First in a series on the evolution of time measurement: celestial, flow, and mechanical clocks [Recalibration]

Time is elusive. We are comfortable with the concept of time, but in many ways it defies understanding. We cannot see, hear, or touch time; we can only observe its effects. Although we are unable to grasp time with our traditional senses, we can clearly "feel" the passage of time as we watch night turn to day, the seasons change, or a child grow up. We are also aware that we can't stop or reverse the continuous flow of time, a fact that becomes more obvious as we get older. Defining time seems impossible, and attempts to do so by philosophers and scientists fall far short of their goal.