Space exploration began with the launch of Sputnik and Astrophysics was born as the application of physics to the phenomena observed by Astronomy, which etymologically means laws of the stars.
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| Day |
In astronomy, a unit of time defined as 86,400 seconds, where the second is in turn defined in terms of the frequency utilized in a caesium atomic clock. This definition of the day is closely linked with the Earth’s rotation period, though that rotation is not completely uniform.
See also: solar day, sidereal day.
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