People who succeed have momentum. The more they succeed, the more they want to succeed, and the more they find a way to succeed. Similarly, when someone is failing, the tendency is to get on a downward spiral that can even become a self-fulfilling prophecy. – **Tony Robbins**

# Parallax Method and Range of Length

**Example 2.2** A man wishes to estimate the distance of a nearby tower from him. He stands at a point A in front of the tower C and spots a very distant object O in line with AC. He then walks perpendicular to AC up to B, a distance of 100 m, and looks at O and C again. Since O is very distant, the direction BO is practically the same as AO; but he finds the line of sight of C shifted from the original line of sight by an angle θ = 40^{0} (θ is known as ‘parallax’) estimate the distance of the tower C from his original position A.

**Example 2.3** The moon is observed from two diametrically opposite points A and B on Earth. The angle θ subtended at the moon by the two directions of observation is 1^{o}54′ . Given the diameter of the Earth to be about 1.276 ×10^{7} m, compute the

distance of the moon from the Earth.

**Example 2.4** The Sun’s angular diameter is measured to be 1920′′. The distance D of the Sun from the Earth is 1.496 × 10^{11} m. What is the diameter of the Sun ?