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I was trying to figure out how people came to know about time then I realized that people started keeping track of time to know about sunset and sunrise. But I can't figure out how did time came into the physics world. Other physical quantities can be somewhat directly observed by our 5 senses but what caused time to be considered as physical quantity?

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  • $\begingroup$ I think the time which any event might take was measured relatively with "some other event" at first. Though time may not have been defined at those stages. I don't have any reference for my statement but this seems quite probable to me. So for explaining any time consuming event the standard of measurement(some other event) might have been considered. Therefore maybe at that time "time" may have been defined indirectly. $\endgroup$ – Jasser Jan 7 '15 at 7:33
  • $\begingroup$ The notion of time is discussed in considerable detail by Aristotle in book 4 of his "Physics". Perhaps you would like to have a look. $\endgroup$ – fdb Jan 7 '15 at 9:46
  • $\begingroup$ Also this: belate.wordpress.com/2011/02/17/… $\endgroup$ – fdb Jan 7 '15 at 9:47
  • $\begingroup$ The notion and perception of time is much older than physics, and it has to be present in any physics theory. So the question is equivalent to the question "when physics started", and this on my point of view has no exact meaning. $\endgroup$ – Alexandre Eremenko Jan 7 '15 at 11:07
  • $\begingroup$ I think this is almost as asking when did we become aware of the passing of time. Too philosophical. $\endgroup$ – hjhjhj57 Jan 10 '15 at 7:40
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I was trying to figure out how people came to know about time then I realized that people started keeping track of time to know about sunset and sunrise.

People first kept track of time to know about the seasons, not sunrise and sunset. Knowing when to plant and when to harvest was a critical step in humanity's neolithic advancement. This knowledge was quite lucrative, and it most likely precedes writing. Understanding of how the constellations change over the course of a year and of the cyclical nature of the phases the Moon was much more important in this regard than was understanding when sunrise and sunset occur.

Knowledge of time of day only became lucrative after one of the worst inventions ever developed by mankind, those awful things we now call "meetings." Even then, the notion of accurate time of day is a fairly recent invention, circa the 16th or 17th century.

There are two distinct notions of time of day, one based on the position of the Sun and the other based on the position of the stars. Sundials "ticked" with the Earth's rotation with respect to the Sun. Modern clocks tick with the Earth's rotation with respect to the stars, but with the tick rate adjusted so that, on average, there are 86,400 seconds from noon to noon. (A mean sidereal day is 236 seconds shorter than a mean solar day.)

As with many other things in science, our understanding of time has changed over the course of centuries. Galileo, Kepler, and Newton were critical in our classical (non-relativistic) understanding of time. While the equation of time was known to the ancients, Kepler was one of the first to argue that time as measured by the stars (or by a mechanical clock) was better than time as measured by the Sun (or by a sundial). Galileo performed experiments in which time was the independent variable of motion. Newton cemented these ideas.

Relativity turned these very solid notions upside down. Contrary to what Newton thought, time is not the absolute, universal independent variable of motion.

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