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How did Hipparcos improve astronomers’ measurements of stellar distances?

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Diagram illustrating how astronomers calculate the distance to a star using parallax: Earth is shown in two positions on opposite sides of its orbit around the Sun, with lines extending to a nearby star X and further to distant background stars. The apparent shift in star X's position, measured as angle A, is used to determine its distance, with the baseline labeled as 186,000,000 miles. — Parallax Calculating stellar distances.

The Hipparcos satellite, launched by the European Space Agency in 1989, significantly advanced the measurement of stellar distances by utilizing parallax through direct triangulation from space.1

By operating outside Earth's atmosphere, the satellite avoided the distorting effects that limit ground-based observations. Using a "split" Schmidt telescope, Hipparcos continuously scanned the sky along two lines of sight. Light from stars was directed onto a focal plane containing 2,688 narrow parallel slits, allowing for the precise determination of angular separations.1

This methodology enabled the satellite to compute individual stellar positions to within 0.001 arc second, an improvement of 20 times over previous terrestrial estimates. By measuring the positions of approximately 118,000 stars, Hipparcos provided a highly accurate foundation for the cosmic distance scale. This data allowed astronomers to refine the calibration of Cepheid variables—essential "standard candles" for measuring distances to other galaxies—and improve estimates for the Hubble constant.1