JON LOMBERG / SCIENCE PHOTO LIBRARY JON LOMBERG / SCIENCE PHOTO LIBRARY
Parallax measurement of distance. Diagram showing the principle of measuring astronomical distances using parallax. Parallax is the shift of a nearby object against a distant background when viewed from two different positions. Here, a nearby star is viewed from either side of Earth's orbit (lower centre), the largest possible baseline. Parallax measurements are given in parsecs, where an object with a 6-month parallax shift of 2 seconds of arc is at a distance of one parsec. One astronomical unit (AU, the Earth-Sun distance) is 150 million kilometres. A parsec is 30,900 trillion kilometres. Parallax can be detected for stars up to 100 parsecs away.
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