# Some Units and Definitions

**Some Definitions and Units**

** Practical Units.**

(1) **Length :**

(i) 1 fermi = 1 *fm *= 10^{–15}*m*

(ii) 1 angstrom = 1Å = 10^{–10}*m* = 10^{–8}*cm* = 10^{–7}*mm* = 0.1 *m**mm*

(iii) 1 micron = *m**m* = 10^{–6}*m*

(iv) 1 astronomical unit = 1 *A.U.* = 1. 49 * 10^{11}*m* ≈ 1.5 * 10^{11}*m* ≈ 10^{8}*km*

(v) 1 Light year = 1 *ly* = 9.46 x 10^{15}*m*

(vi) 1 *Parsec* = 1*pc* = 3.26 light year

(2) **Mass :**

(i) Metric tonne : 1 Metric tonne = 1000 *kg*

(ii) Quintal : 1 Quintal = 100 *kg*

(iii) Atomic mass unit (*amu*) : 1 *am**u* = 1.67 x 10^{–27}*kg*

(3) **Time : **

(i) Year : It is the time taken by earth to complete 1 revolution around the sun in its orbit.

(ii) Lunar month : It is the time taken by moon to complete 1 revolution around the earth in its orbit.

1 *Lunar month* = 27.3 days

(iii) Solar day : It is the time taken by earth to complete one rotation about its axis with respect to sun.

1 Solar year = 365.25 average solar day

average solar day 1/365.25 the part of solar year

(iv) Sedrial day : It is the time taken by earth to complete one rotation about its axis with respect to a distant star.

1 Solar year = 366.25 Sedrial day = 365.25 average solar day

Thus 1 Sedrial day is less than 1 solar day.