# Some Units and Definitions

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.

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