Time

For millions of years, our ancestors paid very close attention to the moon. A full moon made it easier for them to move and hunt at night. At some point, someone noticed that it was always the same number of days from one full moon to the next. At some point, someone noticed that the seasons repeated themselves every twelve full moons.

This is time: recurring, measurable, predictable. 

The longest day of the year, when the shadow thrown by a stick stuck into the ground is the shortest, is always the same number of days away from the shortest day of the year, when the shadow is the longest, and halfway between those days, in the spring and again in the fall, the day will be exactly as long as the night. In a life full of dangerous, terrifying uncertainty, it was discovered that some things repeated themselves after periods of time which never changed. That is: these periods of time changed so slowly that it's only been relatively recently that we've begun to suspect that they change, let alone being able to measure the changes.

Besides showing things about times of the year, that same stick stuck into the ground, if it was kept in exactly the same place and observed very carefully, could be used to divide the day into equal parts of time, even though the days themselves got longer, and then shorter, and then longer again. The more interesting someone found all of this to be, the more they would want the stick never to be moved, or, alternatively, to replace it with something bigger and heavier. Maybe something made of stone. 

Whatever else Stonehenge may have been used for, it, along with some other ancient temples, has an opening through which the sun can shine on the summer solstice, and only then, causing interesting optical effects. Provided that the weather on summer solstice is not too cloudy.

From the moon and the seasons to sundials to water clocks to mechanical clocks and watches to quartz clocks and watches to atomic clocks and ever smaller and more numerous and affordable and convenient devices connected to the best authorities of time and ever more accurate, only to be rendered relative and un-absolute by Einsteins's physics, ...I don't know how to finish that sentence. Days and years have turned out not to exactly match the movements of planets and stars on which they are supposedly based, and rather than adjust our units of time we have found it more convenient to adjust the number of units which measure those movements. Which in a way means that, the more absolutely we define and measure time, the more mysterious becomes the question: what is it we are defining and measuring?