The motion across the sky of the Sun, Moon, planets, and stars stems primarily 
from three repeating motions:

(1) The rotation of Earth around its north-south axis.  One such rotation takes 
about 24 hours, and is the basis for what we call a DAY.

(2) The movement of the Moon around Earth.  One such revolution takes about 27.3 
days, and is the basis for what we call a MONTH.

(3) The movement of Earth around the Sun.  One such revolution takes about 365.25 
days.  This is the basis for the what we call a YEAR.

Since all these motions are going on simultaneously, along with many other more 
minor motions such as Earth's precession and nutation, the result is a panoply of 
things moving across the sky at differing rates.  The modern definitions of days, 
months and years - and their subdivisions, such as hours, minutes and seconds - 
take all of these motions into account as accurately as possible.  When computing 
the locations and appearances of other objects in the universe, their own 
individual motion must be added to our own motions too.