The distance to celestial objects is key to calculating their intrinsic properties like mass and luminosity. Distance, unfortunately, is also one of the most difficult parameters to measure. The most direct method is called parallax: When a celestial body is viewed from different, widely separated vantage points, its angular position with respect to background stars appears different. Parallax is traditionally used to triangulate the distances to nearby stars by measuring their apparent angles six months apart, at the two opposite sides of the Earth's orbit around the Sun.
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