514 Adams’ Recent Astronomical Discovery. | Oct., 
this influence were constant, so also would the moon’s mean motion 
be constant. But it is clear that when the earth is in or near 
perihelion, the sun’s disturbing effect must be greater than when 
the earth is mm or near aphelion. Hence arises the annual 
equation, the effect of which is that the lunar month is longer 
in our winter than in our summer, the moon’s orbit attaining its 
maximum dilation in January (when the earth is near perihelion), 
and its maximum contraction in July. The period of the inequality 
is an anomalistic year, and its maximum amount is 10’, by which the 
moon 1s sometimes before and sometimes behind her mean place. 
Now since the annual equation is due to the variation of the 
sun’s disturbing influence from the mean value it would have if the 
earth’s orbit were circular, it is clearly possible that any variation 
in the eccentricity of the terrestrial orbit would, besides affecting 
the amount of the annual equation (which it would inevitably 
do), affect the total amount of the moon’s motion im successive 
anomalistic years. But the eccentricity of the earth’s orbit has 
been continually diminishing since the date of the earliest recorded 
observations ; and since the major axis of the orbit has throughout 
remained appreciably unaltered, the absolute extent of the ellipse 
in which the earth moves has been continually increasing, so that the 
probability prior to exact investigation is that, owing to this cause, 
the sun’s disturbing action has been diminishing, and therefore the 
earth’s influence, and with it the moon’s mean motion, increasing.* 
It will be as well to state the actual extent of the ellipticity of 
the earth’s orbit, as some misconception appears to prevail on this 
point, and in works on astronomy reference is continually made to 
this feature, as if it were much more marked than it is in reality. 
Assuming the earth’s mean distance from the sun to be 91,500,000 
miles, her least distance is somewhat under 90,000,000 miles, her 
greatest somewhat over 93,000,000 ; the centre of her orbit being 
1,533,600 miles from the sun. Thus the eccentricity of her orbit 
though small is appreciable. The ellipticity of her orbit, however, 
is quite insignificant by comparison. For the major axis of the 
orbit is 183,000,000 miles, and the minor axis 182,974,000 miles, 
the difference being less than one-7,000th part of either dimension, 
so that a finely drawn circle one foot in diameter would differ from 
a correct representation of the earth’s orbit by less than the thick- 
ness of its own bounding line. Again, the eccentricity of 1,533,600 
miles is subject to an annual diminution of 40 miles, owing to 
which change the semi-minor axis undergoes an annual increase of 
* Tt does not necessarily follow, from the increase of the minor axis, that the 
average distance of the earth from the sun would be increased. The lagging of 
the earth near aphelion, and her rapid motion near perihelion of the more 
eccentric orbit might be more than sufficient to compensate for the smaller absolute 
extent of that orbit. Hxamination proves the contrary; but it is not to be assumed 
as self-evident, as is commonly done. 
