Book 8, Question 72
Why Do Planets Sometimes Look Like They Go Backward?
Planets sometimes look like they go backward because Earth is passing them as both circle the Sun.
Planets sometimes look like they go backward because of a trick of perspective! When Earth catches up to and passes an outer planet, that planet seems to loop back against the stars for a while. Scientists call it retrograde motion.
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Quick answers
How often does Earth pass Mars?
About every two years, Earth passes Mars as both planets circle the Sun.
What is this called?
This backward drift is called retrograde motion.
Source: NASA
More fun facts
- Most of the time, Mars drifts slowly in one direction against the faraway stars.
- About every two years, Earth passes Mars as both planets circle the Sun.
- While Earth is passing, Mars seems to move backward in the sky.
- This backward drift is called retrograde motion.
- Mars can trace out a loop in the sky during this time.
- The backward motion is greatest right when Earth passes Mars.
- Other planets in our solar system can show retrograde motion too.
Sources: NASA
Questions kids ask
Does Mars really turn around?
No. Mars keeps going the same way around the Sun. It only looks backward because Earth, moving on an inner track, is passing it.