Standard ruler
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A standard ruler is an astronomical object that has a known size. By measuring its apparent angular size on the sky, one can determine its distance from Earth.
An object with a known diameter would look smaller at farther distances, the angular size being proportional to the inverse of the distance to the object. We can therefore estimate the distance to the object by knowing its actual diameter and measuring its angular size.
The angular diameter of a spiral galaxy, for instance can be easily measured. The size of such galaxies are known and do not differ very much. One can therefore use their size, and the angular size that we measured to estimate their distance. the relation of the angular diameter, actual diameter and distance is given by:
α=206265 D/d, where alpha is the angular diameter measured in arcseconds, D is the actual diameter of the galaxy and d is the distance of the galaxy to the observer.
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