A day is

escribe

o rotate

the word generally used to

the time it takes for the Earth

on its axis. The rotation of the

Earth causes night and day. Daytime occurs

for the side of the Earth facing the Sun, and

night takes place when that side is facing away

from our star, The rotation of the Earth also

causes the stars and other objects in the sky to

appear to rise and set -- this apparent motion

is always in the directior from east to west

across the sky.

Solar and sidereal days

The true rotational period of the Earth is 23hr

56min 4.09sec. This period is called a sidereal

day (from the Latin word s~us meaning star)

and is equal to the period between successive

passages across the meridian of a star. The

meridian is an imaginary line reaching from

the north point on the horizon, through the

overhead point and down to the south point

on the horizon. The meridian is not fixed but

refers to the sky that an observer sees from

anywhere on the Earth s surface. In other

words, ira star is due south (on the meridian)

at midnight on a particular night, it will reach

the same position just under four minutes

sooner the following night.

A solar day (from the Latin sol meaning

Sun) is 24 hours long and is the interval

between successive meridian passages of the

Sun. Another way of saying this is that it is the

time between successive noons. However, the

solar day is a little longer than the sidereal

day. This is because the Earth is orbiting the

Sun, and we see the Sun from a slightly

different position from day to day. In other

words, it is like viewing a tree (the Sun) from

a moving car (the Earth). The tree will appear

to move against the background although it is

actually the car that is moving,

During the course of one orbit around the

Sun (a year), our star will appear to travel

completely round the sky. The apparent

motion of the Sun is in an easterly direction.

As a result, the solar day is slightly longer than

the sidereal day because the Sun has to catch

up each time.