439 
Here we see that the actual motion of the moon, which has, of necessity, 
upon the received hypothesis, a velocity of more than 65,000 miles an hour, 
is not only disregarded or forgotten ; but, that the moon should require 
to move with any such great velocity, is even made an argument against the 
probability of the Ptolemaic system,- — though the facts, and consequently 
the argument as regards the moon’s motion, computed from the duration of 
an eclipse, upon the two rival hypotheses, are precisely the other way. It is 
only, as we have already seen, if the earth be at rest that the moon can be 
regarded as passing through the earth’s shadow at the approximate rate 
of 2,000 miles an hour whereas, upon the Copernican hypothesis, and 
regarding the earth’s velocity in its orbit as 65,000 miles an hour, the rate 
of the moon’s motion is actually, of necessity and ex hypothesi , even greater 
than the rate of 60,000 miles an hour, which was urged as so “very 
unlikely ” as to amount to an argument against the Ptolemaic system ! 
14. But, not only has the real velocity of the moon been thus disregarded, 
— and in fact it is not only disregarded, but apparently denied, by the argu- 
ment employed in the passage above cited, — but in no other part of these 
lectures is the moon’s real motion or path even once mentioned. Its motion 
is exclusively spoken of as only about 2,000 miles, or more precisely as 
2,288 miles an hour, in a nearly circular monthly orbit. 
15. But, since in Ferguson’s Astronomy the real path and rapid motions 
of the moon as she accompanies the earth round the sun, and also the great 
variations in her velocities at one time and another, are recognized ; let us 
examine by what kind of reasoning or argument her actual velocities 
are practically set aside and become resolved into the small mean motion 
of only 2,290 miles an hour. It can scarcely be said that anything like 
adequate argument is attempted. What Ferguson says is solely directed to 
meet a single “ difficulty,” which alone appears to him to require to be 
removed. His words are : — 
“ The moon’s path being concave to the sun throughout, demonstrates 
that her gravity towards the sun at her conjunction, exceeds her gravity 
toward the earth. And if we consider that the quantity of matter in 
the sun is almost 230,0001 times as great as the quantity of matter 
in the earth, and that the attraction of each body diminishes as the square 
of the distance from it increases, we shall soon find that the point of equal 
attraction between the earth and the sun is about 70,000 miles nearer the 
earth than the moon is at her change. It may then appear surprising that 
the moon does not abandon the earth when she is between it and the 
sun, because she is considerably more attracted by the sun than by the 
earth at that time. But this difficulty vanishes when we consider, that 
* But, even upon a geocentric hypothesis, the rate of the moon’s motion 
cannot be so very simply ascertained. The true solution of the problem 
will depend upon the breadth of the earth’s shadow, the distance and size of 
the sun, and the motion of the earth’s shadow in one direction or another ; 
which will again depend upon whether the earth, if supposed to be at rest in 
space, has an axial rotation or not, and whether the moon is moving faster 
than the sun in one direction, or slower than the sun in another. In fact, 
unless both the earth and the sun were at rest in space, the duration 
of a lunar eclipse, on either hypothesis, could not give precisely the rate 
of the moon’s motion, even if we knew the precise breadth of the earth’s 
shadow. If only the earth were at rest and in the centre, the duration 
of eclipses could only indicate the difference between the velocity of the sun 
and moon. 
t Increased to 352,280 times, taking the earth’s radius as 95,000,000 
miles. ( Airy's Lects., p. 215.) 
