Q.10) 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 ?
A) $124 \text { earth days }$
B) $88 \text { earth days }$
C) $225 \text { earth days }$
D) $172 \text { earth days }$
Solution:
From Kepler's 3rd law, $T^2 \propto R^3$.
Let $T_M=687$ days (Mars), $R_M=4 R$, and $T_{M e}=$ ?
Using the ratio:
$$
\left(\frac{T_M}{T_{M c}}\right)^2=\left(\frac{R_M}{R_{M c}}\right)^3=4^3=64
$$
So,
$$
\frac{T_M}{T_{M e}}=8 \Rightarrow T_{M e}=\frac{T_M}{8}=\frac{687}{8}=85.875 \approx 88 \text { days }
$$
Hence, the answer is option (2) 88 Earth days.