Many different methods are used to measure distance in space.Apollo 11 astronauts left a laser reflector on the moon, making possible to measure the centimetre the precise distance the moon and earth.

                 The distances of other bodies in our solar system, such as the planets & asteroids, can be measured by RADAR,that is reflecting radio waves off them.However,this method wonn't work for stars,which are much further away.For these, astronomers use a technique that measures tiny differences in the position of a star when it is observed from opposite side of the earth's orbit,that is at intervals six months apart.Since the diameter of the earth's orbit is known, the change in angular position of a star at either side of the orbit can be used to calculate it's distance known as the "PARALLAX" method this works for objects at distances of upto about 1000 light years.

For still more distant objects,such as galaxies astronomers use indirect methods.One of these involves cepheid variables, a type of star which has the convenient property of varying in brightness at regular intervals.We know that the length of the intervals depend on a star's luminosity,which means that by measuring a cepheid variable's brightness it is possible to calculate how far away it- and the galaxy that it is a part of- is from us.   

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