Hipparcos has more than doubled the number of known variable stars, and has discovered many thousands of new double or multiple star systems. Striding the light-years by parallax The study of stars with candidate planets is a dramatic example of Hipparcos's new determinations of the distances of stars by the parallax principle. Many other discoveries will flow from it. Parallax is an unfamiliar name for a familiar concept, akin to stereoscopic vision. People judge distances in nearby scenes from the difference in direction of the two eyes when focused on an object.
Military rangefinders use the same principle, with more widely separated optics.
Astronomers adapt the Earth's orbit to make a huge rangefinder. At opposite seasons, the Earth is on opposite sides of the Sun, at vantage points 300 million kilometres apart. As a result, the bearings of stars change a little. Nearby stars shift more than very distant stars, and astronomers can measure their distances by trigonometry. Until now, unavoidable inaccuracies in observing the directions of stars from the ground have meant that the distances of only the closest stars can be measured directly by parallax, out to about 100 light-years.