imes as heavy as
Mercury, and he considers that this is worthy of great confidence. There
is a considerable difference between this result (which makes the earth
about thirty times as heavy as Mercury) and that of Le Verrier; and, on
the other hand, Haerdtl has, from the motion of Winnecke's periodic
comet, found a value of the mass of Mercury which is not very different
from Le Verrier's. Mercury is, however, the only planet about the mass
of which there is any serious uncertainty, and this must not make us
doubt the accuracy of this delicate weighing-machine. Look at the orbit
of Jupiter, to which Encke's comet approaches so nearly when it retreats
from the sun. It will sometimes happen that Jupiter and the comet are in
close proximity, and then the mighty planet seriously disturbs the
pliable orbit of the comet. The path of the latter bears unmistakable
traces of the Jupiter perturbations, as well as of the Mercury
perturbations. It might seem a hopeless task to discriminate between the
influences of the two planets, overshadowed as they both are by the
supreme control of the sun, but contrivances of mathematical analysis
are adequate to deal with the problem. They point out how much is due to
Mercury, how much is due to Jupiter; and the wanderings of Encke's comet
can thus be made to disclose the mass of Jupiter as well as that of
Mercury. Here we have a means of testing the precision of our weighing
appliances. The mass of Jupiter can be measured by his moons, in the way
mentioned in a previous chapter. As the satellites revolve round and
round the planet, they furnish a method of measuring his weight by the
rapidity of their motion. They tell us that if the sun were placed in
one scale of the celestial balance, it would take 1,047 bodies equal to
Jupiter in the other to weigh him down. Hardly a trace of uncertainty
clings to this determination, and it is therefore of great interest to
test the theory of Encke's comet by seeing whether it gives an accordant
result. The comparison has been made by Von Asten. Encke's comet tells
us that the sun is 1,050 times as heavy as Jupiter; so the results are
practically identical, and the accuracy of the indications of the comet
are confirmed. But the calculation of the perturbations of Encke's comet
is so extremely intricate that Asten's result is not of great value.
From the motion of Winnecke's periodic comet, Haerdtl has found that the
sun is 1,047.17 times as heavy as Jupit