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  Motions of the Sky:  Cycles and Seasons
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.


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 Earth’s rotation with respect to stars, we refer to celestial meridians, imaginary north-south lines passing from observer’s 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.

The Moon travels around the Earth and the Earth travels around the Sun
Motions of the Earth and the Moon

Earth’s 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. 

Mean Solar Time Minus Apparent Solar Time
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 time.



 Mean solar time is the apparent solar time averaged over a year.

Zones around the planet orienting from GreenwichZone 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 15 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 730' 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 Earth’s 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 each day.

the Earth straight on from the Sun or Slightly off center

The 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 day.
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