### Our Earth is a Clock

The yardstick used to measure time is the period it takes the earth to make one complete rotation. This period is very uniform...it will not vary by as much as a fraction of a second in your lifetime. To mark one rotation requires some kind of index mark. When the index mark is the sun, the resulting time is SOLAR TIME (or sun time); when the index is a star, the time is SIDEREAL TIME (or star time).

SOLAR TIME
Our common day, like that of prehistoric man, is still timed to the sun because its presence lightens and warms our days, and its absence darkens and cools our nights. To tell time we consider the sun to be like the hand of a huge earth clock as it seems to go around us once each 24 hours. Of course, we know that it is the earth that goes around the sun.

The solar day begins when the sun is at the lower transit, Figure 1. There are 24 hours in one solar day which, in everyday life, we divide for convenience into two sets of 12 hours each, a.m. for the morning hours and p.m. for the afternoon and evening hours.

Time in astronomy is straight 24-hour time, starting at the zero hour, midnight. The actual keeping of time is based on the hour angle of the sun from the observer's meridian.

Hour angle is always measured to the west, and examples of how this works are shown in Figure 1. The "plus 12 hours" is needed to have the day start at midnight.

It is definitely not easy to visualize the rotation of the earth. Hence, astronomy texts adopt the system of showing things as they appear to be rather than what they are. This makes a diagram like Figure 2. It represents an observer standing on a fairly flat piece of non-moving ground, while the sun, moon and stars turn around him. Although an entirely different concept than the true situation, the base rules work just as before. The day starts with the sun at the lower transit, and solar time is the hour angle of the sun plus 12 hours.

KINDS OF SOLAR TIME
Your Spilhaus Space Clock will be much more meaningful to you if you have a better understanding of the different kinds of Solar Time.

APPARENT SOLAR TIME
This is true sun time, governed by the passage of the sun across the sky. It is the time which you can read from a sundial. Although true sun time, it is not uniform time because the earth travels in an ellipse around the sun, making some days longer than others. See Figure 3.

MEAN SOLAR TIME
To obtain uniform time, the total time in a solar year is divided into 365 days of equal length. This average type of time is paced by an imaginary sun known as the mean sun. The difference between the real sun and the mean sun ranges from zero to about 15 minutes fast or slow, the difference being calculated by the equation of time. The equation of time is used to convert apparent time to mean time, a time problem seldom encountered by the amateur star gazer.

STANDARD TIME
Both apparent and mean solar time give a different time for every different location, (Figure 5) It was not until 1884 that the people of the world got together and agreed on a world-wide standard of time. This system divides the world into 24 standard time zones, each comprising an area of some 15 degrees of longitude. All of the area in a zone uses the same standard time as determined by the mean solar time of its central meridian, Figure 6. The Zero zone is centered at Greenwich, England. Zones to the west are numbered Plus 1 to Plus 12; the number of the zone added to the standard time of the zone equals Greenwich mean time, as shown at bottom of Figure 7. The United States has four zones, Plus 5, 6, 7 and 8, better known as Eastern, Central, Mountain and Pacific standard time, as shown in Figure 8. Standard time does not vary more than about an hour from mean solar time at any location. At the central meridian of each zone, standard time coincides exactly with mean solar time.

LOCAL MEAN TIME (L.M.T.)