= Annual stellar aberration amplitude
{title2=$\delta\simeq v_\oplus/c$}
At a sightline perpendicular to <Earth>'s instantaneous orbital <velocity>, a heliocentrically transverse ray acquires an apparent angular displacement towards the observer's motion. A <telescope> of <length> $L$ moves transversely by $v_\oplus L/c$ during the light crossing, giving the first-order result. The exact transverse Lorentz boost gives $\sin\delta=v_\oplus/c$. For a circular orbital <radius> $R$ and year $Y$, $v_\oplus=2\pi R/Y$. The <amplitude> is half the full opposite-season excursion for a suitable sightline; it is not annual parallax, which depends on stellar distance.
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