CHAMRERS'S INFORMATION FOR THE PEOPLE. 



fall on half the surface at once, reach only to the 

 arctic circle ; and the turning of the earth on its 

 axis has no effect to bring any part of the space 

 within that circle into the light. On the north 

 temperate zone, the sun's rays fall slanting and 

 with little effect ; and the part within the light at 

 any one time is small compared with the part 

 in darkness, which causes short days and long 

 nights. As the earth moves round in the direc- 

 tion of the arrows, keeping its axis parallel to its 

 original position which it does on the principle 

 of a spinning-top more and more of the northern 

 hemisphere comes within the light, until, at a 

 quarter of the circuit, when the axis is at right 

 angles to a line drawn from the sun, the rays fall 

 direct on the equator, and reach to both poles, 

 so that every part of the whole surface is twelve 

 hours in light and twelve in darkness. This 

 is the vernal equinox (22d March). In the con- 

 tinued progress of the earth, the effect of the 

 inclination of the axis is to turn the north pole 

 towards the sun, instead of from it, and bring 

 more and more of the northern hemisphere into 

 the hemisphere of light ; and when the globe 

 comes to the opposite point from where it started, 

 the inclination is directly towards the sun, and its 

 rays reach 23^ beyond the pole, so that the 

 whole of the arctic circle turns round in continual 

 day, and the temperate zone is longer in light 

 than in darkness it is midsummer (2 1st June). 

 The earth's progress through the remaining half 

 of its orbit has just the reverse effect on the posi- 

 tion of the northern hemisphere with regard to the 

 sun's rays ; at the middle of it there is another 

 equality of day and night the autumnal equinox 

 (22d September) and at the end, things are in the 

 position from which they started. It is evident at 

 a glance that the hemisphere around the lower or 

 south pole must undergo the same vicissitudes as 

 the northern, only in reverse order, the summer in 

 the one corresponding to the winter of the other. 



The earth is in perihelion on the 1st of January ; 

 it is then about 3,000,000 miles nearer to the sun 

 than on the ist of July ; the sun's disk is slightly 

 broader, and we might expect this circumstance 

 to mitigate the severity of our winter, and to add 

 to the heat of summer in southern latitudes. But 

 owing to the action of the law already given, by 

 which the velocity of a planet increases with its 

 nearness to the sun, the earth passes over the 

 perihelion half of its orbit in less time than over 

 the other half, and thus the effects of greater 

 proximity are counteracted. 



It is this real motion of the earth in its orbit 

 that causes the apparent motion of the sun in the 

 ecliptic already described. If we conceive a wide 

 circle, described outside the orbit, to represent the 

 sphere of the fixed stars, when the earth is in its 

 position marked 'winter,' the sun will appear to 

 be at a point in this circle beyond the earth's 

 'summer position; and as the earth moves to- 

 wards the vernal equinox, the sun will seem to 

 travel along the outer circle in the direction of 

 the autumnal. It is to this plane that the orbits 

 of all the other planets are referred ; they cross 

 it at small angles, and the points of crossing are 

 called nodes. 



A year, in the usual sense of the term, is the 



time that the s.un takes to move from either 



equinox back to the same equinox, or from either 



tropic back to the same tropic. This embraces a 



10 



complete circle of the seasons, and brings the 

 earth into the same position with respect to the 

 sun ; hence it is called a solar, equinoctial, or 

 tropical year. If the equinoctial points remained 

 fixed, this period would coincide with a complete 

 revolution of the sun in the ecliptic, or which is 

 the same thing of the earth in its orbit. But v 

 owing to a cause which it belongs to physical 

 astronomy to explain, the equinoctial points have 

 a slow backward motion on the ecliptic of 50" 

 annually ; when the sun, therefore, leaving the 

 equinoctial point Aries one spring, arrives at that 

 point next spring, he has yet 50" to travel before 

 he has completed a circuit among the stars, which 

 makes a sidereal year. The return of the sun to- 

 the same equinoctial point thus precedes its return 

 to the same point in the ecliptic ; and this fact is 

 known as the precession of the equinoxes. The 

 length of the equinoctial or tropical year is 365 

 days, 5 hours, 48 minutes, 50^4 seconds ; of the 

 sidereal year, 365 days, 6 hours, 9 minutes, 10*4 

 seconds. 



The equinoxes thus retrograde i in 71 '6 years ; 

 and in 25,868 years they will make a complete 

 revolution of the ecliptic. Celestial longitudes 

 being counted from the point Aries, are slowly 

 increasing : since the first catalogues were formed, 

 the longitudes of the fixed stars are all greater by 

 30. 



The Calendar. How the civil year, which must 

 contain an exact number of -whole days, is adjusted 

 to the natural year, which contains fractions of a 

 day, is explained in CHRONOLOGY. 



THE MOON. 



Next to the sun, the moon is to us the most 

 striking of all the heavenly bodies. Its disk is 

 almost equal to that of the sun, the mean apparent 

 diameter being 31' 7". The real diameter is 2153, 

 miles. 



The moon's orbit being elliptical, and the earth 

 in one of the foci, its distance varies to the extent 

 of 26,228 miles ; and this causes a corresponding 

 variation in its apparent diameter. The moon's 

 disk is thus sometimes larger than that of the sun, 

 so as to cause a total eclipse of the latter, when 

 it passes over it. The moon's path in the heavens 

 does not coincide with the sun's path or ecliptic,, 

 but crosses it at two opposite points, at an angle 

 of 5 9'. These two points are called the mooris 

 nodes. These points change their position, so as 

 to make a complete revolution of the ecliptic, in 

 a retrograde direction, in i8'6 years. When the 

 moon is nearest to the earth, it is in perigee; and 

 when at its greatest distance, it is in apogee. 



A month. The moon goes round the earth in 

 her orbit in about 27^ days ; and this motion 

 makes her seem to us to move eastward among 

 the stars at the rate of a little more than the length 

 of her own apparent diameter in an hour. The 

 time that the moon takes to make one complete 

 revolution round the earth that is, to return to 

 the same place among the stars is called a side- 

 real month or lunation. But while the moon is 

 performing this journey, the sun has also advanced, 

 though at a slower pace, in the same direction ; 

 and it takes the moon upwards of two days more 

 to overtake the sun, as it were, and get again into 

 the same situation with respect to that luminary 

 and the earth. When it has done so, it has 



