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 the distance of the moon from the Earth is d = ct / 2 .