THE MOON AND LUNAR PHENOMENA. 79 



and industry. To Meton, the Athenian, the invention is ascribed, of the celebrated luni- 

 solar period of 6940 days, supposed to be equal to 19 years, or 235 lunations. This 

 period, called the Metonic cycle, was adopted in the year 432 B. c. It was published 

 amid the applause of the Greeks at the Olympic games, who decreed a statue to the 

 inventor, and declared him victor of the first class. The coincidence, however, is not 

 exact, for 6940 days exceed 19 tropical years by about 9^ hours, and exceed 235 lunations 

 by 7^ hours, an error which the Calippic period was designed to rectify, by the leap of a 

 day in four Metonic cycles, or in an interval of seventy-six years. This brought 940 

 lunations into correspondence with 27,759 days, within 5 h 54 m , an inaccuracy which became 

 important by accumulation in the civil usage of the period, and eventually entailed the 

 necessity of the Gregorian reform of the calendar. 



While, in the course of her monthly circuit, the moon passes between the sun and the 

 earth, and deflects a shadow upon the latter, the compliment is returned in another part 

 of her orbit, by the earth's shadow being cast upon the face of the satellite. A lunar 

 eclipse, as well as a solar one, would occur every month if the moon revolved in the same 

 plane with the earth, but she escapes the terrestrial shadow owing to the inclination of 

 her orbit 5 to the ecliptic, carrying her above or below it, and only suffers an eclipse, 

 when, besides being in opposition to the sun, she is in or near the plane of the earth's 

 path. The shadow of our globe is computed to extend 800,000 miles into space. It is 

 long enough, therefore, to reach a body three times the distance of the moon. The 

 diagram represents the immersion of the satellite in it. The lunar globe, however, when 



wholly immersed, is rather obscured than 

 hid from view, owing to the inflexion of 

 the rays of light to her orb from the 

 terrestrial atmosphere. She appears fre 

 quently of a dark copper colour, because 

 the red rays, which have the greatest 

 momentum, are those which principally 

 reach her. During the eclipse of September 2d, 1830, the moon exhibited a deep 

 blood-red hue, as seen from the metropolis. Upon comparing the ancient observa 

 tions of eclipses recorded by Ptolemy, with those of Albutegnius in the ninth cen 

 tury, and of modern astronomers, Halley discovered the acceleration of the mean 

 lunar motion, a phenomenon which Laplace referred to its true cause, that of a 

 diminution of the eccentricity of the earth's orbit, owing to the disturbing forces of the 

 planets. The terrestrial orbit is gradually changing from an ellipse into a* circle, its 

 eccentricity decreasing at the rate of about forty-one miles annually, thereby accelerating 

 the mean motion of the moon ; and should the decrease proceed equably, the earth's path 

 will be reduced to a circle in 37,527 years. But, like all the other phenomena depending 

 on gravitation, there is redress laid up in store for this perturbation, through the dis 

 turbing forces of the planets beginning to act in a contrary direction, which will produce 

 a change towards eccentricity in the earth's orbit, and proportionably retard the mean 

 motion of the moon. It may require thousands of ages for one part of this cycle of 

 change to transpire ; but the fact itself is not without interest to us, as one of the most 

 sublime and beautiful results which the mind has mastered, and an illustration of the 

 permanence of the system under all its disturbances. While lunar eclipses have thus 

 been watched by the eye of science, and knowledge enlarged by their means respecting 

 the condition of the system, they have created no small alarm among barbarian races. 

 The Landers give an interesting account of an eclipse of the moon, Sept. 2. 1830, during 

 their stay at Boossa in Central Africa : " The earlier part of the evening had been 

 mild, serene, and remarkably pleasant. The moon had arisen with uncommon lustre, 



