ce next succeeded in determining the absolute dimensions of the
orbits. What is the distance of the sun from the earth? No scientific
question has occupied the attention of mankind in a greater degree.
Mathematically speaking, nothing is more simple: it suffices, as in
ordinary surveying, to draw visual lines from the two extremities of a
known base line to an inaccessible object; the remainder of the process
is an elementary calculation. Unfortunately, in the case of the sun, the
distance is very great and the base lines which can be measured upon the
earth are comparatively very small. In such a case, the slightest errors
in the direction of visual lines exercise an enormous influence upon the
results. In the beginning of the last century, Halley had remarked that
certain interpositions of Venus between the earth and the sun--or to use
the common term, the transits of the planet across the sun's disk--would
furnish at each observing station an indirect means of fixing the
position of the visual ray much superior in accuracy to the most perfect
direct measures. Such was the object of the many scientific expeditions
undertaken in 1761 and 1769, years in which the transits of Venus
occurred. A comparison of observations made in the Southern Hemisphere
with those of Europe gave for the distance of the sun the result which
has since figured in all treatises on astronomy and navigation. No
government hesitated to furnish scientific academies with the means,
however expensive, of establishing their observers in the most distant
regions. We have already remarked that this determination seemed
imperiously to demand an extensive base, for small bases would have been
totally inadequate. Well, Laplace has solved the problem without a base
of any kind whatever; he has deduced the distance of the sun from
observations of the moon made in one and the same place.

The sun is, with respect to our satellite the moon, the cause of
perturbations which evidently depend on the distance of the immense
luminous globe from the earth. Who does not see that these perturbations
must diminish if the distance increases, and increase if the distance
diminishes, so that the distance determines the amount of the
perturbations? Observation assigns the numerical value of these
perturbations; theory, on the other hand, unfolds the general
mathematical relation which connects them with the solar distance and
with other known elements. The determination of