A Reason for the Season Stormy Stories Activity Index

<< Many believe that Medicine Wheel, a circular rock formation in Wyoming's Bighorn Mountains, is a solar calendar built by the Plains Indians to track the sun throughout the year. Photograph by Tom Melham

Become a season sleuth. Learn the long and short of the changing of the seasons, then test your knowledge with our cosmic map.

In most regions of the Earth, the cycle of the year can be separated into four distinct seasons: summer, autumn, winter, and spring. Each season brings with it distinct changes in weather, temperatures, and length of daylight. Ever wonder why this is so?

If your answer is yes, you're not the only one. Throughout the ages different cultures have speculated why the seasons happen as they do.

The ancient Greeks thought that the seasons owed their existence to an unfortunate marriage between Persephone (per–SEFF–uh–nee), the daughter of the goddess of the harvest, and Hades (HAY–deez), the god of the underworld. For four months out of the year Persephone would travel to the underworld to visit her husband; stricken with grief, her mother Demeter (Duh–MEE–ter) would let the fields grow cold and lifeless.

If you asked ancient Navajo Indians, they would tell you that the seasons are caused by Estsanatlehi, the wife of the sun god Tsohanoai. As the seasons pass from spring to winter Estsanatlehi (whose name means "Changing Woman") becomes older and older, until she is reborn with the next spring.

The Earth rotates on an axis that is tilted. In other words, our planet never stands upright—it is always leaning to the side. The direction of this lean never changes. As the Earth travels along its orbit, it sometimes leans toward the sun and sometimes away from the sun.

Xpedition Xtra: To simulate the tilt of the Earth as it goes around the sun, point at a motionless object across the room and walk in a circle around a friend while he/she remains motionless. Remember to keep pointing at the object across the room; this way you are always pointing in the same direction, no matter how you move. As you "orbit your friend", observe that the direction you are pointing sometimes goes directly toward your friend, sometimes directly away from him and sometimes to the side; the Earth behaves in the same way toward the sun.

Because the direction of the Earth's tilt changes in relation to the sun, the northern and southern halves of our planet get differing amounts of sunlight over the course of the year. When the Northern Hemisphere of the Earth is leaning toward the sun, it receives direct rays of sunlight and is warmer, while the Southern Hemisphere receives more indirect rays.

When the northern part of the Earth is leaning away from the sun, the situation is reversed—the Northern Hemisphere gets cooler, more indirect sunlight while the southern half receives direct rays. Because of this, the seasons in the Northern and Southern Hemispheres are reversed, about six months apart from each other.

The changing position of the Earth's tilt is the reason for the differences in temperature and length of daylight that distinguish the seasons. When the Northern Hemisphere is leaning toward the sun, the warmth of direct rays causes spring and then summer in that part of the globe. When the Northern Hemisphere is leaning away from the sun, the cooling effects of more indirect sunlight cause autumn and winter.

Because the astronomical position of the Earth causes the seasons, the start of spring, summer, autumn, and winter is marked by special days that correspond to different points in the Earth's orbit:

The summer solstice is the first day of summer and the longest day of the year, occurring in the Northern Hemisphere when the North Pole is leaning more directly toward the sun than it does on any other day. During the period marked by the summer solstice, the Northern Hemisphere is warmed by more direct sunlight and days are long and hot.

The winter solstice, by contrast, is the first day of winter and the shortest day of the year. As you might have guessed, the winter solstice in the Northern Hemisphere occurs when the North Pole is leaning away from the sun. When the North Pole is pointing away from the sun, the Northern Hemisphere receives only indirect sunlight; that is why winter is so much colder than summer. Brrrr!

Equinoxes, on the other hand, occur during transition periods when the North Pole is pointing neither directly toward nor directly away from the sun; these days are marked by equal periods of light and darkness. The autumnal equinox is the first day of autumn and occurs when the North Pole begins to lean away from the sun; the vernal equinox is the first day of spring and occurs when the North Pole begins to lean toward the sun again.

Now that you know the basics of the seasons, see if you can identify the four days of the astronomical calendar on our cosmic map.

Younger Xpeditioners: Over the course of the upcoming year, draw a picture of a place in your yard or near your home at 8 p.m. on the evening of each of the four days of the astronomical calendar—the autumnal equinox (September 22, 2000), the winter solstice (December 21, 2000), the vernal equinox (March 20, 2001), and the summer solstice (June 21, 2001). These dates may vary, but not by much. Include as many natural things as you can—flowers, insects, birds, etc.—and the activities they're engaged in, as well as things you see people doing—gardening, strolling, playing games.

When you're finished with your picture, share it with someone at home and point out all the things that were going on during that evening. Keep each of your pictures in a safe place, and when you have a complete collection of four season pictures, share them with your family and talk about how certain things change with the coming of the different seasons and certain things remain the same.

Older Xpeditioners: Plan a series of seasonal celebrations for your family or neighborhood. Each celebration could feature foods particular to that season, such as strawberries in summer or roasted chestnuts in winter. If you have particular flowers that are in bloom or trees that have leafed out, make identification tags for them; be sure to include both the name of the plant and the season.

Also, find photos of other seasonal celebrations around the globe (some good starting points might be summer and winter solstice celebrations, plus revelries associated with the equinoxes) and give a brief presentation of them, explaining to your family and friends where each celebration takes place and how its special characteristics express unique qualities of that particular season. If you have friends you can reach on the Internet who live in other areas of the world, find out what things they do to celebrate the different seasons and share that information with your gathering.

Parents: This activity can help kids learn about the science behind the seasons. It can also serve as a jumping point for learning why and how people have marked these very different points in the Earth's yearly cycle. Ask kids for their ideas on why the seasons have historically been so meaningful to people throughout the world. Traditionally, of course, the seasons have been important to the growing of crops and the gathering of food. Even now, however, when many of us live near cities and are no longer involved with farming or agriculture, we still celebrate the seasons. Ask kids why they think this is.