Parallax is the apparent shift of a nearby object against a distant background when the observer moves, and it is the only direct way to measure stellar distance. Astronomers observe a star six months apart, from opposite sides of Earth's orbit, and measure the tiny angle it appears to move. The distance in parsecs is simply one divided by that angle in arcseconds. The Gaia mission has applied the technique to more than a billion stars, anchoring the bottom rung of the cosmic distance ladder with unprecedented precision.