Introduction
Time Scales
One of the most important parts of physics is how the basic measurements that we use are defined. The less concrete a basic measurement is the larger the errors can be when using them. In history, time has been defined in many different ways and with many diverse scales ranging from the second, the sidereal day, and the solar day to the lunar month, the century, and many more.
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In general time is defined as "the duration between two events" (University Physics by Bauer) and that duration is related to a time scale. This brings us to one of the most basic units of measurement: the second. The second has been measured in several ways in the past including the approximate time interval of one heartbeat, as a fraction of one rotation of the earth, and most recently in correlation with a certain number of cesium atom oscillations.
As we explore several of the ways that mankind has kept time and the general physics behind each method I hope you will learn a little about how important the physics of time really are.