A Retrospective of Time

Time is the 4th dimension (after the 3 spacial ones) and is inseperable from them (space-time). It flows in only one direction (although theoretically, a black hole with naked singularity would allow backward time flow). What's interesting is it can only be measured by indirectly observing the universes' mechanics. This typically means the use of the sun, moon, Earth, and stars which notably carries inherent innaccuracies. Passing time can be measured much more accuratly thanks to physics such as a water clock, sand in an hour glass, the rotation and gravity of the Earth for pendulum clocks, and Swiss time pieces utilizing stored energy in conjunction with springs and gears. However all these techniques require regular maintenance to avoid innaccuracy (winding, refilling, and replacing), not to mention relying on a previously agreed time. More recently, "atomic" time has revelutionized the time keeping indestry by being both accurate and self-sufficient. The vibration of molecules is what makes this possible.