If you want to mark the passage of time, you need something to occur with regularity, repeatedly, that you can use as your measure of keeping time. Throughout history, we’ve really had two options for this:
to use an artificial device made by humans, such as a mechanical, chemical, or atomic device,
or to use a natural astronomical device, such as a planet, moon, star or pulsar.

from Forbes – Tech http://ift.tt/1MhSTfx
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