Light-time correction

<h2 id="calculation">Calculation</h2>
<p>A calculation of light-time correction usually involves an <a href="/facts/Iteration/AMc1p7BL">iterative</a> process. An approximate light-time is calculated by dividing the object's geometric distance from <a href="/facts/Earth/HLLmDfj0">Earth</a> by the speed of light. Then the object's velocity is multiplied by this approximate light-time to determine its approximate displacement through space during that time. Its previous position is used to calculate a more precise light-time. This process is repeated as necessary. For planetary motions, a few (3–5) iterations are sufficient to match the accuracy of the underlying <a href="/facts/Ephemeris/vjQCBG29">ephemerides</a>.
</p>
<h2 id="discovery">Discovery</h2>
<p>The effect of the finite speed of light on observations of celestial objects was first recognised by <a href="/facts/Ole_R%25C3%25B8mer/1lqXzA7F">Ole Rømer</a> in 1675, during a series of observations of <a href="/facts/Eclipse/zXeAKwyj">eclipses</a> of the <a href="/facts/Moons_of_Jupiter/Rq3GMt28">moons of Jupiter</a>. He found that the interval between eclipses was less when Earth and Jupiter are approaching each other, and more when they are moving away from each other. He correctly deduced that this difference was caused by the appreciable time it took for light to travel from Jupiter to the observer on Earth.
</p>

<ul><li>P. Kenneth Seidelmann (ed.), <i>Explanatory Supplement to the Astronomical Almanac</i> (Mill Valley, Calif., University Science Books, 1992), 23, 393.</li>
<li>Arthur Berry, <i>A Short History of Astronomy</i> (John Murray, 1898 – republished by Dover, 1961), 258–265.</li></ul>

Light-time correction open-in-new

Light-time correction