ASTRONOMt. 



ing precisely halfway between the great- 

 est and least height of those stars that 

 never set. Since H M O, the visible part 

 of the heavens, contains 130, and it is 90, 

 between the pole, P, and the equator, 

 E Q ; if, therefore, we take away P E 

 from the semicircle H M 0, there remains 

 90 for the other two arcs, P H and E O, 

 that is, the elevation of the pole and the 

 equator are, together, equal to 90, so 

 that the one being known and subtracted 

 from 90, the other also is found. Hence 

 it is known, that " the elevation of the 

 pole at any place is the complement of 

 the elevation of the equator," or what 

 tli at elevation wants of 90. Hence also 

 " the elevation of the equator is equal 

 to the distance from the pole, P, to the 

 zenith, M ;" for the elevation of the equa- 

 tor is the difference between that of the 

 pole and 90. 



When we travel towards the north, we 

 perceive that the north pole does not re- 

 main stationary, but rises towards the ze- 

 nith, in proportion to the space that we 

 pass over. On the contrary, it sinks just 

 as much when we travel towards the 

 south : from which we infer that the sur- 

 face of the earth is not plane, as would ap- 

 pear to a superficial observer, but curved. 



The heavenly bodies appear to describe 

 a complete circle round the earth every 

 24 hours ; but, besides these motions, 

 which are common to them all, there are 

 several which possess motions peculiar to 

 themselves. The sun is farther towards 

 the south during winter than during sum- 

 mer ; he does nottherefore keep the same 

 station in the heavens, nor describe the 

 same circle every day. The moon not 

 only changes her form, diminishes and 

 increases, but, if she is observed in rela- 

 tion to certain fixed stars, it will be found 

 that she proceeds to the eastward, mak- 

 ing progress every day, till, in about a 

 month, she makes a complete tour of the 

 heavens. There are eight other stars, 

 which are continually changing their 

 place ; sometimes they seem to be mov- 

 ing to the westward, sometimes to the 

 eastward, and sometimes they appear sta- 

 tionary for a considerable tine : these are 

 called planets. There are other bodies 

 which appear only occasionally, move for 

 some time with very great velocity, and 

 afterwards advance beyond the regions 

 visible to us: these are comets, of which 

 one is now (November, 1807,) apparent. 

 The greater number of the heavenly bo- 

 dies always reUi.i the same, or nearly the 

 Same, relative distance from each other, 



and are, *> on that account, called fixed 



stars. 



OF THfc FIGURE AND MOTION OF THE 

 EARTH. 



The earth, as we have observed, was 

 long considered as a large circular plane, 

 spreadingout on all sides to an indefinite 

 distance ; but it is now ascertained that it 

 is of a spherical figure, nearly resembling 

 that of a globe. The evidence for this 

 fact is decisive, without having recourse 

 to scientific principles, by considering that 

 the celebrated navigators, Magellan, Sir 

 Francis Drake, Lord Anson, and captain 

 Cook, have all, at different times, sailed 

 round the earth. They set out from Eu- 

 ropean ports, and, by steering their 

 course westward, arrived at length at the 

 very place from whence they departed, 

 which could not have happened, had the 

 earth been of any other than a spherical 

 or a globular figure. This form is also 

 apparent, from the circular appearance 

 of the sea itself, and the circumstances 

 which attend large objects, when seen 

 at a distance on its surface. For when a 

 ship goes out to sea, we first lose sight 

 of the hull, or body of the vessel, see 

 fig. 4 ; afterwards that of the rigging ; 

 and at last can discern onlr the top of 

 the mast, which is evidently owing to 

 the convexity of the water between the 

 eye and the object ; for, otherwise, the 

 largest and most conspicuous part would 

 be visible the longest. Another proof is 

 taken from the shadow of the earth upon 

 the face of the moon, during- the time of 

 a lunar eclipse ; for the moon, having no 

 light but what it receives from the sun, 

 and the earth being interposed between 

 them, the moon must either wholly, or 

 in part, become obscure. And since in 

 every eclipse of this kind, which is not 

 total, the obscure part always appears to 

 be bounded by a circular line, the earth, 

 itself, for that reason, must be spherical ; 

 it being evident that none but a spherical 

 body can, in all situations, cast a circular 

 shadow. 



It is not ascertained who was the first 

 person that asserted the figure of the 

 earth to be spherical, but the opinion is of 

 very great antiquity. For, when Babylon 

 was taken by Alexander the Great, it was 

 known that the philosophers in that city 

 had been long in the habit of calculating 

 eclipses, which they could not have ac- 

 complished without a knowledge of the 

 true figure of fhe earth. Thales, who 



