The Hubble Space Telescope's Wide Field Camera 3 has the potential to provide a precision of 20 to 40 microarcseconds, enabling reliable distance measurements up to 5,000 parsecs (16,000 ly) for small numbers of stars. In the 1990s, for example, the Hipparcos mission obtained parallaxes for over a hundred thousand stars with a precision of about a milliarcsecond, providing useful distances for stars out to a few hundred parsecs. Astronomers usually express distances in units of parsecs (parallax arcseconds) light-years are used in popular media.īecause parallax becomes smaller for a greater stellar distance, useful distances can be measured only for stars which are near enough to have a parallax larger than a few times the precision of the measurement. The amount of shift is quite small, even for the nearest stars, measuring 1 arcsecond for an object at 1 parsec's distance (3.26 light-years), and thereafter decreasing in angular amount as the distance increases. These shifts are angles in an isosceles triangle, with 2 AU (the distance between the extreme positions of Earth's orbit around the Sun) making the base leg of the triangle and the distance to the star being the long equal length legs. As the Earth orbits the Sun, the position of nearby stars will appear to shift slightly against the more distant background. The most important fundamental distance measurements in astronomy come from trigonometric parallax, as applied in the stellar parallax method. A similar diagram can be drawn for a star except that the angle of parallax would be minuscule. The lower diagram shows an equal angle swept by the Sun in a geostatic model. In the upper diagram, the Earth in its orbit sweeps the parallax angle subtended on the Sun. Parallax is an angle subtended by a line on a point.
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