The sidereal messenger of Galileo Galilei : and a part of the preface to Kepler's Dioptrics containing the original account of Galileo's astronomical discoveries . tance from the boundary of the light (theterminator), by comparing their distance with thecomplete diameter of the Moon, I learnt that it some-times exceeded the one-twentieth (^oth) part of thediameter. Suppose the dis-tance to be exactly -i^\hpart of the diameter, and letthe diagram represent theMoons orb, of which c A F isa great circle, E its centre,and c F a diameter, whichconsequently bears to thediameter of the Earth theratio
The sidereal messenger of Galileo Galilei : and a part of the preface to Kepler's Dioptrics containing the original account of Galileo's astronomical discoveries . tance from the boundary of the light (theterminator), by comparing their distance with thecomplete diameter of the Moon, I learnt that it some-times exceeded the one-twentieth (^oth) part of thediameter. Suppose the dis-tance to be exactly -i^\hpart of the diameter, and letthe diagram represent theMoons orb, of which c A F isa great circle, E its centre,and c F a diameter, whichconsequently bears to thediameter of the Earth theratio 2:7; and since the diameter of the Earth, ac-cording to the most exact observations, contains 7000Italian miles, CF will be 2000, and CE 1000, and the-2Vth part of the whole, CF, 100 miles. Also let CFbe a diameter of the great circle which divides thebright part of the Moon from the dark part (for,owing to the very great distance of the Sun from theMoon this circle does not diflPer sensibly from a greatone), and let the distance of a from the point c be-^Vth part of that diameter; let the radius E A bedi-awn, and let it be produced to cut the tangent line. 30 THE SIDEREAL MESSENGER. G c D, which represents the ray that illumines thesummit, in the point D. Then the arc CA or thestraight line c d will be 100 of such units, as c e con-tains 1000. The sum of the squares of DC, CE istherefore 1,010,000, and the square of de is equal tothis; therefore the whole ED will be more than 1004;and A D will be more than 4 of such units, as c E con-tained 1000. Therefore the height of A D in the Moon,which represents a summit reaching up to the Sunsray, G c d, and separated from the extremity c bythe distance c D, is more than 4 Italian miles ; butin the Earth there are no mountains which reach tothe perpendicular height even of one mile. We aretherefore left to conclude that it is clear that the- prominences of the Moon are loftier than those ofthe faint \ wlsli iu tMs pkcc to as
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Keywords: ., bookauthorgalileigalileo1, bookcentury1800, booksubjectastronomy
