Stellar Parallax (Distance to Stars)
Bessel measured the tiny yearly shift of the star 61 Cygni in 1838: the first reliable distance to a star, about 10.3 light-years, and the answer to an old objection to a moving Earth.
Open in the interactive tree →Observing 61 Cygni in 1837-38 with a heliometer (a telescope for measuring tiny angles), Friedrich Bessel published the first reliable stellar parallax in 1838, which put the star about 10.3 light-years away. Thomas Henderson (Alpha Centauri) and Friedrich Struve (Vega) measured nearby stars at almost the same time. Until then, the fact that no star shift had been seen was used as an argument against heliocentrism. Parallax is the first rung of the cosmic distance ladder, the anchor for calibrating Cepheid stars, and the method of ESA's Gaia, which has measured the positions and distances of nearly two billion stars.
Prerequisites
- Trigonometry~150 BCDistance follows from the shift angle by simple triangle geometry.
- Heliocentrism (Sun at the Centre)1543A moving Earth should make nearby stars appear to shift.
- The Telescope1608Only a precise telescope could see the tiny yearly shift of a nearby star.
- Fraunhofer: Lines & Gratings1814Bessel's heliometer came from Fraunhofer's workshop, whose precise lenses made tiny angles measurable.
Unlocks
- Cepheid Distance Yardstick1912Cepheid brightness is calibrated against stars whose distance is known from parallax.
- Gaia: 3D Map of the Milky Way2018–2022Gaia's three-dimensional map is built on parallax measurements.