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108 unchanged; and thus Coppernicus was able to give the accompanying diagram of the solar system (fig. 40), representing his view of its general arrangement (though not of the right proportions of the different parts) and of the various motions.

82. The effect of the motion of the earth round the sun on the length of the day and other seasonal effects is

discussed in some detail, and illustrated by diagrams which are here reproduced.

In fig. 41 represent the centre of the earth in four positions, occupied by it about December 23rd, March 21st, June 22nd, and September 22nd respectively (i.e. at the