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The second is the SI base unit of time, commonly understood and historically defined as 1/86,400 of a day – this factor derived from the division of the day first into 24 hours, then to 60 minutes and finally to 60 seconds each. Another intuitive understanding is that it is about the time between beats of a human heart. Mechanical and electric clocks and watches usually have a face with 60 tickmarks representing seconds and minutes, traversed by a second hand and minute hand. Digital clocks and watches often have a two-digit counter that cycles through seconds. In common parlance, a "clock tick" is a second, though most modern clocks are digital electronic, and do not actually tick. The second is also part of several other units of measurement like velocity, acceleration, and frequency. Though the original definition of the unit was based upon the division of the Earth's rotation cycle, the current definition of the second as agreed upon in the current formal definition of the SI system is instead based upon a much steadier timekeeper – the atomic clock. Because the Earth's rotation is slowing ever so slightly, a leap second is added to clock time every once in a while to keep clocks in sync with Earth's rotation. Multiples of seconds are usually counted in hours and minutes. Fractions of a second are usually counted in tenths or hundredths.