Explain parallax method to measure distance between the earth and planet.

Explanation

Parallax method is used to determine distance between the earth and planet (s) very far from the earth.
Two position from the earth (observations) are A and B. Planet (s) is observed from A and B.
Distance between A and B is b.
Angle subtended from A and B S is `Delta angle ASB= theta` parallax angle.
As planet is very far from the earth `(b)/(D) lt lt 1` hence. angle `theta` wll be very less.
In this situation AB can be considered as arc of circle with centre S and radius D.
`:. AS=BS=D`
`AB=b`
`angle ASB=theta`
`:.` angle `theta` (in radian) `=(arc)/("radius")=(AB)/(AS)`
`:. AS=(AB)/(theta)=(b)/(theta)`
`:. D=(b)/(theta)`
By using this equation distance between the earth and planet can be determined.