What is the history of the second as a time unit

The second originated as a subdivision of the day in ancient astronomy. Historically:

• Babylonian system: Day divided into 24 hours, each hour into 60 minutes, and each minute into 60 seconds (sexagesimal base).
• 17th century: Mechanical clocks could finally measure individual seconds accurately.
• 20th century: In 1960, the second was defined by the ephemeris time—the Earth’s orbital motion.
• Modern definition (1967, SI unit): 1 second = duration of 9,192,631,770 vibrations of the cesium-133 atom’s radiation transition.

Result: Today, atomic clocks define time with accuracy better than one second in millions of years.