The Book of Stars: Being a Simple Explanation of the Stars and Their Uses to Boy Life

by A. Frederick Collins

Mean Solar Time.—Apparent solar days which are of unequal length were all right as long as sundials were the only timepieces, but when clocks and watches came into use days which were equal in length were needed and needed badly, for a clock couldn’t be made which would keep Sun time.

So astronomers who watched the stars by night lay awake during the day wondering how they could make the Earth travel round the Sun at the same speed every day in the year and just as though it was not tilted. At last they solved the problem.

And how do you think they did it? It was as easy as rolling off a log—when you know how. They simply imagined that the Earth traveled at a uniform speed and that it stood straight, as shown in [Fig. 74]. In other words they took the mean length, which is another way of saying the average length of all the apparent solar days which make up a year, and divided it up equally. Further, the men who got up this scheme said that every day should have not only the same length, but that it should have 24 hours, and of course you know that hours are divided into minutes and minutes into seconds. Mean solar time, then, is really imaginary Sun time and this is the time used everywhere and watches and clocks are set by it.

Equation of Time.—To get the exact mean time each day you have to know just what the apparent solar time is, and then you have to know what the difference in time is between the apparent solar time and the mean time, that is how many minutes and seconds to add to or subtract from the solar time of each day to get the mean time.

This difference of time is called the equation of time. A table prepared by Professor Todd of the Amherst College Observatory is given in Appendix M and can be used for all ordinary purposes.

Standard Time.—A meridian, as you know, is an imaginary line running due north and South and hence we can have a meridian whenever we want it and as many as we like.

When the Sun crosses the meridian of those who are on it, it is noon to them, but to no one else, for the Sun has already crossed the meridians to the east of it and has yet to cross those to the west of it. When the Sun crosses the meridian which passes through New York City it is noon there and when the Sun crosses the meridians which pass through St. Louis, Denver and San Francisco, it is noon at those places, and this is true of every other place and of every other hour of the day. This is the reason why every place had its own, or local time, before the year of 1883.

As an illustration it takes the Sun about three hours to cross the United States from the Atlantic to the Pacific coasts.

Now as long as people traveled on foot, or by horse, they moved so slowly that local time did not worry them, but when railroads came into use there was all kinds of trouble for the traveler; if he was going west his watch was faster than the local time of the towns he passed through and if he was going east his watch was always slower than the local time. If he wanted the right time he had to set his watch at every town he passed through; of course he couldn’t very well do this and he was always in a stew.

The railroad companies were just as much put to for it was next to impossible to make a timetable to fit the local time of each town and still keep up a running schedule; and the result was that the railroads finally got up a system of their own which they called railroad time. This was all well enough for everybody but the poor traveler, who, not knowing the difference in time between local time and railroad time, nearly always found he was either an hour too early or—as it usually happened—a minute or two too late to catch his train.