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Mercury spins once in 59 Earth days and circles the Sun in 88. How long does the Sun take to return to the same spot in its sky?
A176 Earth days
B59 Earth days
C88 Earth days
D243 Earth days
Answer & Solution
Correct answer: A. 176 Earth days
1. A spin period is measured against the distant stars, not against the Sun.
2. A solar day is the time for the Sun to come back to the same place in the sky.
3. Mercury turns three times for every two trips around the Sun.
4. Because the planet moves so far along its orbit while it turns, the Sun lags well behind.
5. The Sun returns to the same spot only after two Mercury years.
6. Two years of 88 Earth days each gives 176 Earth days.
7. The 59-day answer is the spin against the stars, which is a different quantity.
8. The 243-day answer is the spin period of Venus and has nothing to do with Mercury.
_Source: OpenStax Astronomy (CC BY 4.0), section 9.5 Mercury_
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