<P> "Mercury in retrograde" is an example of the term used as a noun for retrograde motion . Retrograde is also sometimes used as an intransitive verb meaning to become, to appear, to behave--or appear to move--in a retrograde fashion . </P> <P> Although planets can sometimes be mistaken for stars as one observes the night sky, the planets actually change position from night to night in relation to the stars . Retrograde (backward) and prograde (forward) are observed as though the stars revolve around the Earth . Ancient Greek astronomer Ptolemy in 150 AD believed that the Earth was the center of the Solar System and therefore used the terms retrograde and prograde to describe the movement of the planets in relation to the stars . Although it is known today that the planets revolve around the sun, the same terms continue to be used in order to describe the movement of the planets in relation to the stars as they are observed from Earth . Like the sun, the planets appear to rise in the East and set in the West . When a planet travels eastward in relation to the stars, it is called prograde . When the planet travels westward in relation to the stars (opposite path) it is called retrograde . </P> <P> In Earth's 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 . </P> <P> As seen from Earth, all the other objects in the Solar System appear to periodically switch direction as they cross the sky . Though all stars and planets appear to move from west to east on a nightly basis in response to the rotation of Earth, the outer planets generally drift slowly eastward relative to the stars . Asteroids and Kuiper Belt objects (including Pluto) exhibit apparent retrogradation . 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 inferior conjunctions with the Sun . They are unobservable in the Sun's glare and in their "new" phase, with mostly their dark sides toward Earth; they occur in the transition from morning star to evening star . </P>

A constellation appears to move from east to west in the sky during the night because