The Universe in the Classroom

To Every Season There is a Reason

Fact #5. The Sun rises higher in summer than in winter. If you drive south on a sunny day in northern winter, the Sun is in your eyes more than it would be in summer. The change in the height of the Sun is pretty substantial. Over the course of the year, the angle between the Sun and the zenith (straight up) varies by 47 degrees -- a fourth of the way across the sky. You can simulate this behavior with the Solar Motion Demonstrator (see Using Your Solar Motion Demonstrator).

To keep track of the Sun's position, you can watch the shadow of a street sign, telephone pole, or tree. When the Sun is lower, shadows extend a longer distance (see figure 3). The shadow will be shortest around noon (the exact time varies because our clocks are not exactly synchronized to the Sun). If you measure the shadow every day at noon, you'll notice that its length changes from day to day. The daily change is greatest at the equinoxes.

seasonal cycle of shadows seasonal cycle of shadows
Figure 3
The seasonal cycle of shadows. On the left is the Sun as it appears in summer, high in the sky. Shadows are short. On the right is the Sun as it appears in winter, lower in the sky. Shadows are long. When the height of the Sun changes, so does the angle its rays strike the ground, the incidence angle. The incidence angle is smaller in summer than in winter. You can calculate the incidence angle if you know the height of the flagpole and the length of the shadow, using the trigonometric formula angle = arctan(shadow/height).

Like the length of the day, the shadow length varies with latitude as well as season (see figure 4). In the temperate zones, the shadow is shortest at the summer solstice, indicating that the Sun is at its highest point in the sky. In the tropics, at latitudes below 23.5 degrees, the Sun is overhead twice a year; on those days, telephone poles don't cast a shadow.

Angle of sun
Figure 4
The angle of the Sun at various northern latitudes. The angle varies as the Sun rises and sets, but reaches the maximum shown here around noontime. A solar incidence angle of 90 degrees means the Sun is on the horizon; an angle of 0 degrees means the Sun is directly overhead; negative angles, which occur only in the tropics, mean that shadows point toward the equator. In the tropics, the Sun is overhead twice during the year; at other latitudes, the Sun is never overhead. In the arctic and antarctic, the Sun is on or below the horizon for part of the year. If you want to know your latitude, measure the solar angle on the vernal or autumnal equinox. To do so without looking directly at the Sun, measure the shadow of a stick placed in the ground and use trigonometry to calculate the angle (see figure 3).

Many cultures kept track of the Sun's position by observing its position on the horizon at sunrise or sunset. Stonehenge, the Big Horn Medicine Wheel in Wyoming, and the Sun Dagger of the Anasazi were clocks that told their builders when it was the summer solstice. You can make the same observations. Look out your window and draw the position of the sunrise or sunset with respect to landmarks, such as buildings, mountains, or trees. If you do this even for a few days, you'll notice that the sunrise moves. Around the solstice, the Sun seems to stop moving for several days, and then reverses direction. In fact, the word solstice is Latin for "Sun stands still."

Fact #6. The Sun seems to be the same size in winter as in summer.

Fact #7. Different stars and constellations appear in the sky in winter than in summer, a fact used by many cultures to keep track of the seasons. The Sotho-Tswana peoples of southern Africa ran a contest: The first person to see the star Canopus in the predawn sky won a cow. The appearance of Canopus, in late May, means the dry season is about to start in southern Africa.

The appearance of the sky changes gradually. At northern temperate latitudes, the constellation Orion rises about 11 p.m. in September, 9 p.m. in October, and 7 p.m. in November. Every night, stars rise 1/365 day (four minutes) earlier than they did the previous night. This occurs because the Sun and stars don't move at the same rate.

Fact #8. Not only do the Sun and stars move at slightly different rates, they move in slightly different directions. Every day, the stars rise and set along arcs parallel to the celestial equator. If you hold your arms in an 'L' shape and point one arm at the North Star, the other arm points at the celestial equator. The belt of Orion lies almost exactly on the celestial equator; other bright stars near the celestial equator are Altair and Procyon.

The celestial equator is directly above the Earth's equator. In Kenya and Ecuador, the stars rise straight up, cross the sky in straight lines, and set straight down. At the north or south pole, the celestial equator is on the horizon; stars never rise or set, but move in circles parallel to the horizon.

The Sun almost moves along a path parallel to the celestial equator -- but not quite. In Kenya and Ecuador, the Sun rises straight up only on the equinoxes; on other days, it rises at an angle. Over time, the difference causes the Sun to move slowly with respect to the stars. The Sun is north of the celestial equator in northern summer and south of the celestial equator in northern winter.

If you look at the constellations just after sunset or just before sunrise, you can tell the location of the Sun relative to the stars. In January, for example, the Sun appears in front of Sagittarius; in February, it's in front of Capricorn. (This whole idea of the Sun being in front of a constellation might seem strange. After all, there are no stars during the day, right? Actually, the stars are still there during the day. You can't see them because the sky is so bright. On the Moon, the astronauts could see stars even in broad daylight because the Moon has no blue sky to get in the way.)

If, like the astronomers of antiquity, you plotted the Sun's path on a star map, you could tell that the Sun slowly moves along a path called the ecliptic, marked by the constellations of the zodiac.

The Sun is like a freeway driver who uses the right lane on Monday, the middle lane on Tuesday, and the left lane on Wednesday. The driver takes the same basic path every day, just as the Sun rises and sets every day. But the driver is also slowly moving across the road to the left, just as the Sun is slowly moving across the sky on the ecliptic. You can see this behavior with the Solar Motion Demonstrator (see Using Your Solar Motion Demonstrator).

The ecliptic and celestial equator intersect in the constellations of Pisces and Virgo. The intersection isn't just some astronomical abstraction. It defines the seasons, because the Sun reaches the intersection points at the time of the equinoxes. Ancient Roman art often had an 'X' to represent the intersection. The intersection occurs at an angle of 23.5 degrees. Chinese astronomers may have known this angle as early as 1100 B.C.

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