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Loading contentMeasuring the precise positions and motions of the stars — parallax, proper motion, and the space astrometry that has mapped over a billion stars. The foundation on which distances across the galaxy are built.
Measuring positions, distances, and motions.
The apparent shift of a nearby star against distant background stars as the Earth orbits the Sun. The size of that shift gives the star's distance by simple geometry — the first and most fundamental rung of the distance ladder.
The slow drift of a star across the sky over years, as it and the Sun move through the galaxy. Combined with radial velocity and distance, it yields a star's true motion through space.
Measuring stellar positions, distances, and motions from space at extraordinary precision. ESA's Hipparcos and then Gaia have charted more than a billion stars, transforming our three-dimensional map of the galaxy.
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Facts on this topic will be cited from these primary and reference sources.
Mission data, planetary science, space telescopes, and public-domain imagery.
Most NASA-produced imagery is in the public domain; individual items are checked for usage terms before publication.