An infinite number of great circles can be drawn through the poles they will all the perpendicular to the celestial equator, they are similar to the vertical circles in the horizon system, these great circles are called hour circles, because they are related to time, only one hour circle can be drawn through a given point the sun moon and a planet or a star, each body on the celeste sphere has it own hour circle the angular distance of a body from the celestial equator to measure along the hour circle is called declination, the horizon and zenith, similarly, there is an infinite number of points with the same declination.

Centuries of observation have shown that the sun seems to describe a great circle as seen from the moving earth, the word comes from the fact that the sun and the moon must be on this circle for an eclipse to occur, the elliptic is inclined to the celestial equative at an angle of about 23 and half inclination is responsible for seasons, there are two intersections of the northward in march is called the vernal equinox, the other where the sun crosses the equator moving southwards in September in the autumnal equinox, vernal means spring and autumnal means full, equinox means that the day where the sun crosses equator moving southwards in September is the autumnal equinox, vernal means spring point of highest of the ecliptic is 231 and the lowest is 230 the former is called the summer solstice and the latter is called the winter solstice, sun stands still summer begins when the sun reaches the solstice at time 21 and winter begins when the sun reaches the winter solstice about December 21

Spring begins when the sun is at the vernal equinox march 21 and the autumn begins about September 23, when the autumnal equinox is reached, because of large size and mass of the earn its rate of precession is very slow, once around in 25,000 years that can been known since the time of Hipparchus in the second century, equatorial plane of the earn is tipped about to the plane of orbit, north celestial pole which id defined by the directions of the axis, is thus from the pole and ecliptic right ascension of the north pole of the ecliptic is 18 hour and its declamation is 66, processional causes the north celestial pole is describe a small circle in radius about the pole of the period of 25,900 years.

The cone shaped muton traced in space by the axis of rotating body when a force trends to tip the direction of the earth apercus said the wobbling motion of the toy is an example the earth undergoes precession because the gravitation force of the sun tends to pull the plane of the earth equator into elliptic and the moon or even the planets exert similar forces, the greater the processional force the more rapids is the procession, period of the earth is approximately twenty five thousand years precession has no effect on the obliquities of the elliptic or the latitude of the place on the earth as to commonly believed, it does not however change the seasons and cause the north celestial pole to move in a round a north pole of the ecliptic, variations in the processional force do cause slight variations in the obliquity of the elliptic, these are called notations.

Astronomy is the science of heavenly bodies and their nature of motions, much of the Ptolemy's almagest consists of trigonometric tables, diagrams, formulas and proofs of long illustrative example, he predicated accurately the planetary system, Ptolemy's book almagest geography remained standard text books for at least 14 centers, he has greek Egyptian, he divided almagest into 13 parts first almagest defined Ptolemaic systems that is solar system centered upon earth the methodic excellence of almagest caused the supremacy of systems when the sixteenth century, catalogue of the stars as the earliest which has come down to us, includes stars and gives longitudes and latitudes and magnitudes of each Ptolemy the mod of presently called, stereographic projection, every point of the space as presented by its projection depends on its sphere equator form the opposite role, particulars system of projection has remarkable and useful properties, his geographical treatise collared the whole of mathematical geography just as almagest collared mathematical astronauts aglets was called the bile of astronomy his geography is the bible of geography.

## 0 Comments