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RELATIVE ORIENTATION OF EARTH AND CELESTIAL GLOBES AND THE MEASUREMENT OF TIME The earth's rotation upon its polar axis can be represented by holding the celestial globe firmly on its axis and rotating the earth globe, by means of the earth rotating knob, counter-clockwise as viewed from the North pole. The same effect can be achieved by holding the earth globe stationary and rotating the celestial globe clockwise. As the earth rotates, a reference longitude on the earth will cross a celestial reference meridian. The period between two such crossing is called a DAY. When the celestial meridian is the prime celestial meridian, the period is a SIDERIAL DAY. When the celestial meridian |
is that of the sun, the period is an APPARENT SUN DAY. A measurement of the angle between the terrestial reference longitude and the celestial reference meridian represents a measurement of the time of day. The time ring on the Uniglobe is divided into 360 degrees on its upper scale, with longer graduations every 10 degrees. The lower scale is divided into 24 one-hour segments (each of which contains 15 degrees), and graduations at the half hour and 15 minutes points. There are several time systems which are used to measure the passage of time, depending on what specific earth and celestial reference lines are used. Figure 19 shows the various reference lines and the terminology used to define the different time systems. |
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SIDEREAL TIME Siderial time corresponds to placing the time ring with 1200 at the autumnal equinox. This position corresponds to setting the zero mark at the vernal equinox. Local sidereal time (LST) can then be read for any longitude at its intersection with the time ring. Greenwich Sidereal Time (GST) is read at the intersection of the Greenwich meridian with the time ring. Figure 20 shows the setting of the time ring for sidereal time, and the reading of GST as 0400 hours. QUESTIONS 1. When GST is 0400 what is LST at Cairo, Egypt? Answer: (0604). 2. When GST is 0400 what is LST at San Francisco? Answer: (1945), or 7:45 P.M. |
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