We can tell this, you may say, by a clock or a watch, but the best of clocks and watches are always a little fast or a little slow, and time today is measured by the fraction of a second. So we get the _apparent time_ from the Sun, change it into _mean time_ and set our clocks and watches by it. There are several ways by which we can obtain Sun time, but all of them are based on the same principle. The way that was used by people who first began to think about these things, and it is also a good way for you to try, is like this: [Illustration: FIG. 162.—DIAGRAM SHOWING HOW TO FIND SOLAR NOON.] First you must have a good _horizon_, that is, you must be able to see the Sun rise and set without any mountains or other things in the way. When you begin your observations for getting solar time note exactly where the Sun rises and where it sets on the horizon. You can easily do this by using hills, houses and trees for marking the places. Now with your eye draw a line between these two points. Notice also where you are standing and let your line of sight meet the imaginary line which joins the places where the Sun rises and sets just as near the middle as you can, and all of which is clearly shown in Fig. 162. Now this line will run due north and south and hence it is a _meridian line_ which you are to use to observe the Sun. When the Sun reaches that point in the sky where it is directly over the middle of the imaginary line, joining the places where it rises and sets, it is exactly noon, _Sun time_. The next day observe the Sun in the same way and when it crosses the meridian line again the Earth will have turned round once and you will have a part of time measured off called a Sun or _solar day_. When the time is taken between two succeeding crossings of a meridian by the Sun a day so measured is called by astronomers an _apparent solar day_, and when the Sun is on the meridian it is called _apparent noon_. Now the word _apparent_ means to seem, that is, something which is obtained by observation. An apparent solar day is then the length of a day measured by the Sun, and while we might suppose that the Sun at least would always give a day of the same length, this is not the case for the reason that the Earth does not travel in all parts of its orbit round the Sun at the same rate of speed, and, further, it is tilted on its axis; together these things make the days as measured off by the Sun unequal and hence they are called apparent solar days. =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.