Year (yr)
The year, symbol yr, is the time Earth needs to go around the sun—about 365.242 days. People have tracked years for farming and religion for a long time, creating calendars. The Julian and then the Gregorian calendars made the year’s length more exact by adding leap years. Today, the year is used everywhere for history, science, work, and culture. Years are closely linked to how people see seasons and life’s cycles.
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.