PLANETS IN ANCIENT TIMES - planets appear as points of...

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PLANETS IN ANCIENT TIMES Five planets were known to the ancient Greeks: Mercury, Venus, Mars, Jupiter, and Saturn. It was not recognized that earth was a planet that orbitted the sun. In the night sky they did not look different from the many thousands of stars that were visible at that time. (Today, especially in cities, we see many fewer stars because their light is masked by the artificial lights of human society.) How did they distinguish these special five (and give them names corresponding to gods)? The word planet means "wanderer," and the planets differed from the stars in their motions across the sky. Our object is to discuss that motion. ANGULAR MOTION First keep in mind that by motion we do not mean motion in three-dimensional space, such as the motion of a baseball across a field or a car along a road. The stars and
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Unformatted text preview: planets appear as points of light in the sky, and we have no direct way of knowing how far away they are. We can specify their position by giving angles that define a direction -- the direction for pointing to them. Motion means moving from one angular position to another, and the amount that the object moves is given as an angle. The "speed" of the motion is how fast its angular position changes. It would be given in degrees per second (or per year). [If somebody was in front of you and then moved to the right side of you, his angular position would have changed by 90 degrees. If this took place in 5 seconds, the angular speed would be 90/5 = 18 degrees per second.]...
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