ABSTRACT
This chapter discusses that some notions on the diurnal rotation and annual revolution of the Earth will be expounded, in order to allow a first 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, will be considered in the chapter as invariable, and with absolutely constant angular velocity. A rotational time scale can be defined by using the Sun, which for everyday life is certainly much more important than the equinox. A word of caution is in order because each particular set of fundamental stars determines a slightly different equinox. In order to avoid carrying too many decimals, and to start the day at midnight, a Modified Julian Date (MJD) has been introduced, having its zero date on 1858 Nov.
