LATITUDE AND LONGITUDE. 



196 



So that supposing as before, AB the real diameter of 

 a distant object, the eye being at C, then the angle 

 ACB will represent and express its apparent diameter ; 



now if the eye be further removed to D, its apparent 

 diameter will be expressed by the angle ADB, which 

 is evidently less than ACB. Again if the Real Diam- 

 eter AB, be increased to EB, its apparent diameter will 

 likewise increase, since the angle EDB is greater than 

 ADB. In order therefore, to ascertain the true altitude 

 of the sun's centre, we must add its semi-diameter as 

 seen from the earth, now since this varies with the dis- 

 tance, and that the earth and sun are always at different 

 distances, it has been found convenient in practice to 

 employ a mean semi-diameter of 16 minutes. 



In the Nautical Almanac also we find the sun's 

 apparent diameter for every seven days of each month. 



In regard to the moon's apparent diameter, it varies 

 very considerably during its revolutions round the earth, 

 and ig calculated in the Nautical Almanac to every 

 12th hour of each month, and the semidiameter is there 

 calculated as seen from the earth's centre : there is 

 also an increase, called the augmentation, in proportion 

 to the moon's altitude, for she is a semidiameter of 

 the earth nearer when in the zenith, than when in 

 the horizon, and this bears a sensible proportion to the 

 moon's distance from the earth's centre, the distance 

 being about 60 semidiameters of the earth. This aug- 

 mentation becomes insensible when applied to the sun, 

 the distance of the earth from the sun being almost in- 

 finite compared to the semidiameter of the earth. 



In regard to the Planets or Stars, the apparent semi- 



