The second was a historical stepchild of the Babylonian system of dividing the day and night into twelve hours each, and subsequent Hellenistic extractions by Ptolemy and his ilk, using astronomical calculations to create a sexagesimal (based on the number 60) time measurement system. Around 1660, the Royal Society of London recognized the “second” as the time it took for a pendulum one meter in length to complete one ‘beat” or swing. Several 20th century revisions were adopted, then rejected, before the current definition, “the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom” (at a hypothetical 0 degrees Kelvin). The exact duration of a second in the universe remains elusive, because it will be longer or shorter depending on the temperature of the caesium-133 atom.

The kilogram was originally just 1,000 grams, when the gram was defined as the mass of 1 cubic centimeter of pure water at the temperature of melting ice. When this turned out to be a variable measurement depending on variables such as water density and pressure, various redefinitions resulted in the current measurement, being defined as the mass of the “prototype”, that piece of alloy in France with the scratches on it. It may be redefined in the future using atomic constants.

To put it simply, though the definitions have since changed, the basis of these measurements are the size of the earth (meter), the mass of water (kilogram) and the time taken for a one-meter pendulum’s swing (second).

The SI unit of measure for length is the meter (m). It is defined as the lenght of the path traveled by light in a vacuum during a time interval of 1/299,792,458 second.

The SI unit of measure for mass is the kilogram (kg). It is the only unit still defined as a physical artifact, the mass of the artifact cylinder of platinum iridium alloy kept by BIPM at Paris, France.

The SI unit of time is the second (s). It is defined as the duration of 9,192,631,770 cycles of radiation corresponding to the transition between the two hyperfine levels of the graound state of cessium-133 atom.