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Grade 9 Applied Science –Space Theories of the Solar System The human understanding of the skies has change throughout history. Stars have been important culturally, as means to improve and make order in every day life and it has also greatly help to improve our technology and scientific methods. Ancients Constellations are patterns in the sky. Different cultures have seen these as different forms according to their beliefs or mythologies. Astronomical constellations are regions in the sky used as a map (NOTE: There are 88 recognized regions) Throughout the ages the skies have been used to set order. Calendars are based on the star cycles and monuments have been erected to help determine the growing seasons and such (e.g., Stonehenge, Machu Pichu) Seafaring cultures devised methods of using star to navigate the open oceans advancing trade and leading to globalization. By determining the angles to the North Star (in the north) compared to the horizon sailors could determine their latitude on the globe. The use of the stars allowed for greater observations and thus more interest. New models were developed. Geocentric Model (geo earth centered) Ptolemy (Ancient Greek) discovered that the world was round (sphere – knowledge lost until the renaissance) The sun and all planets orbit the earth in perfect circles Retrograde motion was explained by epicycles (apparent backwards movement of the planets Accepted as valid until the renaissance Heliocentric Model (helios sun centered) Copernicus first suggested that the earth and planets orbited the sun Galileo used telescopes to show the orbiting moons of Jupiter (Galilean moons) Keppler: discover mathematically that planetary orbit were elliptical (predicted planet positions – of Uranus and Neptune) Newton – Work on the law of gravity describes planetary motion New technologies in astronomy have confirmed and pushed forward our modern galactic theories of the universe Grade 9 Academic Science – Unit 3 Space Planet Retrograde Motion The word retrograde applies to the apparent backward motion of a planet. An old encyclopedia of astrology describes this retrograde motion as "…the effect of a slow-moving train as viewed from another train traveling parallel to it but at a more rapid rate, wherein the slower train appears to be moving backwards. However, it is not a matter of their actual speed of travel, but the rate at which they change their angular relationship." Retrograde motion is the apparent motion backwards in the sky as viewed from the Earth. Normal motion of planets in the sky is from west to east, but sometimes, outer planets seem to slow to a stop, reverse direction for a time, and then resume their original direction. Copernican Revolution In 1540, Copernicus proposed that the Sun, not the Earth, was the center of the Solar System. Such a model is called a heliocentric system. In this new ordering, the Earth is just another planet and the Moon is in orbit around the Earth, not the Sun. The stars are distant objects that do not revolve around the Sun. Instead, the Earth is assumed to rotate once in 24 hours causing the stars to appear to revolve around the Earth in the opposite direction By banishing the idea that the Earth was the centre of our Solar System, Copernicus led to a simple explanation of both the varying brightness of the planets and retrograde motion. 1. The planets in such a system naturally vary in brightness because they are not always the same distance from the Earth. 2. The retrograde motion could be explained in terms of geometry and a faster motion for planets with smaller orbits (i.e., the planets further from the sun are moving more slowly in their orbits than those closer to the sun). Since the Earth travels faster in its orbit than the superior planets, it overtakes and passes them at times during their mutual orbits around the Sun. For example as the Earth begins to overtake Mars, Mars will appear to slow its eastward motion among the stars. Then just as the Earth overtakes it, Mars will appear to loop slightly westward for a short time. Once the Earth is well past Mars, that planet will resume its eastward motion among the stars. View retrograde motions at http://www.go.ednet.ns.ca/~larry/orbits/retrogrd.html http://www.lasalle.edu/~smithsc/Astronomy/retrograd.html http://www.astro.uiuc.edu/projects/data/Retrograde/ http://www.flex.com/~jai/astrology/retrograde.html http://www.astro.ubc.ca/~scharein/a310/SolSysEx/retro/Retrograde.html http://www.scienceu.com/observatory/articles/retro/retro.html http://www.mhhe.com/physsci/astronomy/applets/Retro/frame.html Mercury’s retrograde happens three times a year and it lasts for about three weeks. In 2007, you can watch the Mercury retrograde between February 13 and March 7.+