I (Brian Tung) wrote:
The Earth's orbit, however, is only slightly elliptical. Its distance
from the Sun varies only by a little more than 3 percent throughout the
year. The variation in the Sun's apparent motion doesn't change very
much as a result (about 6 percent, twice the variation in distance)[*],
and its effect is never so great as to ever be *greater* than the
rotational motion (which, after all, is 366 times greater on average).
Generally speaking, we just don't notice.

That is *not* the case with Mercury. First of all, its orbit is much
more elliptical than the Earth's. Its distance from the Sun varies by
some 40 percent, and the Sun's apparent motion, as seen from Mercury,
therefore varies by almost a factor of 2[*].
[*] In both the cases, I mean the apparent motion *due solely to the
planet's orbital motion*. Sorry about any possible confusion.

Brian Tung
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