102 THK I'MX i:KSh: A VAST SWSri-.M Ol" PARIS Just as the sun has a number of satellites, so the planets have varying numbers of satellites of their own, which are called moons. Mercury Is the Dwarf among the Planets. Mercury, having a mass about one-twentieth of the mass of the earth and having a diameter only half again as great as the moon's, is the dwarf among the planets with the possible exception of Pluto. Mercury is so close to the sun that it is visible only in a few certain positions in its orbit and then only shortly before sunrise or after sunset. Mercury makes the circuit of the sun in eighty-eight days, and its time of rotation around its own axis is the same, so that its day is the same length as its year and it always presents the same face to the sun. It is too small for its gravitational attraction to hold atmospheric particles to its surface. This conclusion, based upon Newton's law of gravitation, is confirmed by the observation of Aldrich, who found that the earth with its atmosphere reflects about 44 per cent of the sunlight w^hich reaches it, whereas the moon with no atmosphere reflects only 7 per cent. It was then found that Mercury also reflects only 7 per cent of the sunlight which reaches it. Because Mercury is so close to the sun, it receives about seven times the radiation per unit area that the earth does. For this reason and because so little radiant energy is reflected, the side of Mercury facing the sun must be a parched desert too hot to support life, while the opposite side is always very dark and must be very cold inasmuch as there is no atmosphere or bodies of water to act as an equalizing medium. Certainly the conditions on Mercury would make life, as we know it, impossible. Venus Is the Brightest of the Planets. Taking the planets in the order of their proximity to the sun, we come now to the planet Venus. At times it appears as a bright evening star and at other periods as a brilliant morning star. It is nearly the same size as its nearest planetary neighbor, the earth. At times of greatest brilliancy, Venus can be seen in full daylight. Inasmuch as Venus reflects 59 per cent of the incident light, it is known to have an atmosphere. The clouds which surround Venus are so dense that the few brief glimpses we have had of its body have been insufficient to enable man to determine the period of rotation about its axis. Spectroscopic evidence indicates that there is little oxygen in the atmosphere of Venus, which in turn indicates that there is little vegetation like that of the earth on Venus, because it is the process of
THE PLANETS AND THEIR MOTIONS 103 photosynthesis of plants on the earth that maintains the oxygen supply of the earth's surface. It is probable that there are great oceans of water on Venus, which vaporize to form its dense clouds. Inasmuch as Venus is closer to the sun than the earth is, it receives more radiant energy, which must result in greater evaporation of water to form clouds. Both Mercury and Venus show the phases characteristic of our moon. Galileo predicted that Venus would show these phases if it were to revolve around the sun rather than around the earth. His actual observations of the full and crescent phases of Venus were therefore very important. The Earth and Its Satellite. Next we come to the earth, or rather the earth and moon — a sort of double planet. Our moon is so much larger than the satellites of the other planets in relation to the sizes of their primaries that it may be considered as a small planet itself. Thus, though the earth and its history will be discussed in the next unit, a separate discussion of the moon may well be taken up at this point.