The radius of Martian orbit around the Sun is about 4 times the radius of the orbit of Mercury. The Martian year is 687 Earth days. Then which of the following is the length of 1 year on Mercury?
- 172 earth days
- 124 earth days
- 88 earth days
- 225 earth days
Answer
(3) 88 earth days
Solution
Kepler's third law: T2 r3
TMarsTMercury = ( rMarsrMercury )3/2 = 43/2 = 8
TMercury = 6878 86 earth days, i.e. nearly 88 earth days (the orbit radius ratio is only "about" 4).
Answer: (3)