Topsy turvy earth

Monday, August 3, 2009
by Diane Boudreau

The Earth isn’t well-behaved. For starters, our planet doesn’t stand up straight. It also doesn’t move in perfect circles. It tilts and wobbles. Sometimes it orbits the sun in an egg-shaped pattern. All this wiggling and wobbling has an effect on weather.

In 1924, a Serbian scientist named Milankovitch came up with the idea that Earth’s orbit affects the amount of radiation it gets from the sun. The changes in radiation affect Earth’s climate. In particular, they dramatically affect the big ice sheets we call glaciers.

The Milankovitch Theory uses three characteristics of Earth’s orbit to find out how much solar radiation the planet receives. These three characteristics are eccentricity, axial tilt, and precession. Eccentricity describes the shape of Earth’s orbit. Earth revolves around the sun, but it doesn’t always move in a perfect circle. Over a period of 100,000 years, Earth’s orbit slowly changes from circular to egg-shaped and then back to circular.

The second characteristic is called axial tilt. Imagine a straw pushed through the center of an orange. When the straw is straight up, the orange doesn’t tilt in any direction. But if you tilt the straw at an angle, the orange tilts in the same direction.

The same thing happens with the Earth. The “straw” is Earth’s polar axis—an imaginary line that runs from the North Pole through the planet to the South Pole. As Earth orbits the sun, the axis is never straight up and down. It always tilts at a small angle.

Picture a line drawn from the center of the sun to the center of Earth. The polar axis crosses this line. If the axis were straight up and down, these two lines would form a right angle (90 degrees). In reality, these two lines make an angle greater than 90 degrees because of the axial tilt. Currently, Earth’s axis tilts about 23.5 degrees beyond the center. Over long periods of time, this angle ranges from 21.5 to 24.5 degrees.

Earth also spins, like a top spinning across the floor. Because Earth rotates on its axis, and that axis is tilted, the Earth’s spin is wobbly. It is like the top just before it topples over. This wobbly motion is called precession.

There are two effects of precession. First, the wobbling motion changes where the North Pole points. Today, the North Pole points toward the North Star. But as Earth rotates, the North Pole moves across the sky. In 11,000 years, the North Pole will be pointing toward a star called Vega.

The second effect has to do with seasons. During the next 11,000 years, as the North Pole moves toward Vega, today’s winter season will become the summer season. In 2008, the winter solstice in the northern hemisphere happens on December 21. But 11,000 years from now, in the year 12,198, the date of the winter solstice will be June 22!

The winter solstice is the shortest day of the year. The summer solstice is the longest day of the year—the day with the most hours of sunshine. The days get shorter and longer because of the Earth’s tilt. For example, when the northern hemisphere tilts away from the sun, the days in that hemisphere get shorter. They get shorter and shorter the further north you get. Above the Arctic Circle, the sun doesn’t rise at all on the winter solstice. But on the summer solstice, the sun doesn’t set.

Researchers measure eccentricity, axial tilt, and precession using the science of astrophysics. Geographers and climatologists use these numbers to determine how much solar radiation Earth receives today. They also study how much solar radiation came to the planet in the past. Milankovitch measured solar radiation during the summer. He had good reason. During the summer, solar radiation affects how fast ice sheets grow or melt. Low solar radiation during the summer keeps temperatures cool and helps glaciers to grow. Of course, wet winters also are important to growing glaciers. Increased solar radiation during the winter causes rain and snow. The rain and snow fall on top of the ice sheet. When they freeze, the glaciers grow bigger.

The opposite conditions cause ice sheets to melt. No ice builds up on the glacier if it does not rain or snow during the winter. High solar radiation during the next summer means warmer weather. A warm summer makes ice melt off the glacier.

When Milankovitch thought up his theory in 1924, he had to do the math by hand. It took days to figure out the total amount of incoming solar radiation. Milankovitch didn’t have time to study the entire planet.

Today’s scientists have computers that can do the math much faster than people can. As a result, they can now study the entire planet.