Units for measuring time

L’unité de temps du Système International est la seconde.

The unit of time in the International System of Units (SI) is the second.
The main units of time in astronomy are the second (defined using the vibrations of the Caesium atom) and the year (defined by the Sun). Time is given in Universal Time (UT), which is the mean time on Earth, at longitude zero.

Unlike the second, which has a very precise definition, for astronomers there are several types of year and day because celestial objects all move in relation to each other. For example, a day is the time the Sun takes to return to the same point in the sky, but astronomers also use a “day” in which stars return to the same position. This “sidereal” (star) day is 4 minutes shorter than the Solar day.
Another difficulty when measuring time is to choose as of what moment the days or years are counted. As there are different calendars on Earth, astronomers have different times according to the origin and the system of reference used. Time measurement is something that is highly complex, as shown in this class.

Another difficulty when measuring time is to choose as of what moment the days or years are counted. As there are different calendars on Earth, astronomers have different times according to the origin and the system of reference used. Time measurement is something that is highly complex.