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.