Standard ruler
Encyclopedia
A standard ruler is an astronomical
object whose approximate size is known. By measuring its apparent angular diameter in 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. The distance to the object can therefore be estimated by knowing its actual diameter and measuring its angular size.
The angular diameter of a spiral galaxy
, for instance can be easily measured by observation. The size of such galaxies are approximately known and do not differ very much. One can therefore use their size, and the angular size that is measured to estimate their distance. the relation of the angular diameter, actual diameter and distance is given by:
where α is the angular diameter measured in arc seconds, D is the actual diameter of the galaxy and d is the distance of the galaxy to the observer.
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...
object whose approximate size is known. By measuring its apparent angular diameter in the sky, one can determine its distance from Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
.
An object with a known diameter
Diameter
In geometry, a diameter of a circle is any straight line segment that passes through the center of the circle and whose endpoints are on the circle. The diameters are the longest chords of the circle...
would look smaller at farther distances, the angular size being proportional to the inverse of the distance to the object. The distance to the object can therefore be estimated by knowing its actual diameter and measuring its angular size.
The angular diameter of a spiral galaxy
Spiral galaxy
A spiral galaxy is a certain kind of galaxy originally described by Edwin Hubble in his 1936 work The Realm of the Nebulae and, as such, forms part of the Hubble sequence. Spiral galaxies consist of a flat, rotating disk containing stars, gas and dust, and a central concentration of stars known as...
, for instance can be easily measured by observation. The size of such galaxies are approximately known and do not differ very much. One can therefore use their size, and the angular size that is measured to estimate their distance. the relation of the angular diameter, actual diameter and distance is given by:
where α is the angular diameter measured in arc seconds, D is the actual diameter of the galaxy and d is the distance of the galaxy to the observer.