apparent retrograde motion is what a planets backward motion in the sky is called.
apparent retrograde motion is what a planets backward motion in the sky is called.
what is a planets backward motion in the sky called?
apparent retrograde motion
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".
apparent retrograde motion
because the ratchet can only stop backward motion at discrete points (i.e., at tooth boundaries), a ratchet does allow a limited amount of backward motion. this backward motion—which is limited to a maximum distance equal to the spacing between the teeth—is called backlash. in cases where backlash must be minimized, a smooth, toothless ratchet with a high friction surface such as rubber is sometimes used. the pawl bears against the surface at an angle so that any backward motion will cause the pawl to jam against the surface and thus prevent any further backward motion. since the backward travel distance is primarily a function of the compressibility of the high friction surface, this mechanism can result in significantly reduced backlash.
ratchet (device)