📐 Stellar Parallax

📖 The Physics

Hold a finger up and blink each eye in turn — the finger jumps against the background. That shift is bigger when the finger is close. Astronomers use the same trick on a huge scale.

They photograph a nearby star from the two ends of Earth's orbit, six months apart, and it shifts against the far background stars. That tiny shift is its parallax, and it gives the distance directly.

Move the star farther away and the apparent wobble gets smaller — the parallax angle shrinks. A star at 1 pc has a parallax of exactly 1″. (The shift here is hugely exaggerated; real parallaxes are under 1″.)