Some Definitions and Units

 Practical Units.

(1) Length :

(i) 1 fermi = 1 fm = 10^–15m

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

(iii) 1 micron = mm = 10^–6m

(iv) 1 astronomical unit = 1 A.U. = 1. 49 * 10¹¹m ≈ 1.5 * 10¹¹m  ≈ 10⁸km

(v) 1 Light year = 1 ly = 9.46 x 10¹⁵m

(vi) 1 Parsec = 1pc = 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 amu = 1.67 x 10^–27kg

(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.