| During the daylight hours there is generally only one celestial object visibile, the sun. To determine longitude during the day, two observations of the sun are made with a known period of time between observations to develop two zenith circles about two different positions of the sun. Here it is easiest to position the celestial globe for the earliest | time reading and to plot the second position of the sun on the celestialglobe as an object at the same declination as the sun's first position but moved westward by an amount determined by the time interval between sightings. The observer's position is then determined in the same way as it is done with two stars. (See Figure 33). |
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PRECESSION OF THE EQUINOXES The positions of stars with respect to the earth change slowly in a regular way. This change is associated with the change in the direction of the axis of rotation of the earth. The North pole now points approximately towards the star Polaris. About 4,000 years from now, the polar axis will be pointing near the direction of Vega, a star that now passes overhead in San Francisco. This change in direction of the axis of the earth is similar to the motion of a spinning top as it precesses about the vertical. In the case of the earth, the vertical is a direction perpendicular to the orbit of the earth about the sun during the year. This direction is called the pole of the ecliptic, and the polar axis of the earth moves about this direction once in about 25,800 years. See Figure 34. Presently, the earth's axis maintains an angle of approximately 23 1/2° with respect to the pole of the ecliptic, however, during the 25,800 year precessional cycle, the angle of the polar axis to the pole of the ecliptic decreases approximately 3°. Thus, the path of the polar axis shown on the Uniglobe represents a spiral. |
The change in the direction of the polar axis means that the direction in space of the sun as seen from the earth when it is passing over the equator on its way north (i.e., the direction of the vernal equinox) is changing also. The vernal equinox, which lines on the ecliptic, is moving slowly alobng the ecliptic in a westerly direction at a rate of 30° in about 2,150 years. Of course, the solstices and the autumnal equinox also move in the same manner. The sun, in its apparent path through the sky, will appear to pass through or near the same constellations as it now does. A natural division of the precessional cycle is into 12 equal parts of about 2,150 years each: each part is called an age. The 12 parts are then given the names of the 12 constellations of the zodiac. To specify a certain age, one is making a statement about the position of the vernal equinox on the ecliptic. The age of the Bull and the age of the Ram have significance to historians of cultures 2,000 to 6,000 years old. The position of the vernal equinox today is moving from the constellation Pisces toward the constellation Aquarius and this leads to the expression heard today that we are entering the age of Aquarius. |
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