436 
a circle), and again by 2, — which will give us a semi-diameter or radius 
of 238,800 miles, — in other words, the moon’s mean distance, — as we find it 
stated in Professor Airy’s Six Lectures. 
3. It is and has long been usual, however, to speak of the moon’s distance 
from us, in round figures, as about 240,000 miles, or as 60 semi-diameters 
of the earth ; which is thus arrived at : — She passes through the earth’s 
shadow when eclipsed in four hours, and is therefore considered as describing 
the breadth of the earth or 8,000 miles in that time. Consequently in 
one day (or six times four hours) she describes six times the breadth of 
the earth ; and taking thirty days as representing the period of each 
lunation, the moon will describe 6 times 30, or 180 times, the breadth 
of the earth in a month. One-third of this will be the diameter of her orbit, 
namely, 60 diameters of the earth, and she is consequently distant from us 
60 semi-diameters of the earth, or 240,000 miles. 
4. We find this mode of computing and speaking of the motion, and 
path, and distance of the moon, in the most modern astronomical works. 
I have made use of the ipsissima verba of the present Astronomer Royal, 
taken from the fourth edition of his Lectures. But it is by no means a 
merely modern view. It dates back far beyond our own day or even 
the time of Newton, Kepler, or Copernicus. In fact, it really belongs 
to the Ptolemaic system ; and it rightly belongs to it ; for it will be 
found, upon due consideration, that in all respects the deductions which 
have been drawn from the one initial fact of observation, that a lunar 
eclipse lasts about four hours, depend for their approximate accuracy 
upon a geocentric hypothesis, with the earth at rest in the centre of the 
moon’s orbit. 
5. According to Ptolemy and other astronomers about his time, the moon 
was regarded when in syzygiis, that is, when in conjunction with and in 
opposition to the sun, or when dark and full, as distant from us 59 semi- 
diameters of the earth. Huygens regarded its distance as 60 semi-diameters, 
Copernicus as 60^-, Street as 60f, and Tycho-Brahe (if we correct the error 
due to his peculiar theory of Refractions) as 60|. In the Principia, B. III., 
Prop. IV., Theor. IV., the distance is taken as 60 ; which is the basis of 
Newton’s original calculations of the force of the moon’s gravitation towards 
the earth, measured by the fall from a tangent to the moon’s circular orbit, 
described with this radius. 
6. As regards Ptolemy and others, who believed the earth to be at rest, 
their deductions as to the path of the moon in a month, in an orbit nearly 
circular round the earth, and consequently as to the extent of the moon’s 
radius or mean distance, based upon the duration of a lunar eclipse, and the 
moon’s consequent rate of motion, were necessarily very nearly accurate, if 
they were correct in the primary assumption that the breadth of the earth’s 
shadow is nearly three times the breadth of the moon. To them, and 
upon the geocentric hypothesis, the velocity or rate of motion, and the 
monthly orbit of the moon in a nearly circular path, were real and actual. 
Not so, upon the Copernican system. 
7. It is obvious, upon a moment’s consideration — if we regard the earth 
as a planet in rapid motion round the sun, flying from west to east, or from 
right to left, with a velocity of 65,000 miles an hour, while the moon, when 
at the full, is moving in the same direction so swiftly that she passes through 
and beyond the earth’s rapidly-moving shadow in the course of four hours — 
that the moon is really moving not at the comparatively slow rate of merely 
2,01)0 miles an hour, but with an enormous velocity, 2,000 miles an hour 
swifter than the earth itself, that is, with a speed of no less than 67,000 miles 
an hour, during a lunar eclipse. 
8. But the whole problem of the moon’s motion and path is otherwise 
