 Annual stellar aberration amplitude

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.