![]() ![]() Note: The first time the animation plays it may pause at times, while the rest of the file loads. In the animation below, that is not a concern. This can also be seen in the photograph above, in that the dots representing the position of the planet on different dates are further apart where it is moving faster, and closer together where it is moving slower but because some nights were cloudy the spacing of the dates is not uniform, and some dots are further apart or closer together than they would be if every date could be shown. After passing that point the planet resumes its normal eastward motion, at a faster and faster pace. It speeds up as it passes from left to right (in westward or retrograde motion), only to slow down again as it nears the right (western) stationary point. As it approaches the left (eastern) side of the retrograde loop it slows down, and is moving slowest at the "stationary" point which defines the left end of the loop. Note that as the animation begins Mars is moving rapidly toward the left. The position of Mars is shown every other day (as shown by the date in the lower right corner), along with the trail traced out by the planet over time. The illustration below is an animation of the retrograde motion shown above. In the image, this is represented by the fact that the dots are relatively close together near the stationary points, and further apart elsewhere. As a result, the ends of the retrograde motion are referred to as "stationary" points. As the planets stop moving east and start moving west their motion slows and may even appear to stop if they have a more-or-less straightline motion. In each "loop" the planet appears to move to the east (from right to left) prior to opposition (when it is in the opposite direction from the Sun), slows down and reverses its path when near opposition (the middle of the loop), then resumes its eastward motion after passing opposition. ![]() Uranus' images (the fainter line of dots above Mars) form a more compact straight-line pattern, because it moves far more slowly in its much larger orbit and its orbit isn't tilted as much relative to ours. In the illustration above Mars' images form a large loop, because its orbit is somewhat tilted and it moves relatively fast. (the "current" iteration referred students to their textbook, but the next iteration will replace that with a proper discussion) All the planets go around the Sun in the same (eastward) direction. Even though some planets have retrograde rotation, no planet has retrograde revolution. Do not confuse revolution, or motion around an orbit, with rotation, or turning about an axis. All of the planets move around the Sun in a direct (eastward) direction, but three of them (Venus, Uranus and Pluto) rotate in a retrograde (westward) direction, and are said to have a retrograde rotation. Retrograde rotation: This is a rotational motion which is backwards relative to the orbital motion that an object has. No planet has a retrograde orbital motion, but some of the moons of the outer planets move backwards relative to the general motion of the other moons of those planets, and many comets have retrograde revolution. Retrograde revolution: This refers to an orbital motion which is really backwards relative to normal orbital motions. In this "retrograde" motion, neither planet is actually moving backwards it only appears that way during the time that one laps the other. Both planets move in a direct (eastward) motion around the Sun, but the planet with the inside (smaller) orbit moves faster than the planet on the outside (larger) orbit, and when it passes the slower-moving planet, each sees the other one as apparently moving backwards relative to its usual motion around the sky. ![]() Retrograde motion: The most commonly discussed "retrograde" motion is the apparent backward motion of a planet caused by its being lapped by another planet, or vice-versa. There are three types of retrograde motion commonly referred to in astronomy: Motions in this direction are referred to as direct motions, while motions in the opposite direction are referred to as retrograde. Most of the rotational and orbital motions in the solar system are in the same "eastward" direction. ![]()
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