If Earth turned the other way
- Round
- Theory
- Points
- Not given
- Level
- Bronze
- Taught in
- Section 2.1
Adapted from IOAA 2012 (Rio de Janeiro), theory problem 2. We have restated it in our own words and added the length of the year. The solution was last checked on 17 October 2026.
Find the length of the sidereal day on Earth, taking the year as 365.2422 mean solar days of 24 h. How long would the solar and sidereal days be, in our hours, minutes and seconds, if Earth turned in the opposite direction at the same speed?
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By (2.1),
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The sidereal day is the time for one turn relative to the stars, so it depends only on how fast Earth turns. Turning the other way at the same speed, Earth still has a sidereal day of 23 h 56 min 4.09 s.
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The Sun's eastward motion among the stars would now be against the direction of the turning, so the rates add,
which gives
AnswerThe sidereal day is 23 h 56 min 4.09 s. If Earth turned the other way at the same speed, the sidereal day would be unchanged and the solar day would be 23 h 52 min 9.5 s, shorter than the sidereal day instead of longer.
This problem also appears among the olympiad problems of Section 2.1, Sidereal and solar days, which teaches the ideas it uses. Equation and figure numbers refer to the course.