Three Laws Keplers Three Laws Circular Motion Principles for Satellites Mathematics of Satellite Motion Weightlessness in Orbit Energy Relationships for Satellites In the early 1600s, Johannes Kepler proposed three laws of planetary motion. Kepler was able to summarize the carefully collected data of his mentor - Tycho Brahe - with three statements that described the motion of planets in a sun-centered solar system. Keplers efforts to explain the underlying reasons for such motions are no longer accepted; nonetheless, the actual laws themselves are still considered an accurate description of the motion of any planet and any satellite. Keplers three laws of planetary motion can be described as follows: The path of the planets about the sun is elliptical in shape, with the center of the sun being located at one focus. (The Law of Ellipses) (2)An imaginary line drawn from the center of the sun to the center of the planet will sweep out equal areas in equal intervals of time. (The Law of Equal Areas) (3)The ratio of the squares of the periods of any two planets is equal to the ratio of the cubes of their average distances from the sun. (The Law of Harmonies) The Law of Ellipses Keplers first law - sometimes referred to as the law of ellipses - explains that planets are orbiting the sun in a path described as an ellipse. An ellipse can easily be constructed using a pencil, two tacks, a string, a sheet of paper and a piece of cardboard. Tack the sheet of paper to the cardboard using the two tacks. Then tie the string into a loop and wrap the loop around the two tacks. Take your pencil and pull the string until the pencil and two tacks make a triangle (see diagram at the right). Then begin to trace out a path with the pencil, keeping the string wrapped tightly around the tacks. The resulting shape will be an ellipse. An ellipse is a special curve in which the sum of the distances from every point on the curve to two other points is a constant. The two other points (represented here by the tack locations) are known as the foci of the ellipse. The closer together that these points are, the more closely that the ellipse resembles the shape of a circle. In fact, a circle is the special case of an ellipse in which the two foci are at the same location. Keplers first law is rather simple - all planets orbit the sun in a path that resembles an ellipse, with the sun being located at one of the foci of that ellipse. The Law of Equal Areas Keplers second law - sometimes referred to as the law of equal areas - describes the speed at which any given planet will move while orbiting the sun. The speed at which any planet moves through space is constantly changing. A planet moves fastest when it is closest to the sun and slowest when it is furthest from the sun. Yet, if an imaginary line were drawn from the center of the planet to the center of the sun, that line would sweep out the same area in equal periods of time. For instance, if an imaginary line were drawn from the earth to the sun, then the area swept out by the line in every 31-day month would be the same. This is depicted in the diagram below. As can be observed in the diagram, the areas formed when the earth is closest to the sun can be approximated as a wide but short triangle; whereas the areas formed when the earth is farthest from the sun can be approximated as a narrow but long triangle. These areas are the same size. Since the base of these triangles are shortest when the earth is farthest from the sun, the earth would have to be moving more slowly in order for this imaginary area to be the same size as when the earth is closest to the sun. The Law of Harmonies Keplers third law - sometimes referred to as the law of harmonies - compares the orbital period and radius of orbit of a planet to those of other planets. Unlike Keplers first and second laws that describe the motion characteristics of a single planet, the third law makes a comparison between the motion characteristics of different planets. The comparison being made is that the ratio of the squares of the periods to the cubes of their average distances from the sun is the same for every one of the planets. As an illustration, consider the orbital period and average distance from sun (orbital radius) for Earth and mars as given in the table below. Planet Period (s) Average Distance (m) T /R (s /m ) Earth 3.156 x 10 s 1.4957 x 10 2.977 x 10 Mars 5.93 x 10 s 2.278 x 10 2.975 x 10 Observe that the T /R ratio is the same for Earth as it is for mars. In fact, if the same T / R ratio is computed for the other planets, it can be found that this ratio is nearly the same value for all the planets (see table below). Amazingly, every planet has the same T /R ratio. Planet Period (yr) Average Distance (au) T /R (yr / au ) Mercury 0.241 0.39 0.98 Venus .615 0.72 1.01 Earth 1.00 1.00 1.00 Mars 1.88 1.52 1.01 Jupiter 11.8 5.20 0.99 Saturn 29.5 9.54 1.00 Uranus 84.0 19.18 1.00 Neptune 165 30.06 1.00 Pluto 248 39.44 1.00 (NOTE: The average distance value is given in astronomical units where 1 a.u. is equal to the distance from the earth to the sun - 1.4957 x 10 m. The orbital period is given in units of earth-years where 1 earth year is the time required for the earth to orbit the sun - 3.156 x 10 seconds. ) Keplers third law provides an accurate description of the period and distance for a planets orbits about the sun. Additionally, the same law that describes the T /R ratio for the planets orbits about the sun also accurately describes the T /R ratio for any satellite (whether a moon or a man-made satellite) about any planet. There is something much deeper to be found in this T /R ratio - something that must relate to basic fundamental principles of motion.
Posted on: Sat, 30 Aug 2014 09:40:44 +0000
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