xliv 



INTRODUCTION TO ASTRONOMY. 



the effect of the centrifugal force, gravity was still found to be greater in 

 the polar than in the equatorial regions, owing to the spheroidal figure of 

 the earth. If then a pendulum vibrates faster at the poles and slower at 

 the equator, the inhabitants must regulate their clocks in a different 

 manner from ours, in which the pendulum vibrates once in a second 

 of time. The only alteration required is to lengthen the pendulum 

 in one case, and to shorten it in the other; for the velocity of the vibra- 

 tions of a pendulum depends on its length ; and when it is said that a 

 pendulum at the pole vibrates quicker than one at the equator, it is sup- 

 posing them both to be of the same length. A pendulum which vibrates 

 a second in this latitude is rather more than 39 inches in length. In 

 order to vibrate at the equator in the same space of time, it must be 

 shortened by a few lines ; and at the poles, it must be proportionally 

 lengthened. 



We shall now explain the variation of the seasons, and the difference 

 of the length of the days and nights in those seasons both effects 

 resulting from the same cause. In moving round the sun, the axis of the 

 earth is not perpendicular to the plane of its orbit ; in other words, its 

 axis does not move round the sun in an upright position, but slanting or 

 oblique. This you will understand more clearly if you carry a small globe 

 round a lamp or candle, whicli js to represent the sun Cfg- 10). You 



Fig. 10. 



must consider the ecliptic drawn on the small globe as representing the 

 plane of the earth's orbit ; and the equator, which crosses the ecliptic in 

 two places, shows the degree of obliquity of the axis of the earth in that 

 orbit, which is exactly 23 degrees. The points in which the ecliptic in- 

 tersects the equator are called nodes. The globe at A is situated as it is 

 in the midst of summer, or what is called the summer solstice, which is on 

 the 21st of June. The north pole is then inclined towards the sun, and 

 the. northern hemisphere enjoys much more of his rays than the southern. 

 The sun now shines over the whole of the north frigid zone ; arid not- 

 withstanding the earth's diurnal revolution, which may be imitated by 

 twirling the ball on the wire, it will continue to shine upon it as long as 

 it remains in this situation, whilst the south frigid zone is at the same 

 time completely in obscurity. 



