196 



LATITUDE AND LONGITUDE. 



diameter is so small as to be seldom noticed in calcu- 

 lations ; for the diameter of a planet may be considered 

 as a point in the circumference of a circle of which the 

 earth is the centre. 



Since it is clear from the foregoing figures that the 

 angles under which a star is seen are excessively small, 

 their diameters may be further considered as forming 

 part of the circle, and to contain as many minutes, 

 seconds, &c. as is measured by the arc which subtends 

 those angles. 



The apparent diameter of a planet, therefore, is in 

 the inverse ratio of its distance, or varies inversely as 

 the distance ; so that the apparent diameters and dis- 

 tances being known, the real diameters become deter- 

 mined (as explained at first.) 



By the term dip sometimes called depression of the 

 horizon, we understand "The effect produced on the 

 apparent altitudes of the stars in consequence of the 

 elevation of the eye above the surface of the sea, " 

 or we may define it to be " The angle contained between 

 the apparent horizon and the sensible horizon, lines 

 being supposed drawn from each to the observer's eye. " 



Let BAG represent a portion of the earth's surface, 

 AE the height of the observer's eye ; then is PEG 

 ihe sensible horizon, and EH, ER the apparent hori- 



Z 



H K 



zons ; and consequently the angles FEH or GEK 

 the depression or dip of the apparent horizon below 

 the sensible. 



Now since the altitudes of all celestial bodies, when 



