![earth earth](https://slideplayer.com/slide/2811839/10/images/5/Speed+Galileo+is+believed+to+be+the+first+person+to+measure+speed.jpg)
In contrast, the rotating Earth is not an accurate clock. The most stable pulsars lose only a few seconds every million years and are the best-known timekeepers, even better than atomic clocks. Earth is the only planet to date for which we have achieved such accuracy, although we also have high-precision measurements of the rotation rate of pulsars, the spinning neutron stars often seen at the center of supernova explosions.
![earth earth](https://cdn.numerade.com/previews/93a50792-f8a8-40ae-a6a5-0feb16f669d6_large.jpg)
Information can be obtained about the interior of a planet, and how its atmosphere couples with its surface, from precise length-of-day measurements.
![earth earth](https://cdn.numerade.com/previews/e9c59ef3-163b-4fe1-b375-ac58a6252220.gif)
The same is likely to hold for any dynamically active planet. When the length of day is measured with high precision, it is found that Earth's rotation is not constant. The standard second is the Système International (SI) second, which is precisely 9,192,631,770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of the 133Cs atom. For all applications but the most demanding, the time Earth takes to turn once on its axis, the length of its day, is adequately represented by a constant value equal to 24 hours or 1440 minutes or 86,400 seconds. Interestingly, the reverse is also true: The Earth's atmosphere and oceans have a measurable effect on the planet's rotation rate. All of these processes are thoroughly discussed in Sections 2–5. The Earth's rotation has an enormous effect on the motions of its fluid envelope that accounts for the circular patterns of large storms like hurricanes, the formation of western boundary currents like the Gulf Stream, the intensity of jet streams, the extent of the Hadley cell, and the nature of fluid instabilities. Showman, in Encyclopedia of the Solar System (Second Edition), 2007 1.1 Length of Day