We can determine Venus's radius from simple geometry, just as we did for Mercury and the Moon. 
(Sec. 8.2) At closest approach, when Venus is only 0.28 A.U. from us, its angular diameter is 64". From this observation, we can determine its radius to be about 6000 km. More accurate measurements from spacecraft give a value of 6052 km, or 0.95 Earth radii.
Like Mercury, Venus has no moon. Before the Space Age, astronomers calculated its mass by indirect means—through studies of its small gravitational effect on the orbits of the other planets, especially Earth. Now that spacecraft have orbited the planet, we know its mass very accurately: 4.9 
1024 kg, or 0.82 the mass of Earth.
From its mass and radius we find that Venus's average density is 5200 kg/m3. As far as these bulk properties are concerned, then, Venus seems very similar to Earth. If the planet's overall composition were similar to Earth's, we could then reasonably conclude that the planet's internal structure and evolution were basically Earthlike. We will review what little evidence there is on this subject later in this chapter.
The same clouds whose reflectivity makes Venus so easy to see in the night sky also make it impossible for us to discern any surface features on the planet, at least in visible light. As a result, until the advent of suitable radar techniques in the 1960s, astronomers did not know the rotation period of Venus. Even when viewed through a large optical telescope, the planet's cloud cover shows few features (see Figure 9.5), and attempts to determine Venus's rotation by observing the cloud deck were frustrated by the rapidly changing nature of the clouds themselves. Some astronomers argued for a 25-day period, while others favored a 24-hour cycle. Controversy raged until, to the surprise of all, radar observers announced that the Doppler broadening of their returned echoes implied a sluggish 243-day rotation period. (Sec. 8.3) Furthermore, Venus's spin was found to be retrograde—that is, in a sense opposite that of Earth and most other solar system objects, and opposite that of Venus's orbital motion.
Planetary astronomers conventionally define "north" and "south" for individual members of the solar system by the condition that planets always rotate from west to east. With this definition, Venus's retrograde spin means that the planet's north pole lies below the ecliptic plane, unlike any of the other terrestrial worlds. Venus's axial tilt—the angle between its equatorial and orbital planes—is 177.4° (compared with 23.5° in the case of Earth). Note, however, that astronomical images of solar system objects conventionally place objects lying above the ecliptic plane at the top of the frame. Thus, with the preceding definition of north and south, all the images of Venus shown in this chapter have the south pole at the top. Figure 9.3 illustrates Venus's retrograde rotation and compares it with the rotation of its neighbors, Mercury, Earth, and Mars. Because of Venus's slow, retrograde rotation, the planet's solar day (noon to noon) is quite different from its sidereal rotation period of 243 Earth days (the time for one "true" rotation relative to the stars). (Sec. 1.3) In fact, as illustrated in Figure 9.4, one Venus day is a little more than half a Venus year (225 Earth days). More Precisely 9-1 discusses this interplay between orbital and rotational motion in a little more detail.
Figure 9.3 The inner four planets of the solar system—Mercury, Venus, Earth, and Mars—display widely differing rotational properties. Although all orbit the Sun in the same sense and in nearly the same plane, Mercury's rotation is slow and prograde (in the same sense as the orbital motion), the rotations of Earth and Mars are fast and prograde, while Venus's is slow and retrograde. Venus rotates clockwise as seen from above the plane of the ecliptic in the direction of the north celestial pole.
Figure 9.4 Venus's orbit and retrograde rotation combine to produce a Venus solar day equal to 117 Earth days, or slightly more than half a Venus year. The numbers in the figure mark time in Earth days.
Why is Venus rotating "backward," and why so slowly? At present, the best explanation planetary scientists can offer is that early in Venus's evolution, the planet was struck by a large body, much like the one that may have hit Earth and formed the Moon, and that impact was sufficient to reduce the planet's spin almost to zero. (Sec. 8.8) Whatever its cause, the planet's rotation poses practical problems for Earthbound observers. It turns out that Venus rotates almost exactly five times between one closest approach to Earth and the next. As a result, Venus always presents nearly the same face to Earth at closest approach. This means that observations of the planet's surface cover one side—the one facing us at closest approach—much more thoroughly than the other side, which we can see only when the planet is close to its maximum distance from Earth.
Astronomers hate to appeal to coincidence to explain their observations, but the case of Venus's rotation appears to be just that. The near-perfect 5:1 "resonance" between Venus's rotation and orbital motion is reminiscent of the Moon's synchronous orbit around Earth, and Mercury's 3:2 spin—orbit resonance with the Sun. (Sec. 8.3) However, no known interaction between Earth and Venus can account for this odd state of affairs. Earth's tidal effect on Venus is tiny, and is much less than the Sun's tidal effect in any case. Furthermore, the key word in the above sentence is near. A resonance, if it existed, would require that the number of rotations per orbit be exactly five. The discrepancy amounts to less than 3 hours in 584 days (Venus's synodic period relative to Earth—see More Precisely 9-1), but it appears to be real, and if that is so, then no resonance exists. For now we are simply compelled to accept this strange coincidence without explanation.
