A

Phase angle y y

Earth

FIGURE 3.11. The phase angle and phase of Mars.

to the phase angle divided by 180 degrees. The phase angle is a maximum around the quadratures, but the actual value varies somewhat with the position of Mars in its orbit, because of the eccentricity. The maximum value, however, is 47 degrees, and the fraction of the disk in shadow is 47/180; that is, a little more than a fourth. Mars then exhibits a gibbous phase, rather like the Moorl 3 or 4 days before and after its full phase (fig. 3.12). Most of the time, therefore, Mars appears in full or almost full phase. The bright outer rdee of Mnrs for other

Full

Full

FIGURE 3.12. Gibbous and full phases of Mars.

celestial body) as seen against the darkness of space is called the limb. This would be equivalent to the point A in the section in figure 3.11. The edge corresponding to B, that is, the line separating the sunlit and shadowed areas in a gibbous phase, is referred to as the terminator, as indicated in figure 3.13.

Retrograde Motion of Mars

If a planet is observed on several successive nights, it seems to move in a general west-

FIGURE 3.13. The limb and terminator of a celestial body.

to-east direction relative to the constellations of fixed stars. This apparent mode of movement is known as direct motion of the planet and is a consequence of Earth's orbital revolution around the Sun. The fixed stars do not exhibit such apparent motion because they are very much farther away. On certain occasions, which for Mars are invariably 4 or 5 weeks before every opposition, the direction of movement of the planet through the constellations changes direction. The planet then appears to travel from east to west. This retrograde motion, as it is called,

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