147. THE INNER PLANETS.--In sharp contrast with the giant planets which we have been considering stands the group of four inner planets, or five if we count the moon as an independent body, which resemble each other in being all small, dense, and solid bodies, which by comparison with the great distances separating the outer planets may fairly be described as huddled together close to the sun. Their relative sizes are shown in Fig. 84, together with the numerical data concerning size, mass, density, etc., which we have already found important for the understanding of a planet's physical condition. VENUS [Illustration: FIG. 94.--The phases of Venus.--ANTONIADI.] 148. APPEARANCE.--Omitting the earth, Venus is by far the most conspicuous member of this group, and when at its brightest is, with exception of the sun and moon, the most brilliant object in the sky, and may be seen with the naked eye in broad daylight if the observer knows just where to look for it. But its brilliancy is subject to considerable variations on account of its changing distance from the earth, and the apparent size of its disk varies for the same reason, as may be seen from Fig. 94. These drawings bring out well the phases of the planet, and the student should determine from Fig. 17 what are the relative positions in their orbits of the earth and Venus at which the planet would present each of these phases. As a guide to this, observe that the dark part of Venus's earthward side is always proportional in area to the angle at Venus between the earth and sun. In the first picture of Fig. 94 about two thirds of the surface corresponding to the full hemisphere of the planet is dark, and the angle at Venus between earth and sun is therefore two thirds of 180°--i. e., 120°. In Fig. 17 find a place on the orbit of Venus from which if lines be drawn to the sun and earth, as there shown, the angle between them will be 120°. Make a similar construction for the fourth picture in Fig. 94. Which of these two positions is farther from the earth? How do the distances compare with the apparent size of Venus in the two pictures? What is the phase of Venus to-day?