Length

Length may be distinguished from height, which is vertical extent, and width or breadth, which are the distance from side to side, measuring across the object at right angles to the length.

Length is a measure of one dimension, whereas area is a measure of two dimensions (length squared) and volume is a measure of three dimensions (length cubed). In most systems of measurement, the unit of length is a fundamental unit, from which other units are defined. Once the fundamental units are defined, it is easier to express larger and smaller units of the same physical quantity. In the metric (SI) system these are related to the fundamental unit in multiples of 10 or 1/10. Thus 1 km is 1000 m and 1 mm is 1/1000 metre. In order to measure very large distances, the following units are used.

• Light year

  Light year is the distance travelled by light in one year in vacuum.

• Astronomical unit

  Astronomical unit is the mean distance of the centre of the Sun from the centre of the Earth.
  1 Astronomical unit (AU) = 1.496 * 10¹¹ m

Determination of distance

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