Figure A1.4: Visibility/invisibility of Venus
Figure A1.5: The apparent retrograde motion of an exterior planet as seen from the earth
Figure A1.5: The apparent retrograde motion of an exterior planet as seen from the earth takes Venus to make a complete revolution around the sun (sidereal period) is 225 days, but clearly the earth also revolves around the sun, so from our viewpoint the planet reappears in the same configuration relatively to the sun after a much longer (synodic) period of 584 days. During each cycle, Venus, which gives the impression of swinging like a pendulum, disappears from sight ("behind" the sun) for about 8 weeks, to then reappear as the Evening Star, visible in the west immediately after sunset; then it disappears from sight ("in front of'' the sun), to then reappear as the Morning Star. The motion of the other inner planet, Mercury, is similar to that of Venus.
The motion of the outer planets that are visible to the naked eye, Mars, Jupiter, and Saturn (Uranus is right at the limit of visibility with the naked eye and does not seem to have been observed in antiquity) is characterized by a phenomenon known as retrograde motion. This refers to the fact that from time to time the earth "overtakes" a planet, with the result that the latter seems to go backward in relation to the stars, to then resume its normal path.
As far as the observation of rising and setting on the horizon are concerned, the planets follow a cycle similar to that of the sun, whose extremes, however—as happens with the moon—vary between two positions, maximum and minimum, both in the north and the south.
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