Time: How We Measure It, and Why It Is a Hot Topic

We measure time by counting a perfectly regular natural process. The SI second is defined by the caesium-133 atom: one second is 9,192,631,770 cycles of the radiation from a specific transition in the atom. Atomic clocks built on this definition lose less than a second over millions of years. Time zones exist because the Earth rotates: local noon (Sun highest) happens at different times in different places, so we divide the world into 24 zones of 15° each, one hour apart, so the clock matches the Sun. Time became a 'hot topic' because Einstein showed it is not absolute: a moving clock runs slow, and a clock in stronger gravity runs slow. That is not philosophy - it is measured, and it must be corrected for in GPS every day.