fig. 16-1 The solar system. The orbits of Mercury and Venus are too small to be shown on this scale. Pluto's orbit is by far the most elliptical. Diameters of sun and planets are exaggerated.

fig. 16-2 Relative sizes of planets and sun.

fig. 16-3 The orbits of the planets seen edgewise. Except for that of Pluto, the orbits all lie nearly in the same plane.

fig. 16-4 The tail of a comet always points away from the sun because of pressure from the sun's radiation and from the solar wind of ions. The tail is longest near the sun and is probably absent far away from the sun.

fig. 16-5 The appearance and apparent size of Venus as seen from the earth depend upon the relative positions of the two planets and the sun. The "new Venus" at 1 is 6 times larger than the "full Venus" at 5. Venus appears brightest between 1 and 2 and between 8 and 1. To an observer on the earth, Venus never appears very far from the sun and either rises just before sunrise or sets just after sunset.

fig. 16-6 The appearance of Saturn's rings from the earth varies with Saturn's location in its orbit. Saturn's period of revolution is 29.5 years.

fig. 16-7 Paths of the two Voyager spacecraft.

fig. 16-8 The origin of the moon's phases. As the moon revolves around the earth, we see it from different angles. When it is between us and the sun, we see only the dark side (new moon), and when it is on the opposite side of us from the sun, we see only the illuminated side (full moon). At other times we see parts of both sides of the illuminated surface.

fig. 16-9 The orbit of the moon is tilted with respect to that of the earth. For this reason the moon normally passes above or below the direct line from the sun to the earth. Eclipses occur only on the rare occasions when the moon passes exactly between the earth and the sun (a solar eclipse, with the moon blocking out light from the sun) or exactly behind the earth (a lunar eclipse, with the earth blocking out light from the sun).

fig. 16-10 Four theories of the moon's origin: (1) The moon split away from the earth. (2) The moon was captured as it approached the earth from elsewhere. (3) The earth and moon were formed together from different clouds of particles. (4) Another early planet struck the earth and formed a larger earth plus the moon. The last theory has the fewest objections to it.