The day as a unit then got split into two, for (roughly) sunrise to sunset versus sunset to sunrise, and each half of a day got split into 12 hours. Since prehistory, a day was a very natural way of measuring time, and was highly repeatable, in that the procedure of measuring a day can be performed anywhere on Earth with essentially the same result. Most physical units have to be defined in terms of something measurable, and a good definition of a physical unit is one in which the measurement of the unit is very precisely repeatable. If relativity holds strictly, then the two ways of defining distance/time are equivalent and we can always chose the definition that is the most precise and reproducible. If we could measure distances with higher precision than we can measure time (we don't and it is unlikely that we will have them in the future), then we would make a new physical definition for one meter and use the constant definition of the speed of light to derive one second as the time light takes to traverse a certain distance. The definitions of meter and second are therefor linked for the future by one constant factor.

Hence we now define the speed of light as a simple numerical constant and the meter as the distance light can traverse in a given time. On the other hand, one of our best tested physical facts is that the speed of light is a constant, so we should treat it as one in the way we define our units. That made the speed of light a measured quantity. In the past we also used to define the meter by a wavelength of red-orange light from an optical line of Krypton 86. We have now better atomic clocks than those one can build with cesium atoms and so it can be expected that the definition will change as soon as the national and international bodies responsible for these definitions decide to act on the availability of better clocks. The currently accepted definition is as '9192631770 cycles of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133'. When we invented atomic clocks we had better ways of defining the basic unit of time. In the past we used to define a second as 1⁄86,400 of a solar day and later as 'the fraction 1/31,556,925.9747 of the tropical year for 1900 January 0 at 12 hours ephemeris time' but both are pretty poor ways of measuring time because the Earth's movement in the solar system is subject to perturbations and changes in the mass distribution of the planet (by winds in the atmosphere and ocean currents and even large earthquakes alter the length of a day, even though the change is small compared to the 'noise' by the former). It's a definition of a unit, which is an arbitrary choice.

Caesium period