All the planets orbit the Sun in a counterclockwise direction as seen from the north pole, and this motion is called prograde. This distinguishes it from true retrograde motion, which is the revolution or rotation of an object in the solar system in a clockwise direction as seen from the north pole (i.e., looking "down" on the solar system). Just like the planets, your friend is always going in the same direction, but relative to the trees the situation looks quite different! Because the effect described above is an optical illusion, it is sometimes called apparent retrograde motion. As you catch up, your friend will appear to stop relative to the trees, move backwards, and then move forward again. Watch your friend relative to some distant trees. After ten seconds, start running faster than your friend in the same direction. Have a friend stand 50 yards away and begin jogging in the direction shown. You can see retrograde motion for yourself if you do this experiment. Inner planets exhibit retrograde motion as well, as they catch up with and pass Earth, moving between it and the Sun. This changed when Nikolaus Copernicus (1473-1543) argued that Earth orbits the Sun like all the other planets, providing a more natural explanation for retrograde motion. For such a system the planet indeed had to be going backwards, because the Earth was stationary. D.), who believed that Earth was at the center of the Universe. Retrograde motion of the planets confounded early astronomers such as Ptolemy (c. The changing line of sight from Earth to the planet makes it appear that the planet has stopped and begun to move backwards, though it is still moving in its original direction. This is an optical illusion produced as Earth, which orbits the Sun faster than any of the outer planets, catches up and passes them in its orbit.
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