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LET. XI. AND VICISSITUDES OF THE SEASON S. l8z

his daily progress through the heavens from theeast towards the west ; but this is an opticalfallacy, arising from the opposite motion of theearth : for a spectator being placed in any partof the dark hemisphere, will, by the rotationof the earth upon its axis, be brought graduallyinto the enlightened one; and as the fun firstappears to him in the east, it will seem to ascendhigher and higher towards the west, in propor-tion as the spectator moves in a contrary direc-tion towards the east: so that whether the earthturns round upon its axis once in twenty-fourhours, or whether the fun and all the othercelestial bodies, move round the earth in thattime, the appearances will be exactly the fame.

Every planet, on which observations havebeen made, is found to have a revolution uponits axis ; and as this revolution is the causeof a constant succession of day and night toevery part of their surfaces, so an inclination ofthe axis of any planet to the plane of its orbit,occasions the vicissitudes of the seasons. Thus,Jupiter, whose axis is nearly perpendicular to theplane of his or bit, has equal days and nights con-tinually, from one pole to the other; their lengthbeing each four hours and twenty-eight minutes:but Venus, the Earth, and also Mars, accordingto the late discoveries of Mr. Herscnel, havingtheir axes inclined to the planes of their orbits,in an angle considerably less than that of 90N 4 degrees,