alleryStart learning →About how long was a day on Earth some 900 million years ago?
AAbout 18 hours long
BAbout 12 hours long
CAbout 24 hours long
DAbout 36 hours long
Answer & Solution
Correct answer: A. About 18 hours long
1. Tidal friction exerts a retarding torque that slowly reduces the spin rate of Earth.
2. A slower spin means a longer day, so days were shorter in the distant past.
3. Recent research puts the day at 18 hours some 900 million years ago.
4. A figure of 36 hours would mean Earth has been speeding up, the opposite of what tides do.
_Source: OpenStax College Physics (CC BY 4.0), Ch 10 "Rotational Motion and Angular Momentum", section 10.5 Angular Momentum and Its Conservation_
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