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<p>relative velocity vector onto the LOS direction. Equivalently, radial speed equals the <a href="page.php?w=vector_norm">norm</a> of the radial velocity, <a href="page.php?w=Modulo_%28mathematics%29">modulo</a> the sign.</p>

<p>In astronomy, the point is usually taken to be the observer on Earth, so the radial velocity then denotes the speed with which the object moves away from the Earth (or approaches it, for a negative radial velocity).</p>

<p><big>Formulation</big></p>
<p>Given a differentiable vector  defining the instantaneous <a href="page.php?w=relative_position">relative position</a></p><p>
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