Challenger in Orbit
Apparent motion is equivalent to apparent retrograde motion and it is the motion of a planetary body in a direction opposite to that of other bodies within its system as observed from a particular vantage point.
When we observe the sky from Earth, the Sun, Moon, and stars appear to move from east to west because of the rotation Earth’s rotation or diurnal motion. Equipment in orbit (space shuttles, or the International Space Station) appear to move from west to east. These direct satellites orbit Earth faster than the Earth rotates, so they appear to move in the opposite direction. This phenomenon is not limited to Earth, Mars has a natural satellite(Phobos); from the surface of Mars it appears to move in the opposite direction because its orbital period is less than a Martian day.
The apparent motion of all the of the planets appear to periodically switch direction as they cross the sky( from an Earthlings point of view). 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(direct motion). Earth completes its orbit in a shorter period of time than the planets outside its orbit, so it periodically overtakes them. The planet Earth is passing will first appear to stop its eastward drift, and then drift back toward the west. Nest, as Earth swings past the planet, it appears to resume its normal motion west to east.
Apparent motion puzzled early astronomers and led to the name ‘planets’ which means wanderer. This motion was thought to be a true reversal of movement until Copernicus and his contemporaries were able to demonstrate that it is merely an illusion.
We have written many articles about apparent motion for Universe Today. Here’s an article about sunrise and sunset, and here’s an article about the Earth’s orbit.
If you’d like more info on apparent motion, check out an article about the Apparent Motion of the Sun, and here’s a link to NASA’s Definition of Apparent Motion.
We’ve also recorded a series of episodes of Astronomy Cast about every planet in the Solar System. Start here, Episode 49: Mercury.
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