The Journal of Science, and Annals of Astronomy, Biology, Geology, Industrial Arts, Manufactures, and Technology (Volume 3)

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1866. Excerpt: ... 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 suns disturbing effect must be greater than when the earth is in 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 is 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 in 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 ...