Aberra

Aberra, a stargazer, tilts a brass telescope toward a slanted beam of starlight.

The Physics

From 1725 James Bradley tracked the star Gamma Draconis and found it shifted through the year by about 20 arcseconds, but in a pattern that did not fit parallax. In 1729 he explained it (Phil. Trans. R. Soc. 35, 637): Earth is moving, so the telescope must be tilted slightly forward to catch starlight, like slanting an umbrella when you walk in rain. The tilt is set by Earth’s orbital speed compared with the speed of light.

θ ≈ v/c ≈ 20.5″

• θ: aberration angle (rad, here ≈ 20.5 arcseconds)
• v: Earth’s orbital speed, ≈ 30 km/s
• c: speed of light (m/s)

It was the first direct proof that Earth moves around the Sun, and gave a new estimate of the speed of light.

Honest caveat: This is not parallax: aberration depends on Earth’s velocity, not its position, and is the same size for every star. The small-angle formula is enough here.

On the sticker

Aberra stands by a brass telescope as a beam of light slants down past it against a starry sky. Printed text: ABERRA with the line 20.5 ARCSEC. Pixel-art kiss-cut vinyl sticker, white, in 2″, 3″, 4″ and 6″ sizes.