Fig. 553.—Evidence of the spherical form of the earth.
Fig. 554.—Latitude and Longitude.
The distance of any meridian from the first meridian is termed the longitude, and it is employed in describing the situation of a place on the earth’s surface. Suppose L (fig. 554) a city, its longitude will be 30°, since it lies on a meridian which is 30° from the first. So, for example, the longitude of Oporto is 8° 37´ west, Paris 2° 22´ east, Vienna 16° 16´ east, Bagdad 44° 45´ east, reckoned from the meridian of Greenwich, and so on. At the 180th degree we have proceeded half round the globe, and reached the farthest distance from the first meridian, and are now on the opposite side of the earth, and proceeding in a similar manner in the opposite direction we get west longitude.
It will readily be perceived that a knowledge of the longitude alone is not sufficient to determine the situation of a place on the earth’s surface. When we say, for example, that the longitude of a place is 30°, it may lie on any point whatever of the line, N L S, on the whole hemisphere (fig. 554). This point must therefore be determined more accurately, and hence the first meridian is divided into 90 equal parts north and south of the equator towards the poles. These are called degrees of latitude, and the lines drawn through these round the globe, parallel to the equator, are called circles or parallels of latitude, and diminish as they approach the poles.
Hence, by the latitude of a place we mean its distance from the equator towards the poles, and we speak of north and south latitude according as the place is situated in the northern or southern hemisphere.
So, for example, the point L (fig. 554), which has 30° longitude and 60° N. latitude is in Sweden.
The latitude is also observable by ascertaining the altitude of the polar star above the horizon when in the northern hemisphere. The longitude is found by the chronometer; for if we know the time at Greenwich we can calculate how far we are east or west of it by seeing whether the local time be an hour (say) earlier or later, and that difference shows we are 15° to the east or the west as the case may be.
The earth’s rotation, according to sidereal time, is less than solar time, as we have seen, so we have 365 solar days and 366 sidereal days; so a person going round the world gains or loses a day as he travels east or west according to his reckoning, as compared with the reckoning of his friends at home. We can easily ascertain the earth’s motion by watching the stars rise and set. Now the path in which the earth moves is called an ellipse,—very nearly a circle,—but it does not always move at the same rate exactly. We will now look at the relations of the sun and the earth.
Let us take an example. Suppose we have a rod, at each end of which we fix a ball (see diagram), and let one ball be three times as large as the other, the common centre of gravity will be at c, at one quarter of the distance between the centres, and there the bodies will be in equilibrium. If these masses be set spinning into space they will revolve at that distance from each other, the attraction of gravitation and the force in opposition to it equalizing each other.