As the moon moves in an ellipse about the earth in its focus, she must, in like manner, by the earth's attraction, have her velocity increased from her apo gee to perigee, and decreased as much from her perigee to apogee. Theo° are the principal causes of the variation of the velocities of the earth and moon. But as the sun attracts the moon, as well as the earth attracts it, the attraction of the sun will cause another variation of the moon's velocity. Thus the moon being attracted both by the sun and earth, they will cause great irregularities in her mo tion ; and hence it is very difficult to compute the place of the moon. After finding the mean place of the moon, that is, the place where she would have been if her motion had been uniform, it re quires not less than twenty corrections, in order to get the true place to a sufficient degree of accuracy. Sir I. Newton was the first person who pointed out the sources of these irregularities; but they are of a nature too difficult to admit of a proper illustration. When we view the moon with a telescope, we find that her surface is very rough with mountains and cavities; this appears from the very jag ged boundary of the light and dark parts. Also, certain parts are found to project shadows always opposite to the sun ; and when the sun becomes vertical to any of them, they are observed to have no sha dow; these therefore must be mountains. Other parts are always dark on that side next the sun, and illuminated on the op• posite side ; these therefore must be ca vities. Hence the appearance of the moon constantly varies, from its altering its situation in respect to the sun.
The tops of the mountains on the dark part of the moon are frequently seen en lightened at a distance from the confines of the illuminated part. The dark parts have, by some, been thought seas; and by others, to be only a great number of caverns and pits, the dark sides of which next to the sun would cause those places to appear darker than the rest. The great irregularity of the line bounding the light and dark parts, on every part of the surface, proves that there can be no very large tracts of water, as such a re gular surface would necessarily produce a line, terminating the bright part, per fectly free from all irregularity. Also, if there was much water upon its surface, and an atmosphere, as conjectured by some astronomers, the clouds and va. pours might easily be discovered by our telescopes; but no such phenomena have ever been observed.
On April 9, 1787, Dr. Herschel disco vered three volcanoes in the dark part of the moon ; two of them seemed to be al most extinct, but the third showed an ac tual eruption of fire, or luminous matter, resembling a small piece of burning char. coal covered by a thin coat of white ashes; it had a degree of brightness about it, as strong as that with which such a coal would be seen to glow in faint daylight. The adjacent parts of the volcanic moun tain seemed faintly illuminated by the eruption. A similar eruption appeared on
May 4, 1783. On March 7, 1794, a few minutes before eight o'clock in the even ing, Mr. Wilkins, of Norwich, an emi nent architect, observed, with the naked eye, a very bright spot upon the dark part of the moon ; it was there when he first looked at the moon, and the whole time lie saw it, which was about five mi nutes; it was a fixed steady light, except the moment before it disappeared, when its brightness increased. The same phe nomenon was also observed by Mr. T. Stretton, in St. John's Square, Clerken well, London. On April 13, 1793, M. Piazza, astronomer royal at Palermo, ob served a bright spot on the dark part of the moon ; and several other astronomers have observed the same phenomenon.
It has been a doubt amongst astrono mers, whether the moon has any atmos phere ; some suspecting that, at an occi I tation of a fixed star by the moon, the star did not vanish suddenly, but lost its light gradually, and thence concluded that the moon has an atmosphere. M. Schroeter, of Liliantlian, in the Duchy of Bremen, has endeavoured to establish the existence of an atmosphere, from the fol lowing observations. 1. lie observed the moon, when two days and a half old, in the evening soon after sunset, before the dark part was visible, and continued to ob serve it tilt it became visible. Two cusps appeared tapering in a very sharp, faint prolongation, each exhibiting its furthest extremity faintly illuminated by the solar rays, before any part of the dark hemi sphere was visible ; soon after, the whole dark limb appeared illuminated. This prolongation of the cusps beyond the se micircle, he thinks, must arise from the sun's rays being refracted by the moon's atmosphere. Ile computes also the height of the atmosphere, which refracts light enough into the (lark hemisphere, to pro duce a twilight, more luminous than the light reflected from the earth when the moon is about 32° from the new, to be 1356 Paris feet, and that the greatest height capable of refracting the solar rays is 5376 feet. 2. At an occultation of Jupiter's satellites, the third disappear ed, after having been 1" or 2 of time in distinct ; the fourth became indiscernible near the limb ; this was not observed of the other two. See the Philosophical Transactions, 1792.
Many astronomers have given maps of the moon ; but the most celebrated are of Hevelius in his Selenographia ; in which he has represented the appear ance of the moon in its different states, from the new to the full, and from the full to the new ; these figures Mayer pre fers. I.:mucous and Racciolus denoted the spots upon the surfage by the names of philosophers, mathematicians, and other celebrated men ; giving the names of the most celebrated characters to the largest spots. Ilevelios marked them with the geographical names of places upon the eart h. Elie former distinction is now generally used.