Fifth book of lessons for the use of schools . est the sun is calledits perihelion, that part most distant from the sun itsaphelion. The earth is about three millions of milesnearer the sun at its perihelion than at its are surprised to learn that during the height ofour summer, the earth is in that part of its orbit whichis most distant from the sun, and that it is during theseverity of winter that we are nearest to it. Thedifference, however, of the earths distance from tbe PLANETS. 293 sun in summer and winter, when compared with itstotal distance from the sun, is but inconsid
Fifth book of lessons for the use of schools . est the sun is calledits perihelion, that part most distant from the sun itsaphelion. The earth is about three millions of milesnearer the sun at its perihelion than at its are surprised to learn that during the height ofour summer, the earth is in that part of its orbit whichis most distant from the sun, and that it is during theseverity of winter that we are nearest to it. Thedifference, however, of the earths distance from tbe PLANETS. 293 sun in summer and winter, when compared with itstotal distance from the sun, is but inconsiderable, forthroe millions of miles sink into insignificance in com-parison of 95 millions of miles, which is our meandistance from the sun. The change of temperature,arising from this difference, would in itself scarcely besensible, and it js completely overpowered by othercauses which produce the variations of the seasons;but the explanation of these must be deferred, till wehave made some further observations on the heavenlybodies. The planets are celestial bodies which revolve roundthe sun, on the *same principle as the earth. Theyare divided into primary and secondary. Those, whichrevolve immediately round the sun, are called any of these are attended in their course by smallerplanets, which revolve round them: these are calledsecondary planets, satellites, or moons; such is ourmoon, which accompanies the earth, and is carried withit round the sun. The sun is the general centre ofattraction to our system of planets; but the satellitesrevolve round the primary planets, on account of their 294 FIFTH BOOK. greater proximity. The force of attraction is not onlyproportional to the quantity of matter, but to thedegree of proximity of the attracting body. Thepower of attraction diminishes as the squares of thedistance increase; so that a planet, situated at twicethe distance at which we are from the sun, wouldgravitate four times less than we do. The more dist
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