FIGURE Of THE EARTH.] 



ASTRONOMY. 



919 



west, and are not seen again, until, having passed through 

 the lower hemisphere, they reappear in the east. 



Risiny and Setting. Cosraical, aohronical, and heliacal 

 rising and setting of the stars and planets, are phrases of 

 the old poets, who spoke of the phenomena in reference 

 to the rising and setting of the sun. A star or planet 

 rising and setting with the sun, was said to rise and set 

 cosmically. A star or planet rising at sunset, or setting 

 at sunrise, was said to rise and set achronically. A star 

 or planet appearing a little before the sun, in the morn- 

 ing, after having been so near him as to be hid by his 

 effulgence, was said to rise heliacally, and to set heliacally I 

 when it ceased to be visible after him in the evening, ou 

 account of its proximity to his orb. 



Apparent Solar Day is the time included between the 

 centre of the sun leaving the meridian of any place, to 

 its return to the same meridian again. 



It varies continually in length, owing to the unequal 

 motion of the earth in its orbit, and the obliquity of the 

 ecliptic, being sometimes more and sometimes less than 

 twenty-four hours. The greatest variation occurs about 

 November 1, when the solar day is 16' 17* less than 

 twenty-four hours, as shown by a well-regulated clock. 



Mean Solar Day is the time which would elapse be- 

 tween consecutive returns of the sun to the meridian of 

 any place, if moving in the plane of the equator, with an 

 equable motion. It is the mean of the true solar days 

 throughout the year, and consists of twenty-four hours, 

 as measured by a time-piece, which, on some days of 

 the year, is as much faster than the sun-dial, as on other 

 days the sun-dial is faster than the time piece. 



Sidereal Day is the time which elapses between con- 

 secutive retuias of any fixed stars to the same meridian, 

 or, in other words, the period which the earth takes to 

 accomplish one rotation on its axis. This period is un- 

 \aryinc; and immutable 23 hours, 56 minutes, 4 seconds 

 which would always be the length of the solar day, if 

 the earth stood still in space, and only turned upon its 

 axia. 



In comparison with the immense distance of the stars, 

 the diameter of the earth's orbit is but a point ; and, 

 consequently, in relation to them, the diurnal rotation 

 is performed precisely the same as if our globe had no 

 translation in space. This is not the case in relation to 

 the sun, a nearer neighbour ; and owing to the earth's 

 change of place, somewhat more than one diurnal revo- 

 lution, or twenty-four hours, is required to bring the 

 sun round again to the same meridian. 



Astronomical Day is reckoned from noon to noon ; 

 and, consisting of the sanie length of twenty-fours in all 

 latitudes, is called a natural day. 



A rtificial Day is the time between sunrise and sunset, 

 and varies with the latitude of places. 



Solar or Tropical Year is the time which the earth 

 takes in moving in its orbit, or, apparently, the sun in 

 the ecliptic, from one equinox or tropic to the same 

 again, consisting of 365 days, 5 hours, 48 minutes, 49 

 seconds. 



Sidereal Year is the time occupied by the earth in 

 moving in its orbit, or, apparently, the sun in the 

 ecliptic, from a determinate point in relation to any 

 fixed star, to the same point again, and consists of 365 

 days, 6 hours, 9 minutes, 12 seconds. 



[The accompanying folio plate illustrates some subjects 

 mentioned in the preceding pages, in reference to the 

 history and technicalities of the science. Fig. 1 re- 

 presents an armillary sphere, showing the position of 

 the equator, the ecliptic, and other circles found drawn 

 on celestial globes. The general appearance of one 

 of them is shown in Fig. 2. Fig. 3 gives the names 

 and positions of the equator, poles, zone% &o., of 

 the sphere referred to in the foregoing definitions. 

 Figs. 4 and 5 afford an interesting illustration of the 

 zodiac, and northern and southern constellations, as 

 drawn and used by the ancient Egyptians.* At the 

 present day, the representation of each sign used by 

 astronomers, is almost identical with that figured in the 

 plate. ED.] 



CHAPTER II. 

 ON THE EARTH. 



THE earth, which to the eye of its inhabitants appears 

 an immense plane, stretching out to an indefinite extent 

 in all directions, is, by a variety of circumstances ap- 

 pealing to our reason and our senses, shown to be of a 

 spherical form. 



ON THE FIODRE OF THE EARTH. The notions of the 

 ancients on the figure of the earth were very uncertain 

 and vague. Xenophanes, who lived 500 or 600 years 

 before the Christian era, supposed the earth to be a plane 

 of indefinite extent, whose foundations were infinite in 

 depth. There appears to have been a great repugnance 

 to admit that a planet could remain suspended in space 

 as the earth is ; but as science progressed, and the art of 

 observation became more certain, the ancient astronomers 

 became well acquainted with its spherical figure. The 

 shadow of the earth, projected by the moon in an eclipse 

 of that body, clearly demonstrated its spherical form ; 

 and, on the other hand, the measures of its circumference, 

 which the Grecian and Arabian astronomers have re- 

 corded, indicate clearly that, when reduced to a common 

 modulus, their agreement is sufficiently striking to show 

 that they possessed some knowledge of its dimensions. 

 The natural pride of man, however, placed it in the 

 centre of the universe ; and according to the doctrines 

 of the Ionian school, the solid crystalline orb, to which 

 the stars were supposed to be attached, revolved around 

 it in the space of 24 hours. The celebrated Aristotle, 

 whose philosophy reigned for many centuries, was of 

 this opinion, but he supposed that the motions of the 

 gun, moon, and planets were performed with solid 

 heavens, but at a nearer distance ; and to explain their 

 proper motions, he considered that a presiding genius 

 was placed in each planet. 



The usual arguments brought forward in favour of the 

 spherical figure of tlie earth, are, that navigators sail 

 round it, setting out in an easterly, and returning in a 

 westerly, direction ; but a happy illustration is given in 

 the appearance of a vessel as it approaches the shore, or 

 in leaving a harbour, or a succession of steam-vessels 

 at sea. In the first case, we see at a distance the upper 

 parts of the vessel, and gradually the lower parts, till 

 finally the hull, wiiich is the most conspicuous part of 

 the vessel, in other circumstances, appears from beneath 

 the waters. In the case of leaving a harbour, the con- 

 trary appearance takes place. 



Those appearances arise from the convexity of the 

 water, between the eye and the object ; for if the surface 

 of the sea were a dead level, the largest objects would 

 be visible the longest. 



Other arguments are adduced : Upon the bosom of 

 the ocean, or in the midst of an extensive plain, the 

 boundary of vision is a well-defined circle, and this 

 circular horizon is a certain indication of the circular 

 figure of the body to which it relates. 



Navigators proceeding in the same general direction, 

 east or west, have arrived at the same point from whence 

 they have started. This enterprise, now so common, 

 was first undertaken by Ferdinand Magellan, who sailed 

 westerly from Seville, in Spain, August 10, 1519, passed 

 the extremity of the South American continent, entered 

 the Pacific, aud reached the Philippine islands, where he 

 was killed in a skirmish ; but one of his ships arrived at 

 St. Lucar, near Seville, September 7th, 1522. 



Voyages of circumnavigation demonstrate the con- 

 vexity of the earth, east or west, or that its form must 



. See ante, p. 859. 



