The rotating earth is like a spinning gyroscope suspended in space. The rotating earth is subject to the gravitational pulls of the sun and to some a extent the moon, and gravitational pulls of all the remaining bodies of the universe. The earth is subject to precession with its rotation about its axis and revolution about the sun, these are the three major motions of the earth. One good thing that happens in the present era of time is the northern axis of the earth is aligned almost exactly with a star which was given the name Polaris the pole star, by early Greek astronomers. Since the distance of this star is many times greater than the diameter of the earth's orbit about the sun, the orientation of the north pole of earth to Polaris does not vary more than one or two degrees throughout the year.
There are also three minor motions of the earth of which the navigator should know about or at least heard of they are terrestrial poles, rotational and nutation. The positions of the north and south terrestrial poles, like the magnetic poles are not stationary, but they move in a circular path about 100 feet in diameter. A complete cycle of movement is so slow, it is almost immeasurable, and it can be disregarded for most navigational purposes. The speed of the earth is slowing gradually and the effect of this motion is also disregarded for most navigational purposes. Nutation is irregular in the earth's precession caused by moon and to a extent the other bodies of the solar system.
Like the star Polaris the other stars of the universe they are so far from earth that their positions seem to remain constant with one another and to the earth's rotational axis. This fact led to the early misconception among ancient astronomers that the earth was located at the center of a hollow sphere on which the stars were fixed; this sphere seemed to make one complete rotation around the earth every 24 hours. The idea of such a celestial sphere is still useful today in visualizing the celestial coordinate system.
Besides being responsible for their apparently unchanging locations on the celestial sphere, the distances of the stars from earth also cause some to appear brighter than others. Usually but not always those stars nearer the earth will appear brighter than the more distant. The brightness of a celestial body is expressed in terms of magnitude visible stars are divided into six groups according to their brilliance and the order in which they appear at night. The first group of stars to become visible to the naked eye at twilight, called the first magnitude stars, are considered to be 100 times brighter than the faintest stars visible to the naked eye in full night, the sixth magnitude stars.
The two brightest stars in the sky, Sirius and Canopus, are actually more than 100 times brighter than a sixth magnitude body. They are assigned negative magnitudes of -1.6 and -0.9 respectively. In practice, all stars and planets of magnitude 1.5 or greater are referred to as first magnitude bodies. Just as a point of interest, the moon varies in magnitude from -12.6 to - 3.3, and the magnitude of the sun is about -26.7. The planets Venus, Mars, and Mercury also appear at negative magnitudes at certain times of the year.
The patterns in which many of the brighter stars were arranged reminded ancient man of various common terrestrial forms, and it was natural that these patterns or constellations became known by the names of the creatures they seemed to resemble, Ursa Major (the Big Bear) hard to see this one, Scorpio (the Scorpion), and Aries (the Ram). The prominence of the early Greek and Roman mathematicians and astronomers in establishing astronomy as a science is seen by the fact that most of the constellations and most of the individual celestial bodies visible to the naked eye bear names of Greek or Latin.
In celestial navigation, the earth is usually considered to be a perfect sphere suspended motionless at the center of the universe. All heavenly bodies are assumed to be located on a second celestial sphere of infinite radius centered on the center of the earth. This celestial sphere rotates from east to west, with the rotational axis of the sphere concurrent with the axis of the earth; the north pole of the celestial sphere is designated by the abbreviation Pn' while the south pole is Ps' The sphere completes one rotation with respect to the earth every 24 hours.
To an observer located on the surface of the earth, it appears that all celestial bodies on the celestial sphere rise in the east, follow a circular path across the heavens, and set in the west. The circular path of each body across the heavens is called its diurnal circle; if the heavens were photographed at night by a time-lapse camera, each celestial body would subtend a diurnal circle as it rotated across the heavens.
