The Universe in the Classroom

To Every Season There is a Reason

The Tilt Tells the Tale

If the Earth is round, the Sun must be north of the equator in northern summer, and south of the equator in northern winter. You can check this by plotting the solar angle at different latitudes onto a semicircle using a protractor (see figure 6). The solar rays are parallel, indicating that the Sun is immensely far away. From the Earth's perspective, the Sun is spiraling from north to south and back again along the ecliptic (see figure 7). From the Sun's perspective, however, the equator is moving (see figure 8). Both perspectives are valid.

earth at summer solstice
Figure 6
The round Earth, at the summer solstice. At the solstice, the Sun's rays hit the Tropic of Cancer (23.5 degrees north latitude) straight-on, indicating that the Sun must be north of the equator. On that day, the roundness of the Earth blocks sunlight from hitting the south pole and every latitude greater than 66.5 degrees south. To recreate this diagram, draw a semicircle and mark off every 10 degrees with a protractor. Draw the tangent to the circle at every 10 degree mark. Read off the solar angle for each latitude at summer solstice from figure 4 (it doesn't have to be exact) and draw a ray at that angle.

Science is a process of adding more becauses to every explanation. With every because we come to understand the universe more fully. Winter is cold because the solar heating is less, because the angle of sunlight changes, because the Sun seems to move north and south. Each because requires more ideas and more facts, and we are beginning to exhaust our supply. Ideally, to know why the Sun seems to move, we'd have to fly into space and look at the situation from an wholly different perspective. In lieu of more facts or a new perspective, people have had to guess.

the sun on the celestial sphere
Figure 7
The Sun on the celestial sphere. The top frame shows the daily motion of the Sun around the Earth. The bottom shows the seasonal motion of the Sun through the zodiac. Diagram from Biblical Archaeology Review, September/October 1994. (c) 1994 Biblical Archaeology Society. Reproduced with permission. For information on subscriptions to BAR, write to P.O. Box 7026, Red Oak, Iowa 51591, or call 800-678-5555.

The ancient Greek philosopher Aristotle guessed that the Earth was the center of the universe. The Sun moved north and south because it was mounted on a gigantic transparent sphere that rotated around the Earth. The stars were mounted on a larger sphere. To account for facts 7 and 8, the two spheres rotated differently; their axes of rotation were tilted.

northern summer solstice northern winter solstice
Figure 8
The Earth as seen from the Sun, at northern summer solstice (left) and northern winter solstice (right). This perspective shows how the seasons involve a redistribution of sunlight. From this perspective, the Earth looks like a disc. The Sun shines uniformly on this disc: If a region covers half the disc, it captures half the sunlight. Because of the tilt of the Earth, regions appear to change in size. The Northern Hemisphere appears bigger during the summer than during the winter; therefore, it captures more sunlight during the summer during the winter. Figure generated using the Earthplot computer program by The Black Swamp Software Company.

This theory explains facts 1 to 8, and people adopted it for 2,000 years. It wasn't until the 19th century that astronomers detected fact 9: slight shifts in the positions of stars over the course of a year. These shifts mean that we must be looking at the universe from different vantage points. The Earth, therefore, cannot be fixed at the center of the universe; it must be moving, revolving around the Sun. The stars hardly move at all.

To account for facts 7 and 8, the revolution must occur at a slight tilt to the rotation. If the rotation axis of the Earth is tilted with respect to its orbit, the equator bobs back and forth, causing the Sun to be north for half the year, and south for the other half (see figure 9).
tilt theory of seasons
Figure 9
The tilt theory of seasons. The Earth is the little circle shown at four positions in its orbit, and its axis is the short diagonal line. The axis always points in the same direction, but because the Earth moves in its orbit, the axis leans toward the Sun in summer and away from the Sun in winter. Diagram courtesy of Yerkes Observatory.

This is easy to demonstrate. One person can hold a basketball representing the Sun in the middle of a room. Another person holds the Earth, a styrofoam ball with a knitting needle stuck through it to represent the axis, and a rubber band around it to represent the equator. The bearer of the Earth mimics the orbit of the Earth by carrying the styrofoam around the basketball. All the while, the needle should point in the same direction, just as the Earth's axis is always pointed toward the North Star. Northern summer occurs when the needle is tilted toward the Sun, because then the Sun is to the north. Northern winter occurs when the needle is tilted away from the Sun, because then the Sun is to the south. As the Earth travels, it sees the stars and Sun from different perspectives.

The idea of tilt is what scientists mean by a theory. The word theory confuses some people, who think that "it's only a theory." But a theory isn't just some random hunch; it is a set of principles that explains facts. No one has ever gathered any evidence that disproves the theory of the tilt of the Earth, and no other theory fits into the broader picture of physics and astronomy. Scientists use theories not only to explain facts, but to make predictions. In many cases, scientific predictions are the basis for technological innovation.

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