ABSTRACT
This chapter expounds first notions on the diurnal rotation and annual revolution of the Earth, in order to allow an initial understanding of the several definitions of time used in astronomy. The Earth’s diurnal rotation takes place around a polar axis whose direction, with respect to the distant stars, are considered in this chapter as invariable and with constant angular velocity. A rotational time scale can be defined by using the Sun, which for the everyday life is certainly much more important than the equinox. The civil calendars adopted in many countries find their roots on the length of the tropical year, because the seasons follow the course of the Sun along the ecliptic. However, this length cannot be expressed by an integer number of days, not even by a rational fraction.
