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Measuring the distance of planets
"Betsy's fascination with and scientific curiosity about the night sky have been common traits of humanity throughout recorded history. Since ancient times, humans have been keen observers of the heavens. Today we enjoy and even take for granted astronomy-based devices such as timepieces, calendars, and that poster of the Solar System we all became familiar with back in elementary school. That 'road map' of our astronomical neighborhood, with the Sun at the center and the nine planets orbiting around it, took some of the greatest minds in history thousands of years to draw accurately, and it is still under construction today! "To answer Billy's question, let's go back to the height of the Greek civilization. The Greeks were the first to apply their newly invented branch of mathematics, geometry, to the natural world. Circles were about as perfect as they could imagine, so naturally celestial bodies must move on them, and, logically, all planets must move at the same speed, as they had no reason to think differently. Mainstream Greek thinkers argued that the faster a planet moved across the sky, the nearer it was to Earth simply because it would have a smaller circle to the traverse. You might imagine using a similar idea today when watching airplanes in the sky: one that appears to move slowly is probably farther away than one that appears to be moving fast (ignoring for the moment differences in the speed and directions of the planes). These ideas culminated around A.D. 150 with the Graeco-Roman scholar, Ptolemy, who meticulously sized up our Solar System using years of observations that were stored at the great library of Alexandria; this model survived for more than a millennium. However, there was something very wrong with his picture: the Earth was at the center! "In Poland at the beginning of the 16th century, Nicolaus Copernicus began the scientific revolution that placed the Sun at the center of the Solar System, knocking Earth out of its exalted position. Using geometry and reasoning that the fastest planets moving around the Sun are the closest, he mapped out the Solar System with distances to the planets that are surprisingly close to today's accepted values. His work became the starting point for all those who came later. During the cultural renaissance in Europe, Johannes Kepler discovered three laws that describe how planets move around the Sun. In particular, Kepler was able to relate the time that it takes for a planet to complete its journey around the Sun to its average distance from the Sun. Practically speaking, this means that finding a planet's distance from the Sun is simply a matter of timing its orbital period. When Kepler's laws were combined with Isaac Newton's newly discovered laws of motion and gravity, the field of celestial mechanics was born. "The first practical methods for orbit determination were developed at the beginning of the 19th century. These very powerful techniques led to the prediction of the existence of the planet, Neptune, and are used routinely today with powerful computers to navigate spacecraft through the Solar System. With advances in technology, new ways of mapping the Solar System have become available. Nearby planets have been ranged by bouncing radar off them. Astronomers even reflect laser light from a mirror placed on the Moon by Apollo astronauts and then record the travel time of the light, allowing the Moon's distance to be measured to an accuracy better than an inch! Fortunately, though, scientists have yet to pinpoint why starry skies have such a romantic effect on us. Some things are best left undiscovered." Thanks to this month's Whizard, Dr. Daniel Boice, a senior research scientist in the Space Science Department of the Space Science and Engineering Division. Boice specializes in computational astrophysics and studies of comets. The Lighter Side
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