This week, I will talk about another fundamental SI unit, the second.

Our most basic understanding of time goes like this:

• The Earth turns completely on its own axis in one day.
• The day is split into 24 hours, each hour into 60 minutes, and each minute into 60 seconds.

This definition of the second is a remnant of antiquity reaching back to the ancient Egyptians.  They were the first to split the daytime into 12 hours and simultaneously invented the water clock beside the Chinese.  The length of an hour would, however, change based on the amount of daylight.

Later, the Greeks split the entire day (dawn to dawn), into 24 equal hours.  This was more reliable because the day is steadier than daylight.  Each hour would subsequently be split into 60 minutes, and each minute into 60 seconds, by mechanical clockmakers in the 14th through 17th centuries.  By 1680, the first clock accurate enough to display seconds was made, and by the 1700’s, John Harrison, an English clockmaker, had developed a clock accurate to 1/5 second per day (Fig 1).

In the 1790’s there was an attempt by the French revolutionaries to enact a decimal calendar.  In accordance with their decimal philosophy (think Metric), they created a second as 1/1,000 minute and 1/1,000,000 day.  This was short lived though.  Moving past this decimal calendar, standard clocks began to be constructed using pendulums (sound familiar).

Pendulum technology was improved upon for many years, resulting in a clock that would only lose or gain one second every year (Fig 2).  The pendulum was replaced by a quartz crystal, which can regulate electrical signals, in 1930.  The quartz crystal clocks had about the same accuracy as the pendulum cloaks, but were less susceptible to disturbance.

In 1960, the CGPM proposed a radical change that would forever alter the second.  They approved a plan to make the second independent of the rotation of the Earth.  Up to this point, the second had been defined as 1/86400 the time it takes the Earth to spin once.  However, the spin of the Earth was too variable to keep up with that era’s precision needs, so the CGPM disconnected the second form the Earth.  Although the actual value of the second did not perceptibly change due to this action, the precision was shored up.  This is also why units like the day, hour, and year are not officially part of the SI.

From here, scientists experimented with various technologies to define the second, from which was invented the atom clock.  Various iterations of this technology through 1993 allowed for an accuracy of 1/6,000,000 seconds per year.  This means these clocks would only gain or lose at most 1/6,000,000 of a second after running for one year.  These atom clocks from 1993 were steadily improved upon until 2014.  The 2014 iteration will only gain or lose at most one second after running for 300,000,000 years.  This type of clock is currently responsible for maintaining official times all over the world.

At this point, I am realizing that my explanation of the second is spanning twice the normal blog length.  In order to avoid boring anyone to death and in order to cover everything, I will pick up here next week with a closer look at atom clocks and the technical definition of the second.

Until then.