Greek time periods, for example the mean synodic month, were usually specified quite precisely because they were "calculated" from carefully selected eclipses separated by hundreds of years—individual "mean" synodic months and similar time periods cannot be measured. Nevertheless, with the development of pendulum clocks keeping "mean time" (as opposed to the "apparent time" displayed by sundials), the second became measurable. The seconds pendulum was proposed as a unit of length as early as 1660 by the Royal Society of London. The duration of a beat or half period (one swing, not back and forth) of a pendulum one metre in length on the Earth's surface is approximately one second.