This makes it very convenient for the traveler, for instead of setting his watch at each station he does not need to set it until he has traveled 15 degrees east or west, which is about 700 miles in our northern latitudes, and then he turns it exactly one hour ahead or one hour back, depending on the direction he is going. [Illustration: FIG. 164.—THE EARTH DIVIDED INTO 24 STANDARD MERIDIANS.] There are four standard time meridians running through the United States, as shown in the map in Fig. 165. The one running through New York, Pennsylvania, New Jersey and Delaware is the 75th meridian, meaning of course that it is 75 degrees west of the prime meridian which passes through Greenwich. Time on this meridian and halfway to the meridians on both sides of it is called _Eastern Time_. It is just five hours slower than Greenwich time. [Illustration: FIG. 165.—STANDARD TIME MERIDIANS IN U. S.] The next is the 90th meridian and this one passes through Wisconsin, Illinois, Missouri, Tennessee, Mississippi and Louisiana. Time on and around this meridian is called _Central Time_. The one after this is the 105th meridian and passes through Montana, Wyoming, Colorado, New Mexico and Texas. Time on and around this meridian is called _Mountain Time_. The last of the four meridians, the 120th, passes through the States of Washington, Oregon and California and time on and around this one is called _Pacific Time_. It is three hours slower than Eastern time and 8 hours slower than Greenwich time. Although these meridians are just one hour apart the time is not changed on them nor exactly in the middle of a belt but at some well-known town or city between two of the meridians, as you will see by looking at the map, Fig. 165. Fig. 166 shows the standard time at different cities around the world north of the equator. =Star or Sidereal Time.=—Besides all the different kinds of time described above there is still another and a very important kind of time, and this is obtained by the stars. Just as Sun, or solar time is obtained by noting when the Sun crosses a meridian, so _star_, or _sidereal time_ is obtained by observing when a star crosses a meridian. Now there is a difference between the length of a day when formed by the Sun crossing the meridian twice in succession and when formed by a star crossing the meridian twice in succession. This difference in time, between a solar day and a sidereal day, as they are called, is nearly four minutes. But the point is this: an astronomer can obtain the time from watching a star cross the meridian much more accurately than he can from the Sun, because a star is a mere point of light, and it is easier for him to calculate the mean solar time from the _transit_ of a star than it is for him to go to bed, and besides he would rather do it, too.