INTRODUCTION TO ASTRONOMY. ' 1 fifi SIT"! 



Let the earth now set off from its position in the summer solstice, ancl --? ^ . 

 carry it round the sun : observe that the axis must be always inclined in 

 the same direction, and the north pole point to the same spot in the 

 heavens. There is a fixed star situated near that spot, which is hence 

 called the North Polar Star. The earth at B has gone through one 

 quarter of its orbit, and is arrived at that point at which the ecliptic cuts 

 or crosses the equator, and which is called the autumnal equinox. The 

 sun now shines from one pole to the other, as it would constantly do were 

 the axis of the earth perpendicular to its orbit, the inclination of the axis 

 being now neither towards the sun nor in the contrary direction. At this 

 period of the year, the days and nights are equal in every part of the 

 earth, excepting at the very poles ; but the next step she takes in her 

 orbit involves the north pole in darkness, whilst it illumines that of the 

 south. This change was gradually preparing as the earth moved from 

 summer to autumn ; the arctic circle begins to have short nights, which 

 increase as the earth approaches the autumnal equinox ; and the instant 

 it passes that point, the long night of the north pole commences, and the 

 south pole begins to enjoy the light of the sun. As the earth proceeds 

 in its orbit, the days shorten and the nights lengthen throughout the 

 northern hemisphere, until it arrives at the winter solstice, on the 21st of 

 December, when the north frigid zone is entirely in darkness, and the 

 southern enjoys uninterrupted daylight. Exactly half of the equator, it 

 will be observed, is enlightened in every position, and consequently the 

 day is there always equal to the night. 



Observe that the inhabitants of the torrid zone have much more heat 

 than we have, as the sun's rays fall perpendicularly on them, while they 

 shine obliquely on the temperate, and almost horizontally on the frigid 

 zone ; for during their long day, the sun move* round at no great eleva- 

 tion above their horizon without either rising or setting ; the only ob- 

 servable difference is, that it is more elevated by a few degrees at mid-day 

 than at midnight ; but at the poles themselves, the sun travels round in 

 the course of four-and-twenty hours nearly at the same elevation from 

 the horizon, rising every day a very little higher from the vernal equinox 

 till midsummer, and declining after that period till the autumnal equinox, 

 when their long night begins. 



To a person placed in the temperate zone, as we are hi England, the 

 sun's rays will shine neither so obliquely as at the poles, nor so vertically 

 as at the equator ; but will fall upon him more obliquely in autumn and 

 winter than in summer. Therefore, the inhabitants of the earth between 

 the polar circles and the equator will not have merely one day and one 

 night in the year, as happens at the pole ; nor will they have equal days 

 and equal nights, as at the equator ; but their days and nights will vary 

 in length at different times of the year, according as their respective poles 

 incline towards or from the sun, and the difference will be greater in pro- 

 portion to their distance from the equator. During the other half of her 

 orbit, the same effect takes place in the southern hemisphere as what we 

 have just remarked in the northern. When the earth arrives at the 

 vernal equinox, D, where the ecliptic again cuts the eqilator, on the 22nd 

 of March, she is situated, with respect to the sun, exactly in the same 

 position as in the autumnal equinox, excepting that it is now autumn in 

 the southern hemisphere, whilst it is spring with us ; for the half of the 

 globe which is enlightened extends exactly from one pole to the other : 

 the sun rises to the north pole, and sets to the south pole. On the two 

 days of the equinox the sun is visible at both poles ; but only half of it is 

 seen from either, the other half being concealed by the horizon. 



