has its origins in the Roman calendar, which evolved into the Julian calendar, and then the Gregorian. Scientific The Jiffy is the amount of time light takes to travel one fermi (about the size of a nucleon) in a vacuum. The Planck time is the time light takes to travel one Planck length. The TU (for Time Unit) is a unit of time defined as 1024 µs for use in engineering. The Svedberg is a time unit used for sedimentation rates (usually of proteins). It is defined as 10−13 seconds (100 fs). The galactic year, based on the rotation of the galaxy and usually measured in million years. The geological time scale relates stratigraphy to time. The deep time of Earth's past is divided into units according to events that took place in each period. For example, the boundary between the Cretaceous period and the Paleogene period is defined by the Cretaceous–Paleogene extinction event. The largest unit is the supereon, composed of eons. Eons are divided into eras, which are in turn divided into periods, epochs and ages. It is not a true mathematical unit, as all ages, epochs, periods, eras, or eons don't have the same length; instead, their length is determined by the geological and historical events that define them individually. Note: The light-year is not a unit of time, but a unit of length of about 9.5 petametres (9 454 254 955 488 kilometers). List {| class="wikitable mw-collapsible" |+ !Name !Length !Notes |- | Planck time || || The amount of time light takes to travel one Planck length. |- | yoctosecond || || One septillionth of a second. |- | jiffy (physics) || || The amount of time light takes to travel one fermi (about the size of a nucleon) in a vacuum. |- | zeptosecond || || One sextillionth of a second. Time measurement scale of the NIST strontium atomic clock. Smallest fragment of time currently measurable is 247 zeptoseconds. |- | attosecond || || One quintillionth of a second. |- | femtosecond || || One quadrillionth of a second. Pulse time on fastest lasers. |- | Svedberg || || Time unit used for sedimentation rates (usually of proteins). |- | picosecond || || One trillionth of a second. |- | nanosecond || || One billionth of a second. Time for molecules to fluoresce. |- | shake || || 10 nanoseconds, also a casual term