Apparent retrograde motion- When we observe the sky, the Sun, Moon, and stars appear to move from east to west because of the rotation of Earth (so-called diurnal motion). However, orbiters such as the Space Shuttle and many artificial satellites appear to move from west to east. These are direct satellites (they actually orbit Earth in the same direction as the Moon), but they orbit Earth faster than Earth itself rotates, and so appear to move in the opposite direction of the Moon. Mars has a natural satellite Phobos, with a similar orbit. From the surface of Mars it appears to move in the opposite direction because its orbital period is less than a Martian day. There are also smaller numbers of truly retrograde artificial satellites orbiting Earth which counter-intuitively appear to move westward, in the same direction as the Moon. Apparent path of Mars in 2009-2010 relative to the constellation Cancer. As seen from Earth, all the planets appear to periodically switch direction as they cross the sky. Though all stars and planets appear to move from east to west on a nightly basis in response to the rotation of Earth, the outer planets generally drift slowly eastward relative to the stars. This motion is normal for the planets, and so is considered direct motion. However, since Earth completes its orbit in a shorter period of time than the planets outside its orbit, it periodically overtakes them, like a faster car on a multi-lane highway. When this occurs, the planet being passed will first appear to stop its eastward drift, and then drift back toward the west. Then, as Earth swings past the planet in its orbit, it appears to resume its normal motion west to east. Inner planets Venus and Mercury appear to move in retrograde in a similar mechanism, but as they can never be in opposition to the Sun as seen from Earth, their retrograde cycles are tied to their lower conjunctions with the Sun. Asteroids and Kuiper Belt Objects (including Pluto) also exhibit apparent retrogradation. Interestingly, Galileos drawings show that he first observed Neptune on December 28, 1612, and again on January 27, 1613. On both occasions, Galileo mistook Neptune for a fixed star when it appeared very close—in conjunction—to Jupiter in the night sky, hence, he is not credited with Neptunes discovery. During the period of his first observation in December 1612, Neptune was stationary in the sky because it had just turned retrograde that very day. Since Neptune was only beginning its yearly retrograde cycle, the motion of the planet was far too slight to be detected with Galileos small telescope. The more distant planets retrograde more frequently: Mars retrogrades for 72 days every 25.6 months. Jupiter for 121 days every 13.1 months. Saturn for 138 days every 12.4 months. Uranus for 151 days every 12.15 months and Neptune for 158 days every 12.07 months. The retrogradation of a hypothetical extremely distant (and non moving) planet would take place during a half-year, with the planets apparent yearly motion being reduced to a parallax ellipse. The period between such retrogradations is the synodic period of the planet. Apparent retrograde motion of Mars in 2003 as seen from Earth. Click on image to see animation. This apparent retrogradation puzzled ancient astronomers, and was one reason they named these bodies planets in the first place: Planet comes from the Greek word for wanderer. In the geocentric model of the solar system, retrograde motion was explained by having the planets travel in deferents and epicycles.It was not understood to be an illusion until the time of Copernicus. The accompanying animated diagram shows the retrograde motion of Mars for the year 2003, which occurs against the background of the constellation Aquarius. (source:wikipedia)
Posted on: Sun, 16 Mar 2014 19:32:28 +0000
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