Heavens seem to be very complex because of two
reasons: We are observing the sky from a moving platform, every
celestial object has its own specific motion.
RHYTHMS OF THE COSMOS
All daily cycles of the celestial objects are caused by the rotation
of the Earth. The Earth spins once around its axis about 24 hours
and for that reason we continuously see a changing view of the sky.
The Earth is our reference frame in observing the universe.
To measure the Earths rotation with respect to stars, we
refer to celestial meridians, imaginary north-south lines
passing from observers position. As the Earth rotates, each
star crosses the meridian once in every sidereal day. Some stars
whose number and location depending on the position of the observer
on the Earth neither rise nor set everyday. These are called circumpolar
stars. Circumpolar stars appear to circle around a point called
north celestial pole. There is a similar south celestial
pole in southern celestial hemisphere. The mid-plane perpendicular
the north-south axis between the poles's celestial equator.
||Motions of the Earth and the Moon
Earths rotation forms the basis for our timekeeping system.
The length of the day is a natural unit of time on which all the
living beings have adapted. Subdivisions of the day such as hours,
minutes and seconds are based on the numbering system developed
thousands of years ago.
There are several definitions of time:
Sidereal day: Time interval between two successive meridian
crossings (or upper transits) of any star (or vernal equinox).
Sidereal time at any instant equals the right ascension
of the star that is at upper transit at that instant.
Solar day: Time interval between two successive
meridian crossings (or upper transits) of the Sun.
||Apparent solar time: Apparent Sun is the Sun we
see. The hour angle of its center plus 12 hours is the apparent
solar time. It is variable. Thus, we define mean solar
Mean solar time is the apparent solar time averaged
over a year.
time: Since difference in local time equals difference in longitude,
local mean solar time is later at places east of us and earlier
at places west of us. The inconvenience of continually resetting
our watches as we travel east or west is avoided by the use of zone
time. Standard meridians are marked on the Earth at intervals of
15Ÿ or one hour east and west of the meridian of Greenwich. The
local mean solar time of each standard meridian is the time to be
kept by the timepieces in the entire zone within 7Ÿ30' east and
west of that meridian. Thus the Earth is divided into 24 zones in
which the times differ by whole hours from universal time.
The Sun and the Moon have their
own motions around the Sun in the same direction of the Earths
rotation. For that reason, they shift to east with respect to stars
a little each day. Thus, the Sun rises about 4 minutes later each
day as compared to the stars and the Moon rises almost an hour later
contrast between solar and sidereal days
A solar day
is always longer than a sidereal day.
The length of the solar day is variable, so
the average value of it is known as a mean solar day. A
mean solar day is 3 minutes 56 seconds longer than a sidereal