As far as the fluctuations are concerned, they might also
be explained by supposing that the shape of the asteroid is that of a
flat disk, rotating about one of its larger diameters so as to present,
alternately, its edge and its broadside to the sun. And, perhaps, in
order completely to account for all the observed eccentricities of the
light of Eros, the irregularity of form may have to be supplemented by
certain assumptions as to the varying reflective capacity of different
parts of the misshapen mass.
The invaluable Harvard photographs show that long before Eros was
recognized as an asteroid its light variations had been automatically
registered on the plates. Some of the plates, Prof. E.C. Pickering says,
had had an exposure of an hour or more, and, owing to its motion, Eros
had formed a trail on each of these plates, which in some cases showed
distinct variations in brightness. Differences in the amount of
variation at different times will largely depend upon the position of
the earth with respect to the axis of rotation.
Another interesting deduction may be made from the changes that the
light of Eros undergoes. We have already remarked that one of the larger
asteroids, and the one which appears to the eye as the most brilliant
of all, Vesta, has been suspected of variability, but not so extensive
as that of Eros. Olbers, at the beginning of the last century, was of
the opinion that Vesta's variations were due to its being not a globe
but an angular mass.
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