ABSTRACT
Light and warmth provided by the sun are obvious aspects of our daily existence. Early humans, and likely our prehuman ancestors, must have speculated about the nature of the brilliant, glowing ball that appears to signal the start of day, traverses the sky, and then disappears again. Many early societies must also have speculated about the changing of the seasons-why the sun appears lower in the sky, and for a shorter period of time, each day until a turning point is reached and the sun begins gradually to regain its former vigor, but only until the next turning point, when the cycle starts over again. The apparent movement of the sun across the sky is an illusion created by the rotation of the Earth; that part of the planet we happen to be standing on gradually rotates out of the path of the sun’s rays. Progression of the seasons derives from the revolution of the Earth around the sun, and the Earth’s axis of rotation being tilted somewhat relative to the plane of Earth’s orbit. During the course of the year, our part of the planet receives greater or lesser amounts of sunlight-so is warmed to greater or lesser degrees-and the tilt of Earth’s axis relative to the plane of its orbit creates the appearance of the sun rising higher or lower in the sky. These explanations of the sun’s behavior have been known for centuries and derive from astronomical observations and the geometry of Earth’s orbit. It took far longer to develop the understanding of the source of the sun’s energy.
