The position of the mean sun can be determined by finding the position of the apparent sun from the ephemeris and adding the correction from the tabular ephemeris.

For example: In the ephemeris on April 10, 1975, the sun's position is 1 h 11 m R.A. On this date the position of the mean sun is -1 m from the position of the sun. With the 2400 mark of the time ring set on the vernal equinox, mark on the celestial globe near the time ring the position of the mean sun 1 h 11 m + (-1 m) = 1 h 10 m R. A. (See Figure 22). This is the position of the mean sun on April 10. Now set the 1200 mark of the time ring on the position of the mean sun. Local Mean Sun Time can then be read directly from the time ring at any particular meridian on the earth globe for any orientation of the two globes. In Figure 23, the two globes are oriented at 1320 local mean sun time (LMT) in Dakar, therefore at that instant it is 0800 (LMT) in Mexico City. GMT is the Local Mean Sun Time on the Greenwich Meridian or 1425 in Figure 23.

EXERCISE

1. Set, for April 10th, the relative orientation of the two globes for 1545 LMT in Los Angeles.

What is GMT? 2340 hrs.

THE EQUATION OF TIME

The position of the mean sun and the apparent sun differ during the year by up to approximately 16 minutes. This difference is called the EQUATION OF TIME. For any particular time of the year,

where:

This difference is given in the Ephemeris for any given date.

ZONE TIME

It is apparent from the previous discussion that times as measured at two particular locations separated by a few degrees of longitude are different.

As a matter of convenience, people have agreed to use certain standard longitude lines to measure time for everyday purposes. Thus, there are 24 standard longitude lines at 15° intervals about the earth. In general, people use the standard longitude line nearest to their location to measure their time. These lines are not always precise and may vary somewhat to follow geographic lines or political divisions. Wee find that generally the territory within + or - 7 1/2° of a standard line will be a time zone, but as in the case of the Arizona-California border which occurs close to standard longitude line of 120° W, the time zone boundary follows the state line.

DAYLIGHT SAVING TIME

It has been the practice, since the introduction of zone times, to alter the terrestrial reference line during the year to take maximum advantage of the longer hours of sunlight in the summertime. The conversion of standard time to daylight time is accomplished by using the next most easterly standard reference line. This results in a change of one hour between standard and daylight saving times. For example: when it is 0400 PST in Los Angeles, (120° W line coinciding with 0400 on the time ring) it is 0500 Pacific Daylight Savings time. (105 W line coinciding with the 0500 mark on the time ring).

ORIENTING THE UNIGLOBE FOR THE CURRENT TIME AND THE OBSERVER'S POSITION

Before orienting the Uniglobe for the current time, consult the ephemeris for the current date to determine the position of the mean sun and plot its position near the time ring on the celestial globe. Refer to Figures 24 and 25 for examples for May 1, 1975 Repeat this proceedure for the sun, placing the sun's position on the ecliptic on the celestial globe. To plot the position of the planets and the moon, determine the right ascension and declination for the current date and plot their positions on the celestial globe.

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