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Facts of the Matter

BY RICHARD BRILL

Sunday, July 7, 2002



Why it’s hot when
the sun is so far away


As we enjoy warm summer days and nights, we might think that it's warmer because Earth is closer to the sun than in winter. That would be common sense, but in fact, yesterday Earth was at its greatest distance from the sun, a condition called aphelion.

Although we say that Earth circles the sun, it doesn't really. Earth orbits in an ellipse, which is a flattened circle, so its distance from the sun changes throughout the year.

At perihelion in January, Earth's closest approach, we are 91.7 million miles from the sun. At aphelion we are 94.8 million miles away, a mere 3.1 million miles farther. That's not much difference, but the sun's intensity is 6 percent less at aphelion, and it makes a significant difference in the amount of heat received.

You might wonder then why it is warmer in the summer when we are farther away from the sun. First of all, the seasons are reversed in the Southern Hemisphere; while the Northern Hemisphere is in midsummer, the Southern Hemisphere is in midwinter. Even considering this, Earth's overall temperature, averaged over both hemispheres, is higher by about 4 degrees Fahrenheit in July than in January. There are several factors responsible.

The greatest influence on Earth's temperature at any latitude is the tilt of its axis. Because Earth's axis of rotation is tilted a little more than 23 degrees, the North Pole is pointed toward the sun from the vernal equinox in March until the autumnal equinox in September, causing summer in the Northern Hemisphere as the sun rises earlier, sets later and takes a higher path through the sky. From September to March, the North Pole is pointed away from the sun; the days are shorter, the nights longer in the Northern Hemisphere, while the Southern Hemisphere has the long days and short nights. The longer days and more direct sunlight more than compensate for the reduced intensity due to the sun's greater distance in July.

But that's not the whole story. Earth is warmer overall in July due to the unequal distribution of land on the planet. Oceans and continents are not distributed evenly around the globe, so the summer sun beating down on the extensive land in the Northern Hemisphere raises the temperature more than it does in the Southern Hemisphere six months later.

Land heats up much faster than water. Water has a high heat capacity, meaning that it can absorb a lot of heat without a large temperature change.

Another factor, albeit a smaller one, is the length of the summer in the Northern and Southern hemispheres. In its elliptical orbit, Earth moves more slowly when it is farther from the sun. Since aphelion occurs in July, summer is two or three days longer in the north than in the south in January. This gives the sun slightly more time to heat up the northern continents.

As simple as Earth's yearly journey around the sun is, the effects are many and varied. Without the tilted axis and the ever so slight flattening of the orbit, there would be no seasons, the days and nights would be always the same length, and the sun would rise and set in the same place day after day. It certainly would be much less interesting.




We could all be a little smarter, no? Richard Brill picks up
where your high school science teacher left off. He is a professor of science
at Honolulu Community College, where he teaches earth and physical
science and investigates life and the universe.
He can be contacted by e-mail at rickb@hcc.hawaii.edu



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