Introduction / Context:
Planets rotate at different rates, giving each a characteristic “day” length. Some, like Venus, rotate unusually slowly (and retrograde), which is a favorite fact-question in astronomy sections of competitive exams.
Given Data / Assumptions:
- We compare sidereal rotation periods (one spin relative to distant stars).
- We consider well-known planetary values: Mercury ~58.6 Earth days; Venus ~243 Earth days (retrograde); Earth ~1 day; Pluto ~6.4 days.
- We pick the longest rotation period among the options.
Concept / Approach:
Venus has the slowest rotation of the listed bodies, taking about 243 Earth days to complete one spin, and it spins in the opposite direction (retrograde) compared to most planets. Despite Mercury’s slow rotation relative to Earth, it is still far faster than Venus. Pluto’s rotation (~6.4 days) is also faster than Venus. Therefore, Venus correctly answers the question about the greatest time to complete one rotation.
Step-by-Step Solution:
Recall approximate rotation periods: Venus ~243 d; Mercury ~58.6 d; Pluto ~6.4 d; Earth ~1 d.Identify the maximum value among these: 243 d (Venus).Select “Venus.”
Verification / Alternative check:
Astronomical almanacs list Venus’s retrograde rotation as the slowest; it also gives Venus a solar day (~117 Earth days) due to the interplay of rotation and orbital motion, reinforcing how unusual its spin is.
Why Other Options Are Wrong:
Mercury: Slow, but 58.6 d < 243 d.Earth: 24 hours; much shorter.Pluto: ~6.4 days; still much shorter than Venus.
Common Pitfalls:
Confusing rotation period with orbital period (year). Always compare spin durations, not time to go around the Sun.
Final Answer:
Venus
Discussion & Comments