Parallax Method:

• Parallax Method:

This method is used to determine distance of stars which are less than 100 light years
away .
Let we have to find out the distance (d) of a nearby star N. let A and B are the two
positions of earth revolving around the sun. The position B is diametrically opposite to
position A.
Let F is any star at very large distance from the earth such that its direction and position
w.r.t. earth remains constant.
Now ∠FAN = ∠ANS = θ1 ( Alternate angles)
& ∠FBN = ∠BNS = θ2 ( Alternate angles)
Therefore,∠ANB = ∠ANS + ∠BNS
= θ1 + θ2
As, )*567 %     
89:
;    θ1 + θ2 = AB/AN

<
Hence by knowing θ1 and θ2 we can calculate AN = BN =d.
Size of Moon or Planet:
Let r = distance of planet or moon from the earth.
θ = angle subtended by diameter AB of planet or moon at any point on the earth.

As, angle =     
89:
= θ = #

= θ = >

or d = θ r
Hence, the diameter ‘d’ can be calculated by measuring ‘θ ’ and using ‘r’
Numerical: Find the value of following in radians.(i) 1O (ii) 1’ (iii) 1’’
(i) 1o =

radian = 1.745 10-2 radian
(ii) 1’ = 8 ?

= 1.745 10-2/60 radian @ 2.91 10-4 radian
(iii) 1’’ = A9B:

= 2.91 10-4 /60 radian @ 4.85 10-6 radian