People from time immemorial have been able to measure distances in space, or, as they say in geometry, measure segments of a straight line between two points. Just as long ago, humanity began to take into account the time, that is, measure the periods of time" between two events. At first, this time keeping was done roughly, then more and more accurately. The higher the culture of our life develops, the more diverse human activity becomes, the more important the exact measurement of time becomes.