How the distance of a star or galaxy or a distant planet from earth is measured by astronomers?
It is almost impossible to tell the distances of objects we see in the sky. Almost, but not quite, and astronomers have developed a large variety of techniques. A. TRIGONOMETRIC PARALLAX This method rates an A because it is the gold standard for astronomical distances. It is based on measuring two angles and the included side of a triangle formed by 1) the star, 2) the Earth on one side of its orbit, and 3) the Earth six months later on the other side of its orbit B. Moving Clusters Not many stars are close enough to have useful trigonometric parallaxes. But when stars are in a stable star cluster whose physical size is not changing, like the Pleiades, then the apparent motions of the stars within the cluster can be used to determine the distance to the cluster. C. Secular Parallax Another method can be used to measure the average distance to a set of stars, chosen to be all about the same distance from the Earth. D. Statistical Parallax When the stars have measured radial velocities,
