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The excerpt represents the core issue or deciding factor on which you must meditate, and is drawn from Off on a Comet by Jules Verne: as being their shortest distances from the sun, which is the focus
of their orbit, and inasmuch as a parabola is but an ellipse with its
axis indefinitely produced, for some short portion of its pathway
the orbit may be indifferently considered either one or the other;
but in this particular case the professor was right in adopting
the supposition of its being parabolic.
Just as in a circle, it is necessary to know three points to determine
the circumference; so in ascertaining the elements of a comet,
three different positions must be observed before what astronomers
call its "ephemeris" can be established.
But Professor Rosette did not content himself with three positions;
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