**Units and Measurements – Notes**

**International system of units**

A unit is an internationally accepted basic reference standard against which the measurements of any physical quantity are compared.

The units of fundamental or base quantities are called base units or fundamental units.

The units of derived quantities, expressed as a combination of base units are called derived units.

A system of units is a generally accepted set of units of measurement, which comprises base and derived units.

**Measurement of length**

To measure large distances, we employ an important method called the parallax method.

Distance between two viewpoints or points of observation is called basis.

The distance of the star from the earth, D=b/⍬ (where ⍬ is in radiance)

Diameter of the star, d=D⍺ (⍺ is in radiance).

For the purpose of measurements, degree can be converted into radiance.

10 = 1.75x 10-2 rad (60’)

1’ = 2.91x 10-4 rad (60”)