7), toward
the east. He observed, then, that the seeming loop followed by the
planet was a real looped track (so far, at least, as our observer on
the earth was concerned). Fig. 2 shows the apparent shape of Mars's
loop, the dates corresponding to those shown in Fig. 1. Only it does
not lie flat, as shown on the paper, but must be supposed to lie
somewhat under the surface of the paper, as shown by the little
upright _a, b,_ which, indeed, gives the distance under the paper at
which the part of the loop is supposed to lie where lowest at _m_. The
other similar uprights at M_1, M_2, and M_3 show the depression at
these places. You perceive that the part M_1, M_2, lies higher than
the part M_2, M_3. If the loop were flat, and, like E, the earth,
were in the level of the paper, it would be seen edgewise, and the
advancing, receding, and advancing parts of the planet's course would
all lie on the same line upon the sky. But being thus out of the
level, we see through the loop, so to speak, and it has the seeming
shape shown in Fig.
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