ASP: To Every Season There is a Reason

To Every Season There is a Reason

by George Musser, Astronomical Society of the Pacific

There are two kinds of people: Those who enjoy the ebb and flow of the seasons, the dapples of autumnal color, the hoary headed frosts of winter, the flowers anew of spring, the live murmur of a summer's day. And those who don't find anything romantic in blinding snowstorms, ice-covered roads, or gangrenous frostbite; who would just as soon go south in the winter, and stay there.

Whatever you think of seasons, the annual cycle is imprinted on nearly everything we do. The seasons tell us the time to plant and the time to reap, the time to go skiing and the time to go scuba-diving. From the earliest days of humanity, kings, farmers, and vacation-goers in nearly every culture have set astronomers to work explaining the seasons.

Every society experiences seasons, but the four seasons of the temperate climates don't mean much to people who live in the tropics, where two seasons are more common: the rainy season and the dry season (see Superseasons). In Ethiopia, for example, the rainy season runs from mid-June to mid-September. Why do temperate and tropical climates have different seasonal cycles? This is one of the many questions that any explanation of the seasons has to answer.

The Measure of the Year

It might be easier if I just told you why seasons occur. But then we'd lose out on a chance to learn how scientists came up with the answer. Many people have gotten used to the idea that scientists always have the answer, but they have to work things out just like everybody else. So let's figure out the seasons, starting from what we see:

Fact #1. It's cold in winter and hot in summer. Only the hardiest Alaskan or Lapp would deny that. In the Northern Hemisphere, the coldest temperatures generally occur in January and February, and the hottest in July and August.

Fact #2. The seasons happen on a regular schedule. Northern winter, for example, starts around December and ends around March.

Fact #3. As every duck knows, winters are milder toward the equator. The strength of the seasons varies as you go north or south, but not as you go east or west. As you cross the equator, the seasons reverse. During winter in the Northern Hemisphere, it's summer in the Southern Hemisphere, and vice-versa (see figure 1). By flying from Canada to Brazil, some clever birds avoid winter altogether.

Figure 1
Average daily high temperature in five cities: Liverpool, England (53 degrees north latitude); Los Angeles, Calif. (34 degrees north latitude); Singapore (1 degree north latitude); Johannesburg, South Africa (26 degrees south latitude); and Punta Arenas (53 degrees south latitude), Chile. When it's winter in the Northern Hemisphere, it's summer in the Southern Hemisphere, and vice-versa. Near the equator, the seasonal fluctuations are small.

Fact #4. The days are shorter, and nights longer, in winter than in summer (see figure 2). The longest day of the year is the summer solstice, which occurs in the Northern Hemisphere around June 21. The shortest day is the winter solstice, around Dec. 21 in the Northern Hemisphere. In between the two solstices are the equinoxes, when days and nights are equally long. The northern vernal (spring) equinox occurs on March 21 and the autumnal equinox on Sept. 21. The length of the day also depends on your latitude. In the arctic and antarctic, at latitudes above 66.5 degrees, there are days when the Sun never sets or rises -- the midnight Sun.

Figure 2
The length of the day at various northern latitudes. This chart does not take into account bending of sunlight by the atmosphere, which lengthens the day by about 10 minutes, or the slight noncircularity of the Earth's orbit, which causes Northern summer to be four days longer than Northern winter.

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No. 29 - Winter/Spring 1995

To Every Season There is a Reason

The Measure of the Year