FORM OF THE EARTH—MOTION OF THE GLOBE—RATE AND MANNER OF PROGRESSION—LATITUDE AND LONGITUDE—THE SEASONS.

We have learnt from our books on Geography that the earth is shaped like an orange,—that is, our globe is round and flattened slightly at the “poles,” and we can easily see that the earth curves away, if we only try the experiment mentioned in a foregoing chapter—viz., how far a person standing (or lying) on the ground can see on a level. Our power of eyesight is not limited to three or four miles, but a man of ordinary height standing on the plain cannot see more than three miles, because the earth is curving away from him.

We know that at the seaside we can see ships gradually appear and disappear. When approaching us the masts and top-sails appear first, then the main-sails, and then the ship itself. A sailor climbing up the mast can see farther than the captain on deck, because he can see over the curve, as it were. When the vessel is at a considerable distance we see her “hull down” as it is termed,—that is, only her sails are visible to us, and at last they disappear also. If we want any other proof that the earth is round we can see when an eclipse takes place that the shadow on the moon is circular. So we may be certain of one fact; the earth is round, it is a globe. So much for the rotundity of the earth.

But the earth appears to us, except in very mountainous districts, as being almost a plane. This is because of its extent; and even from very high mountains we can only see a very small portion of the earth, and so, on a globe sixteen inches in diameter, the highest hills would be only about 1/100 of an inch, like a grain of sand.

The motion of the earth is known to most people, though as everything upon the globe passes with it, and a relative fixity is apparent, this is, of course, not real rest. The earth is moving from west to east at a tremendous rate,—viz., nearly nineteen miles a second! We think a train at sixty miles an hour a fast train; but what should we think of an express going more than 68,000 miles an hour! Yet this is about the rate at which our globe whirls around the sun. Her fastest pace is really 18·5 miles a second; the least about one mile per second less.

That is one motion of the earth; the other is its motion on its axis. If we send a skittle ball rolling we perceive it turns round as it proceeds. So the earth rotates on its axis, N S, in the accompanying diagram; the extremities of the axis are called the poles. The line in the middle is the equator, which is divided into 360 equal parts, each being 69-1/10 miles in length; so there are 180 lines, or rings, drawn upon the globe from N to S, and these are meridians. In England the degrees are calculated from the Greenwich meridian. We can thus obtain the distance of localities east or west, as we may briefly show (fig. 554).

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.

The earth, as we know, proceeds with a tremendous force around the sun, not in a circle, remember, but in an ellipse or oval track, from which it never moves year by year in any appreciable degree. Now what prevents this earth of ours from rushing off by itself into space? Why should not the earth fly away in a direct line? The reason is because the sun holds it back. The force of the sun’s gravitation is just sufficient, or we may say so enormously great, that it suffices to retain our globe and all the other planets in their various orbits at the very same distance, and to counteract the force which launches them through space. Therefore, as we have already noticed when considering the sun, it is to that ruler of the day that we are indebted for everything.

What would happen, then, if the earth were suddenly to increase her velocity, or the sun to contract his mass?—We should be flung into infinite space, and in a short time would be frozen up completely. Our present diurnal course would probably proceed, but all life and existence would cease as we whirled with distant planets through infinity.

Suppose, on the contrary, we were to stop suddenly. We have some of us read in a foregoing part of this volume that heat is the motion of molecules in ether, and that when a body strikes another heat is developed by contact and friction. If the earth were to be stopped suddenly, “an amount of heat would be developed sufficient to raise the temperature of a globe of lead of the same size as the earth 384,000° of the Centigrade thermometer. The greater part, if not the whole of our planet, would be reduced to vapour”, as Professor Tyndall says.

In the diagram (fig. 546, on page 497) we shall at once find the explanation of the constantly-recurring seasons, and the amount of our globe which is illuminated by the sun at various times. It will be easily understood that the poles have six months day and six months night. When the earth is at an equinox, one half of the surface is illuminated and the other half in shade, therefore the days and nights are equal. But when the north pole turns more and more towards the sun, the south pole is turning away from it in the same ratio,—the days and nights respectively are getting longer and longer, and at the north and south poles day and night are continuous, for the small spaces round the poles are, during a certain period, wholly in sunshine and shade respectively.

In March (in the diagram, fig. 546) we see that exactly one half of the earth is illuminated, and the other is darkened. So in September, when we have the opposite view. In June the earth is more inclined apparently to the sun, and more of the surface is exposed to it, so the days are longer in some parts. The opposite effect is visible in December.

The summer heat and winter cold are accounted for by the more or less direct force of the sun’s rays, for the more the angle of incidence is inclined the fewer rays reach the object; and if the rays fall at an angle of 60°, the heat is only half what it would be if they came vertically. When the days are shortest the sun is lowest, and therefore gives less heat to the earth at certain periods.

The wonderful precision which has adapted the position of the earth on its axis, will be apparent from the illustration (fig. 557). Here we have a table and some bottles, a candle to represent the sun, and a ball of worsted and a knitting-needle to represent the earth and its axis. Suppose we place the ball in the position at a, with its axis perpendicular to the plane of the orbit. As the earth would turn and go round the sun in this supposed case, we should find the days and nights equal, and the sun would quickly scorch up the tropics, and the other portions would have a never-changing spring or winter all the year for ever. This would not be so pleasant, for variety is the charm of nature, and the salt of life. So we may put a aside, as the earth would be scarcely habitable under the supposed conditions, and try b. Here we find the poles directed to the sun. The whole northern hemisphere would thus be illuminated one half year, and the southern similarly; such rapid changes from heat to cold and back again would not suit us. So we fall back upon c, the actual appearance of the position of the earth, and here we find all the most favourable circumstances existing for us. This inclination gives rise to all the varied phenomena of the pleasant gradations of heat and cold, summer and winter, the charming changes of season, and the wonderful results of the ever-recurring days and nights, months and years, as the earth spins round. So we see that the sun does not really rise and set upon the earth; the globe rotates, and brings us into view of the sun, and as we turn we lose his light.

In the foregoing brief description we have learnt some few facts concerning our earth. We have ascertained that the planet we inhabit is round; we have also seen that the earth moves around the sun and around its own axis, and also that it moves at a tremendous rate; we know that that rate is just counterbalanced by the attraction of gravitation, and the course round the sun gives us varying seasons, day and night. There are many subjects relating to the earth which will be more properly included under Physical Geography. We may here just add the diameter of the earth, and proceed to inquire concerning the moon. The polar diameter of our globe is 7,899 miles; the equatorial diameter 7,925 miles. It is distant from the moon 238,500 miles. We will close this chapter with the letters and characters of the Greek alphabet used in astronomical works to designate the stars.

• ? a = Alpha
• ? = Beta
• G ? = Gamma
• ? d = Delta
• ? e = Eps[=ilon
• ? ? = Zeta
• Ê Ê = Eta
• T ? = Theta
• ? ? = Iota
• ? ? = Kappa
• ? ? = Lambda
• ? = Mu
• ? ? = Nu
• ? ? = Xi
• ? ? = Omicron
• ? p = Pi
• ? ? = Rho
• S ? = Sigma
• ? t = Tau
• ? ? = Upsilon
• F f = Phi
• ? ? = Chi
• ? ? = Psi
• O ? = Omega