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Light-time correction
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<p>Light-time correction is a displacement in the <a href="/facts/Apparent_place/VHqbq1Ga"><i>apparent</i> position</a> of a <a href="/facts/Astronomical_object/BCHLa0Q2">celestial object</a> from its <i>true</i> position (or geometric position) caused by the object's <a href="/facts/Motion/dCnP7MlB">motion</a> during the time it takes its <a href="/facts/Light/jeofpMn4">light</a> to reach an observer. </p><p>Light-time correction occurs in principle during the observation of any moving object, because the <a href="/facts/Speed_of_light/sKSQwdNx">speed of light</a> is finite. The magnitude and direction of the displacement in position depends upon the distance of the object from the observer and the motion of the object, and is measured at the instant at which the object's light reaches the observer. It is <i>independent</i> of the motion of the observer. It should be contrasted with the <a href="/facts/Aberration_of_light/N3yb3D2C">aberration of light</a>, which depends upon the instantaneous <a href="/facts/Velocity/Q3o1LVDe">velocity</a> of the observer at the time of observation, and is independent of the motion or distance of the object. </p><p>Light-time correction can be applied to any object whose distance and motion are known. In particular, it is usually necessary to apply it to the motion of a <a href="/facts/Planet/rbPjE6Bx">planet</a> or other <a href="/facts/Solar_System/QIcLSazQ">Solar System</a> object. For this reason, the combined displacement of the apparent position due to the effects of light-time correction and aberration is known as <i>planetary aberration</i>. By convention, light-time correction is not applied to the positions of stars, because their <a href="/facts/Proper_motion/ECByRQ8c">motion</a> and distance may not be known accurately. </p>