Final answer:
Time is precisely measured by atomic clocks based on cesium atoms' vibrations, offering a precision better than a microsecond per year. These clocks have replaced older methods of timekeeping based on the Earth's rotation, providing a consistent standard for the fundamental unit of time, the second.
Step-by-step explanation:
Time is now precisely measured with primary standard clocks that are based on atomic vibrations. Specifically, modern atomic clocks utilize the vibrations of cesium atoms to keep time with outstanding precision, better than a microsecond per year. The fundamental unit of time, the second, is based on these highly precise atomic clocks. Historically, time was reckoned based on the Earth's rotation and positions of celestial bodies like the Sun and stars, but the solar day can vary as the Earth orbits the Sun. Therefore, atomic clocks have become essential to keep accurate time beyond the irregularities of the solar day.