Second (s)
The second is the fundamental SI unit used to measure time. At first, it was defined as 1/86,400 of the average length of a solar day. However, because the Earth’s rotation is not perfectly consistent, this method was imprecise. In 1967, scientists redefined the second based on atomic properties: one second equals the duration of 9,192,631,770 cycles of radiation from a cesium-133 atom. This atomic standard ensures remarkable accuracy in timekeeping, which is essential for technologies like GPS, communications, and scientific experiments. The second remains a cornerstone for tracking and organizing time in today’s world.
Day (d)
The day, symbol d, is the time it takes Earth to rotate once. A day is 24 hours or 86,400 seconds. Because Earth’s spin changes a little, days can vary, so leap seconds are sometimes added to atomic clocks. The day is a key part of human life, shaping calendars, routines, and events in all cultures. It is one of the most fundamental and widely recognized time units.