Page:The New International Encyclopædia 1st ed. v. 02.djvu/205

ASTRONOMY. seasons are the ones most closely affecting our daily life.

The Moon travels around the earth with an orbital motion quite analogous to that of the earth around the sun, only that the moon com- pletes its revolution in about 20% days instead of about 3G.5 days, as in the solar revolution of the earth. It is this orbital motion that causes the moon, from night to night, to appear in a different position among the stars. Indeed, to complete an entire circuit in so short a period, it is necessary for the moon to have a daily motion of about 13° among the stars. The moon also has a revolution about its own axis of figure, analogous to the earth's diurnal revolution already described. But, curiously enough, the time required for one axial revolution of the moon is exactly equal to the period of its orbital revolution around the earth. The consequence is that the moon always turns the same side toward us, and we never have an opportunity of study- ing the details of the opposite side. It is not a self-luminous body like the sun. The surface simply reflects solar light to us. It is easy to understand that though the sun may be below an observer's liorizon, and therefore invisible to him, yet solar light may travel past the earth to the moon, and be thence reflected back to the observer. Thus the moon appears bright, even though the sun is invisible. Indeed, it is only when the sun is below the horizon that the moon has great brilliancy, for when both are visible at the same time, the solar daylight makes the moon quite dim by comparison.

The phases (q.v.) of the moon are perhaps the most conspicuous phenomena connected with that body. It is clear that since the moon owes its luminosity altogether to light received from the sun, only one-half its surface will be illu- minated at any one time. This would appear as a complete circle "of light in a direct view, but if seen from one side will be foreshortened. We thus see at times the half-circle we call the first or third quarter, and at other times the sickle called t!ie new moon. Absolutely no connection whatever exists between weather on the earth and the changes of the moon's phases. The ap- pearance of the moon's surface in the telescope is much like the earth's would he, if seen by an equally distant observer. The most conspicuous features seem to be volcanic mountain ranges with peculiarly large craters. Often great sur- face-cracks seem to radiate from these craters in all directions.

The moon is the principal cause of the tides (q.v.) that are observed every day along the ocean borders of the earth. According to the law of gravitation, the luoon attracts both the water and the solid earth. But as the force of attraction diminishes rapidly with distance, of course the moon pulls the water on the side of the eartli fiicing it more strongly than it does the solid earth underneath. Still less strongly does it pull the water on the opposite side of the earth. The result is that the moon tends to heap up the water on the side of the earth nearest to it by pulling the water as it were away from the earth. But it also, though in a lesser degree, heaps up water on the opposite side of the earth, in a measure pulling the earth away from the water. Then, as the earth revolves on its axis every day, these two great tidal waves follow the moon, ever tending to be highest at the point of the earth directly facing the moon, and again at the point diametrically opposite. Thus every day there are two high tides, and two correspond- ing low tides. The attraction of the sun has a similar tidal effect, though in a far smaller de- gree, on account of the much greater distance of the earth from the sun. It sometimes happens that the tidal influences of the sun and moon work together. At such times we get our greatest tides. When, on the other hand, these two tide- making bodies are so placed as to work against each other, we get our minimum tides on the earth.

Another of the most interesting astronomical phenomena depending on the moon is the eclipse ( q.v. ). It sometimes happens that in its orbital motion about the earth, the moon gets directly in line between the earth and the sun. In. such a case, the light of the sun will be either partial- ly or totally cut off', giving what we call a solar eclipse. Again, the moon may be in line with the earth and sun, but in such a way that the earth is between the two other bodies. In that case no solar light can reach the moon, to be reflected to the earth. The moon is therefore dark, and we call the phenomenon a lunar eclipse. Of the two, the solar eclipse is by far the more interest- ing and important scientifically. This is true especially of the total solar eclipse, which for any given place on the earth is a very rare phenomenon indeed. It is only during a total eclipse that we can see the solar corona, which is perhaps the most beautiful and grandest spec- tacle in Nature, and at the same time one of her deepest mysteries.

Mercury and Venus have orbits entirely within that of the earth. Mercury is nearest to the sun of all the known planets, and it is also the least in size. Since its linear distance from the sun is so small, it always appears to our view near the sun on the sky. It is therefore usually very difficult to see, on account of the bright solar light, being visible only in the twi- light preceding sunrise and immediately follow- ing sunset. Venus, the other interior planet, is about the same size as the earth, and, like the earth, is surrounded by an atmosphere. Both Mercury and Venus exhibit phases similar to the moon's, and these may be observed easily in the telescope.

Mars. Outside of the earth, the next planet in order of distance from the sun is Mars. It has two satellites, or moons, which are so ex- tremely small that they can be seen only in the most powerful telescopes. Mars has attracted much attention of late years, on account of the extensive observations of its surface markings that have been made by various astronomers, notably Sehiaparelli and Lowell. Details of great interest have been brought to light, but at the present time they are still subjects of con- troversy on account of the inability of other skilled observers to verify them.

The Asteroids form a group of some four or five hundred tiny planets moving with one exception in orbits entirely beyond that of Mars. Their exact size is not known, but it is most probable that the largest of them does not exceed 400 miles in diameter, and the smaller ones are probably no greater than 10 miles. We cannot state positively what may have been their origin. Perhaps some larger planet has broken up into these planetary fragments, or possibly