For measuring large distances such as the distance of moon or a planet from the Earth, special methods are adopted. Radio-echo method, laser pulse method and parallax method are used to determine very large distances.
Laser pulse method: The distance of moon from the Earth can be determined using laser pulses. The laser pulses are beamed towards the moon from a powerful transmitter. These pulses are reflected back from the surface of the moon. The time interval between sending and receiving of the signal is determined very accurately.
If t is the time interval and c the velocity of the laser pulses, then ct the distance of the moon from the Earth is d = ct/2.
