Wednesday, October 19, 2011

The Seasonal Sun

Photo of sun, courtesy of Martin LaBar

The sun is the star at the center of our solar system. Its mass is so large that its gravitational effect holds several planets in orbit around it. Our sun is only a medium-sized star classified as a yellow dwarf, but its diameter is still over 100 times that of Earth's diameter.

In our solar system, Earth is the third planet from the sun. In addition to a plentiful supply of oxygen and water, Earth's position has enabled life to evolve on our planet. Some of our most basic organisms, like plants, algae, and many bacteria, create food for themselves by drawing energy from the sun through a process called photosynthesis.

Earth is not exactly spherical; in fact, it bulges in the center around the equator. Earth's axis is also slightly tilted; the angled axis is what causes the seasons to change as earth makes its annual trip around the sun. The longest day of the year in the northern hemisphere is on June 21st and is referred to as the summer solstice. The shortest day occurs six months later on December 21st and is referred to as the winter solstice. Check out the picture below to see how the Earth is tilted and how that might affect the amount of sunlight/solar radiation the Earth receives at different times throughout the year.

Image of seasons, courtesy of Wally Glutton.

The tilt of the Earth creates another interesting phenomenon: a seasonal change in the height of the sun relative to the horizon. In scientific terms, the height of the sun relative to the horizon is called altitude. The sun's position, as it travels from East to West throughout the day, is called azimuth. Azimuth is simply an angled measurement of the sun's position relative to true north. During the winter in the northern hemisphere, the sun sits very low above the horizon even during the middle of the day; however, in the summer it has the potential to reach 90 degrees from the horizon, hence the term "high noon".

An understanding of the sun and seasons is extremely important to humans. Besides being the source of life, our system of food production is dependent on this knowledge. It makes sense that at the time of the year where the northern hemisphere receives the most solar radiation (summer), we are able to produce a lot of food.

As a fun experiment, pick a time and a day that you will be in class every month (for example, the 2nd Wednesday of every month at 11:00 am). Use a protractor to measure the height (in degrees) of the sun. Use the horizon as the base (0 degrees) and the sun as your target. Chart your monthly altitude measurement and at the end of the school year, you should see that the sun has followed the pattern described above.

For a more in depth experiment, visit the University of Oregon's Solar Radiation Monitoring Laboratory ( and create a detailed graph of your observations.
Terms to look up: Solar System, Photosynthesis, Seasons, Altitude, Azimuth

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