Question

(a) The earth-moon distance is about 60 earth radius. What will be the diameter of the earth (approximately in degrees) as seen from the moon?

(b) Moon is seen to be of ()°diameter from the earth. What must be the relative size compared to the earth?

(c) From parallax measurement, the sun is found to be at a distance of about 400 times the earth-moon distance. Estimate the ratio of sun-earth diameters.

Answer 1

Diameter of the earth as seen from the moon is about 2°.

(b) At earth-moon distance, moon is seen as (1/2)° diameter and earth is seen as 2° diameter. Hence, diameter of earth is 4 times the diameter of moon.

Dearth/Dmoon = 4

(c) Rsun/rmoon = 400

(Here r stands for distance, and D for diameter.)

Sun and moon both appear to be of the same angular diameter as seen from the earth.

(Dsun/rsun)/(Dmoon/rmoon)

(Dsun/Dmoon) = 400

But Dearth/Dmoon = 4

Dsun/Dearth = 100.