Time is a dimension in which events can be ordered from the past through the present into the future, and also the measure of durations of events and the intervals between them. In physics, time is considered one of the fundamental dimensions (along with space and mass), on which all physical laws are based. Time has been a major subject of philosophical speculation throughout history.
Most modern day clocks are based on either an atomic clock or an astronomical clock. An atomic clock uses an atom as a oscillator to keep time, while an astronomical clock uses planetary motion to keep time. The second was defined in terms of radiation emitted by Cesium-133 atoms in 1957. This definition made the SI unit of time become the atomic second. Prior to this, time was measured using mechanical devices such as pendulums and gears which were prone to error over long periods of time. The invention of quartz crystal clocks in 1927 led to more accurate timekeeping for short periods of time, but these were not suitable for keeping long-term accurate time due to drift in the frequency of the quartz crystal used. Atomic clocks use a variety o