perpendicular to its orbital plane, every location (except for the equator itself) experiences more daylight than darkness in the summer. Furthermore, this imbalance becomes more pronounced with increasing distance from the equator (and hence proximity to the poles). In the Northern Hemisphere, June is the sunniest month overall, and December correspondingly the darkest.
You may have heard of the Arctic Circle, a line of latitude circling Earth 66.5 degrees north of the equator (or 23.5 degrees south of the North Pole) and the Antarctic Circle, the Arctic Circle's similarly located counterpart in the Southern Hemisphere. The significance of these imaginary boundaries is that regions closer to the poles than these experience around-the-clock sunlight for a month or more beginning with the onset of summer, called the summer solstice. This is because the tilted axis of Earth's rotation points directly toward the sun on this date, and small portions of the planet do not completely rotate out of the sun's rays until some time has passed. The number of summer solstice hours of daylight is at its peak on this day everywhere on Earth.
At the end of summer, on the autumnal (fall) equinox that takes place on September 21 or 22 in the Northern Hemisphere, the axis or rotation points neither toward nor away from the sun. This has the effect for one day of the Earth not being tilted on its axis at all, and everywhere on Earth receives 12 hours of sunlight and 12 hours of darkness. This also occurs on the vernal (spring) equinox six months later, when the amount of daily sunlight has been increasing from its annual minimum for three months rather than decreasing.
A number of websites, including a page operated by the U.S. Navy (see Resources), integrate these principles and allow you to quickly determine how much sunlight a given location receives on every day of the year. For example, if you enter Portland, Oregon, U.S.A., which has a latitude of just over 45 degrees and is hence just over halfway to the North Pole from the equator, you find that the city is lit for 15 hours and 41 minutes at the time of the summer solstice and for 8 hours and 42 minutes at the time of the winter solstice six months later, meaning that the timing of an Oregon sunset can vary by about three and a half hours. More northerly cities show the same pattern, but a greater amplitude between maximum and minimum amounts of sunlight across the seasons.
About the Author
Kevin Beck holds a bachelor's degree in physics with minors in math and chemistry from the University of Vermont. Formerly with ScienceBlogs.com and the editor of "Run Strong," he has written for Runner's World, Men's Fitness, Competitor, and a variety of other publications. More about Kevin and links to his professional work can be found at www.kemibe.com.
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