A civil, or legal, day ends at the instant of 24 o'clock, midnight, and the next day commences. The time is continuous, the last instant of a day touching the first instant of the next. This is true for all parts of the earth; but something _in addition_ to this happens at a certain meridian called the "date line." Refer again to Fig. 47 which is drawn with 24 meridians representing hours. As we are taking Greenwich for our time, the meridians are numbered from 0°, on which the observatory of Greenwich stands. When you visit Greenwich you can have the pleasure of putting your foot on "the first meridian," as it is cut plainly across the pavement. Degrees of longitude are numbered east and west, meeting just opposite at 180°, which is the "date line." Our day begins at this line, so far as _dates_ are concerned; but the _local day_ begins everywhere at midnight. Let us start to go around the world from the date line, westward. When we arrive at 90° we are one quarter around and it takes the sun 6 hours longer to reach us. At 0° (Greenwich) we are half around and 12 hours ahead of the sun motion. At 90° west, three quarters, or 18 hours, and when back to 180° we have _added_ to the length of all days of our journey enough to make one day; therefore our date must be one day behind. Try this example to change the wording:--Let us start from an island B, just west of the date line. These islanders have their 24-hour days, commencing at midnight, like all other places. As we move westward our day commences later and later than theirs, as shown above. Suppose we arrive at the eastern edge of the 180° line on Saturday at 12 o'clock, but before we cross it we call over to the islanders,--what day is it? We would get answer, "Sunday;" because all our days have been longer, totalling one day in the circuit of the globe. So if we step over the line at 12 o clock Saturday, presto, it is 12 o'clock Sunday. It looks like throwing out 24 hours, but this is not so, since we have lived exactly the same number of hours and seconds as the islanders. In this supposition we have all the _dates_, however, but have jumped half of Saturday and half of Sunday, which equals one day. In practice this would not have been the method, for if the ship was to call at the island, the captain would have changed date on Friday night and thrown Saturday out, all in one piece, and would have arrived on their Sunday; so his log for that week would have contained only 6 days. It is not necessary to go over the same ground for a circuit of the globe eastward, but if you do so you will find that you _shorten_ your days and on arriving at the date line would have a day too much; so in this case you would _double_ a date and have 8 days in that week. In both cases this is caused by compounding your motion with that of the sun; going with him westward and lengthening your days, or eastward meeting him and shortening them. Figure 47 shows Greenwich noon, we