INTRODUCTION TO ASTRONOMY. 



SECTION I. The Earth's Annual Motion. 



JN attempting to give some general notions on astronomy, we shall not 

 begin by entering into an explanation of the system of the celestial bodies, 

 but select that portion which is most interesting to us, the earth ; and when 

 you have formed a distinct idea of the part which it performs in the 

 general | system, lead you from thence to form some conception of the 

 grandeur and immensity of the universe. Let us suppose the earth at its 

 p. , creation to have been projected 



forwards into universal space. 

 We know that if no obstacle 

 impeded its course it would 

 proceed in the same direction, 

 and with a uniform velocity for 

 ever. In Jig. 1, A represents 

 the earth, and S the sun. We 

 shall suppose the earth to be 

 arrived at the point in which it 

 is represented in the figure, 

 having a velocity which would 

 carry it on to B in the space of 

 one month ; whilst the sun's at- 

 traction would bring it to C 

 in the same space of time. According to the laws of motion you would 

 conclude that the earth would move in the diagonal A D of the paral- 

 lelogram A B D C as a ball acted on by two forces will do. But it 

 must be observed that the force of attraction is continually acting upon 

 our terrestial ball, and producing an incessant deviation from its course 

 in a right line, which converts it into that of a curve line ; every point of 

 which may be considered as constituting the diagonal of an infinitely 

 small parallelogram. 



Let us detain the earth a moment at the point D, and consider how it 

 will be affected by the combined action of the two forces in its new 

 situation. It still retains its tendency to fly off in a straight line ; but a 

 straight line would now carry it away to F, whilst the sun would attract it 

 in the direction D S. In order to know exactly what course the earth 

 will follow, another parallelogram must be drawn in the same manner as 

 the first ; the line D F describing the force of projection, and the line 

 D S that of attraction ; and you will find that the earth will proceed in the 

 curve line D G drawn in the parallelogram D F G E : ancl if we go on 

 throughout the whole of the circle drawing a line from the earth to the 

 sun to represent the force of attraction, and another at a right angle to 

 it, to describe that of projection, we shall find that the earth will proceed 

 in a curve line passing through similar parallelograms till it has com- 

 pleted the whole of the circle. It will then recommence a course, which it 

 has pursued ever since it first issued from the hand of its Creator, and 

 which there is every reason to suppose it will continue to follow as long 

 as it remains in existence. It affords an example, on a magnificent scale, 

 of the circular motion which was taught in mechanics. The attraction of 



