Når vi betragter himlen forventer vi at himmellegemerne bevæger sig i en bestemt retning over tid. Tilsyneladende bevæger himmellegemerne sig fra øst mod vest. Det er dog muligt at se f. eks. kunstige satellitter og rumskibe med en østgående bane, bevæge sig i modsat retning En sådan bane kaldes retrograd bevægelse.
When we observe the sky, we expect most objects to appear to move in a particular direction with the passing of time. The apparent motion of most bodies in the sky is from east to west. However it is possible to observe a body moving west to east, such as an artificial satellite or space shuttle that is orbiting eastward. This orbit is considered Retrograde Motion.