c 



Fig. 503. Right 

 angles. 



CHAPTER XXXIV. 

 ANGLES AND MEASUREMENT OF ANGLES. 



THE QUADRANT TRANSIT INSTRUMENT CLOCKS STELLAR TIME 



SOLAR TIME " MEAN " TIME. 



WE must say a few words respecting the various instruments and aids 

 to astronomical observation before proceeding, for astronomy requires very 

 accurate calculations; and though we do not propose to be very scientific in 

 our descriptions, some little idea of the manner in which obsrvations may 

 be made is necessary. The first thing to see about is the 

 ANGLE. 



Suppose we draw four lines on a piece of paper, ab 

 and cd. These intersect at a point, m. We have then four 

 spaces marked out, and called angles. The four angles are 

 in the diagram all the same size, and are termed right 

 angles, and the lines containing them are perpendicular to 

 each other. 



But by altering the position of the lines (see fig. 504), we have two 

 pairs of angles quite different from right angles ; one angle, a' m c' ', is smaller, 

 while a m d' is much larger than the right angle. 

 The former kind are called acute, the latter obtuse 

 angles. We can therefore obtain a great number of 

 acute angles, but only three obtuse, and four right 

 angles around a given point, m. 

 The length of the sides of an angle have no effect on its magnitude, 

 which is determined by the inclination of the lines towards each other. We 

 now may consider the magnitude of angles, and the way to determine them. 

 For this purpose we must describe a circle, which is 

 figured in the diagram. But what is a circle ? A 

 circle is a curved line which always is at the same 

 distance from a certain fixed point, and the ends 

 of this line meet at the point from which the line 

 started. 



If we fasten a nail or hold a pencil on the 

 table, and tie a thread to it, and to the other end 

 cf the thread another pencil, we can describe a line 

 around the first pencil by keeping the thread tightly Fig< s5.-The circle, etc. 

 stretched. This line is at all points at equal distance from the centre point. 



Fig. 504. Obtuse and acute 

 angles. 



