PARALLAX METHOD –
– Parallax method has been used for measuring distance of stars which are less than 100 light years away.(parallax is the name given to change in the position of an object with respect to the back ground , when the object is seen from the two different positions .)
The diameter AB of earth’s orbit around the sun is chosen as the base line as shown in the fig. . N is the nearby star whose distance (d) from the earth is to be measured . F is a far off star whose direction is taken practically the same at all position of earth in its orbital motion .
Suppose A is the position of earth in its orbital motion at any time . using an astronomical telescope we measure <FAN = θ1 , between the directions of light from distant star F and nearby star N . As it clear from the figure <FAN = <ANS =θ1 .
After six months earth is at B which is directly opposite to the point A , let < FBN =θ2, between the direction of light from distant star F and nearby star N is measured again <FBN = <BNS = θ2
Clearly , <ANB = <ANS +<BNS = θ1 + θ2 .
As we know , angle = arc/ radius
Therefore , θ1 + θ2 = AB/AN
Or AN = AB / θ1 + θ2 .
Clearly AB= 2AS = 2AU
If θ1 and θ2 is known then AN can be calculated.
( Note – the parallax method is used for measuring distance of nearby stars only. And parallax method is also used for determining distance of moon from earth ).