In 1956 the modern second was redefined in terms of a year. The reason for this is Earth’s daily rotation is not constant. It speeds up and slows down. The Earth wobbles thru space. Therefore a more desirable definition is useful. Consequently, the period of the Earth’s revolution around the Sun is more advantageous. We can calculate a uniform standard by the observed positions of the celestial bodies along with Newtonian dynamical theories of their motion.
For that reason the 1960 SI scientific second definition appears. Using the above revealed method we define a second as the fraction 1/31,556,925.9747 of a tropical year for 1900 at 12 hours ephemeris time. Noting that how people think a second as a fraction of a day is no longer valid. Simply the scientific term for the second is not a fraction of a day’s rotation but a calculated fraction of a year’s revolution.
Do not freak out! The second did not change much, think of as the accuracy of the second became better. Instead of 1.001 (s) we have 1.00000001 (s). Your clocks still measure time accurate enough so you will not be late for work.
The scientific definition of the second take another turn in the 1960’s. It was decided that the second is to be based on caesium microwave atomic clock, rather than the periodic revolution of the Earth around the Sun. After many years of analysis in 1967 the definition became as follows:
The standard international (SI) definition of a second is 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. Later in 1997 the definition refers to caesium at rest at a temperature of 0 K.
Basically, the definition of the second become a fancy description for an atomic clock. The atomic clock is a technological advancement that eases the masking that the second is still tied to the revolution of the planet Earth. The reason for the improvement is it easier to duplicate and calibrate very sensitive equipment. It is important to note that the second does not change in duration but each advancement in technology defines the second in relative accuracy. We are now looking at 1.0000000001 (s).
During the 1970s space program it was realized that gravitational time dilation caused the second produced by each atomic clock to differ depending on its altitude. This is Einstein stuff. The uniform second define in relativistic term as proper time was produced by correcting the output of each atomic clock to mean sea level. The result is the lengthening of second by about 1×10−10 for things moving in space.
Notice the Earth bound relationship at mean sea level. In order to improve our scientific knowledge we develop understanding that centers around Earth then project that understanding outward. Back to what brought us here Earth Knowledge verses Ancient Knowledge. Everything we know about the modern second is founded from the relative position of Earth. There is nothing ancient or universal about the modern definition of the second.