The Universe in the Classroom

To Every Season There is a Reason


by Brett Gladman, Cornell University

The annual seasonal cycle is one example of how celestial events affect the Earth. Tides, caused largely by the gravity of the Moon, are another example. But the Moon has another influence on the Earth that cause changes noticeable only over longer periods of time.

This influence depends on the fact the Earth isn't exactly a sphere. Since the Earth rotates quickly (1,000 miles an hour at the equator), centrifugal force causes the equator to bulge out and the poles to flatten by a small amount; the same thing happens to a water balloon when you spin it rapidly. The Moon's gravity yanks on the bulging equator. This wouldn't matter if the Moon orbit was aligned with the Earth's equator. But the Moon does not orbit exactly in the Earth's equatorial plane -- it's inclined by about 5 degrees -- and consequently it tries to change the tilt of the Earth.

The Earth's spin resists this change. The tug-of-war between spin and Moon causes the Earth's axis to precess, or wobble. It's the same effect that makes a spinning top wobble: Gravity wants to make the top fall over, and it would if it weren't spinning, but rotation stabilizes the top -- and the result is that the spin axis precesses. In the case of the Earth, the spin axis completes precesses once every 26,000 years.

This precession has two consequences. First, because the axis is moving, the star that is closest to the celestial north pole (extending the Earth's pole out into space) changes. At present, the pole star is Polaris, but by A.D. 14,000 the celestial pole will be close to Vega. Around 3,000 B.C., the pole star was Thuban in the constellation Draco.

Second, precession moves the position of the equinoxes on the sky. The spring equinox occurs when the path of the Sun on the sky crosses the Earth's equatorial plane moving north. Civilizations with astronomical knowledge have used the vernal equinox to signal the beginning of spring. For the ancient Babylonians, the vernal equinox was in the constellation of Taurus; now it is in Pisces, and soon it will cross into Aquarius -- hence the "dawning of the age of Aquarius." This precession is so rapid that astronomers, who use a coordinate system on the sky based on the location of the Earth's pole, must constantly correct for the effect.

The Earth is affected not only by the Moon, but also by the other planets. Over hundreds of thousands of years, the gravitational pull of the other planets causes small oscillations both in the tilt of the Earth's orbit and in how elliptical the orbit is. As the orbit changes, the average amount of sunlight that the Earth receives over a year varies slightly.

This is the basis of the Milankovitch theory of climate change. In this theory, changes in the overall climate, such as the ice ages, are caused by changes in the amount of sunlight received due to variations in the Earth's orbit. Even though the changes are small, the climate is so finely balanced that they can develop into noticeable effects. During times of reduced sunlight, temperatures drop, and the Earth enters an ice age, a sort of superwinter. Right now, it's supersummer.

The Earth's climate is so complicated, however, that this theory is still controversial. Other hypotheses for climate cycles have been put forward, but many of them also involve changes in the Earth's orbit. It's one example of how interaction with the rest of the solar system has had profound consequences for life on Earth.

BRETT GLADMAN is a graduate student in the Department of Astronomy at Cornell University in Ithaca, N.Y. His email address is

The Traditional Seasons of Pohnpei

by Pamela Eastlick, University of Guam

Pohnpei is a beautiful green island of coconut palms, crystal ocean, cool breezes, and tropical sunsets, located near the equator 4,600 kilometers (2,900 miles) southwest of Hawaii. About 32,000 people live there, and despite the cultural changes of the 20th century, they have held onto their traditional lore, especially of the seasons.

The lore, collected and compiled by Stewo Gallen at the Pohnpeian Department of Education, designates two seasons: rahk and isol. Rahk runs from March to September, the rainy season on Pohnpei. It is the traditional season of plenty, when the breadfruit ripens. Five traditional feasts during rahk honor the breadfruit. Isol runs from September through March, the dry season. Historically this is the time of little food, or even famine. Six traditional feasts during isol honor various types of yam. There are 177 locally recognized varieties of yam on Pohnpei, probably more than you'll find at your local supermarket.

