INTRODUCTION TO ASTRONOMY. 



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in its revolution round the sun : it is therefore called the plane of the 

 earth's orbit; and the circle in which this plane cuts the signs of the 

 zodiac is the ecliptic. Let Jig. 9 represent such a plane, S the sun, 

 E the earth with its orbit, and A B C D the ecliptic passing through the 



Fig. 9. 



middle of the zodiac. Therefore the ecliptic relates'only to the heavens ; 

 but it is described upon the terrestrial globe to facilitate the demonstration 

 of a variety of problems in the use of the globes ; and besides, the ob- 

 liquity of this circle to the equator is rendered more conspicuous by its 

 being described on the same globe ; and the obliquity of the ecliptic 

 shows the inclination of the earth's axis to the plane of its orbit. But to 

 return to Jig. 8. 



The spaces between the several parallel circles on the terrestrial globe 

 are called zones ; that which is comprehended between the tropics is dis- 

 tinguished by the name of the torrid zone; the spaces which extend from 

 the tropics to the polar circles, the north and south temperate zones ; and 

 the spaces contained within the polar circles, the frigid zones. 



The several lines which, you observe, are drawn from one pole to the 

 other, cutting the equator at right angles, are called meridians. When 

 any one of these meridians is exactly opposite the sun, it is mid-day, or 

 twelve o'clock in the day, with all the places situated on that meridian ; 

 and, with the places situated on the opposite meridian, it is consequently 

 midnight. To places situated equally distant from these two meridians, 

 it is six o'clock. If they are to the east of the sun's meridian, it is six 

 o'clock in the afternoon, because the sun will have previously passed over 



