Figure 1.1. The bounded geocentric model according
to Peter Apian, from Cosmographia (1539).

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Figure 1.2. Retrograde motion of Saturn relative
to background stars.

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Figure 1.3. (a) (Upper) If circle AB represents the Earth's
orbit, an observer moving from A to B will
detect a larger angle AOB when object O is
closer. By convention, one-half of angle AOB
is the parallax angle. (b) (Lower) Two objects
“O” lying on the Firmament (the supposed eighth sphere of the stars) appear farther apart when the Earth is closer (angle ObO) than when it is more distant (angle OaO).

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Figure 1.4. The bounded heliocentric model of Nicholas
Copernicus, from De Revolutionibus (1543).

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Figure 1.5. Planetary alignments for heliocentric orbits. The position of the Earth at E is shown relative to a Superior Planet (like Mars) in the outer orbit and
an Inferior Planet (like Venus) in the inner orbit. Relative to the Sun-Earth direction, a Superior Planet can be at Opposition (O) or Conjunction
(C); and an Inferior Planet can be at Inferior
Conjunction (IC), Superior Conjunction (SC),
Maximum Eastern Elongation (MEE), or
Maximum Western Elongation (MWE).

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