Straight Lines on a Rotating Planet
Contributed by Jeffrey J. Early | Friday, Mar 09, 2012

You've probably noticed that the earth is rotating---day and night, etc. It's less obvious that this rotation also affects the way things move. If you roll a ball, it rolls in a straight line, but what does 'straight line' mean when you roll the ball so far that it begins to be affected by both the earth's curvature and the earth's rotation? That is exactly what is shown in these three images. Depending on how you roll the ball, it will either almost come back to you, like a boomerang that pulls westward, or it will slide (eventually) to the equator, or it can even make wiggles that cross the equator and come back. Although this is not a practical experiment, thinking about it helps us understand the fundamental forces at work in the ocean and atmosphere and interpret measurements from oceanographic instruments. The author's recent work showed that in all three cases, contrary to conventional wisdom, the ball 'feels' as if is moving in a straight line---despite the fact that gravity is causing the oscillatory motion. This paradox has to do with the nature of gravity.  As Einstein showed, orbiting the earth feels exactly the same as being far from any planets in distant space: it feels like nothing.

Caption by Open-Ocean Staff and Jeffrey J. Early
This material is based upon work supported by the National Science Foundation under Grant No. 0621134 and 1031002. | Disclaimer | Any opinions, findings and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation. | Hide
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