Pohnpeians are mostly farmers and fishermen, and their traditional calendar is suited to their needs. The calendar has 10 months. Each is 36 or 37 days long and shares its name with a bright star or group of stars prominent during that month. The months are closely tied to the tribute feasts for the yam and breadfruit, and also denote the appearance of strong trade winds and the traditional times to catch certain types of fish.

The Pohnpeian days are based on the phases of the Moon. There are 30 Moon days corresponding to the phase. If you told someone that it was the day Rotenpahwel of the month Daliaram, they would know it was a good day to catch coconut crabs. Rotenpahwel is two days after full Moon, when the tides are at their highest and lowest for any month, and Daliaram occurs in late December and January when the tides are at their highest and lowest for the year. Coconut crabs, one of the island's greatest culinary delicacies, must return to the water to lay their eggs and they do so at the time of highest high and lowest low tides. Like the seasons, the calendar reflects the closeness of the Pohnpeian people to the land and sea.

PAMELA EASTLICK is the planetarium coordinator at the University of Guam in Mangilao. Her email address is

Using Your Solar Motion Demonstrator

by Joseph L. Snider, Oberlin College

This device accurately models the motion of the Sun as seen from any place in the Northern Hemisphere at any time of year. Notice the way the months are spread out along the MONTH part of the frame. The region they occupy is determined by the 23.5 degree tilt of the Earth's axis with respect to the plane of its orbit about the Sun. If the tilt were greater, the months would be spread out more along the frame; if it were less, they would be crowded closer together.

The compass disk represents a part of the surface of the Earth. You can imagine a tiny observer (standing on the black dot at the center of the disk) looking out at the horizon in any direction. The round head of the paper fastener represents the Sun. Setting the paper fastener at the desired month adjusts for the time of year. Swinging the frame from east to west moves the Sun in its apparent daily path through the sky. The LATITUDE part of the frame is used to adjust the compass disk to put the imaginary observer at any latitude from the equator (0 degrees) to the north pole (90 degrees).

Hold the device in your left hand so that the green compass disk is horizontal. Imagine that you are standing in the middle of a large open field, at the location of the black dot, with a clear horizon all around you. The geographical directions are marked around the horizon. With your right hand, smoothly swing around the part of the frame which carries the paper-fastener Sun. When the head of the paper-fastener lies below the compass disk, the Sun lies below the horizon, and so it is night where you are. As the head of the paper-fastener passes the edge of the compass disk, the Sun rises, at a definite location around the horizon. As you continue to swing the frame, the Sun gets higher in the sky, reaches a maximum height, gets lower, and finally sets at some definite location on the horizon.

Here are a few ways to use the Solar Motion Demonstrator:

  1. At what times of year are the lengths of day and night equal? On the vernal equinox in March and fall equinox in September, the Sun rises due east and sets due west at every latitude.
  2. What are the relative lengths of day and night? Swing the piece carrying the Sun around at a constant rate, over its entire range. This corresponds to one rotation of the Earth, taking 24 hours. The Sun lies above the horizon for part of this motion (daytime) and below it for the remainder (nighttime). From this, you can estimate the relative lengths of day and night.
  3. What are the reasons for the Earth's seasons? Move the paper fastener to its June position. Swing the Sun and observe the relative lengths of day and night and the maximum height of the Sun. Do the same with the Sun down in its December position. This demonstrates the two most important factors responsible for the seasons: the length of the day and the angle at which sunlight strikes the ground.
  4. Where on Earth does the Sun remain above the horizon for 24 hours? Explore the range of latitudes and times of year for which the paper-fastener Sun remains above the compass disk as you swing it through its entire daily motion. This corresponds to a 24-hour day, with the Sun above the horizon at midnight! Places for which this is true are the "Land of the Midnight Sun". For an observer north of the so-called Arctic Circle at 66.5 degrees latitude, the Sun will never set on at least one day of the year.
JOSEPH L. SNIDER is a professor in the Physics Department at Oberlin College in Oberlin, Ohio. His email address is

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