9.1  Orbital Properties

Venus is the second planet from the Sun. Its orbit lies within Earth's, so Venus, like Mercury, is always found fairly close to the Sun in the sky—Venus is never seen more than 47° from the Sun. Given Earth's rotation rate of 15° per hour, this means that Venus is visible above the horizon for at most 3 hours before the Sun rises or after it sets. Because we can see Venus from Earth only just before sunrise or just after sunset, the planet is often called the "morning star" or the "evening star," depending on where it happens to be in its orbit. Figure 9.1 shows Venus in the western sky just after sunset. The Venus Data box lists some of the planet's orbital and physical properties.

Figure 9.1 The Moon and Venus in the western sky just after sunset. Venus clearly outshines even the brightest stars in the sky.

Venus is the third brightest object in the entire sky (after the Sun and the Moon). It appears more than 10 times brighter than the brightest star, Sirius. You can see Venus even in the daytime, if you know just where to look. On a moonless night away from city lights, Venus casts a faint shadow. The planet's brightness stems from the fact that Venus is highly reflective. Nearly 70 percent of the sunlight reaching Venus is reflected back into space (compare this with roughly 10 percent in the case of Mercury and the Moon). Most of the sunlight is reflected from clouds high in the planet's atmosphere.

We might expect Venus to appear brightest when it is "full —that is, when we can see the entire sunlit side. However, because Venus orbits between Earth and the Sun, Venus is full when it is at its greatest distance from us, 1.7 A.U. away on the other side of the Sun (an alignment known as superior conjunction, where the term "conjunction" simply indicates that two objects are close together on the sky), as illustrated in Figure 9.2. When Venus is closest to us, the planet is at the new phase, lying between Earth and the Sun (inferior conjunction), and we again can't see it, because now the sunlit side faces away from us—only a thin ring of sunlight, caused by refraction in Venus's atmosphere, surrounds the planet. As Venus moves away from inferior conjunction, more and more of it becomes visible, but its distance from us also continues to increase. Venus's maximum brightness, as seen from Earth, actually occurs about 36 days before or after closest approach to our planet. At that time, Venus is about 39° from the Sun, and we see it as a rather fat crescent.

Figure 9.2 Venus appears full when it is at its greatest distance from Earth, on the opposite side of the Sun from us (superior conjunction). As its distance decreases, less and less of its sunlit side becomes visible. When it is closest to Earth, it lies between us and the Sun (inferior conjunction), so we cannot see the sunlit side of the planet at all. Venus appears brightest when it is about 39° from the Sun. (Compare Figure 2.14.)

The Phases of Venus