The Primary Planets. 297 



If then we can measure either of the angles 

 A L C or O A L, which are in effect the same, 

 we shall have the moon's distance from the 

 earth. 



To effect this operation, take a graduated in- 

 strument DAE, having a moveable index with 

 sight-holes, and let it be fixed so that its plane 

 surface may be parallel to the plane of the equa- 

 tor, and its edge AD in the plane of the meridian. 

 So that when the moon is in the equinoctial, and 

 on the meridian A D E, she may be seen through 

 the sight-holes, when the edge of the moveable 

 index cuts the beginning of the divisions at O on 

 the graduated limb D , and let the precise time 

 when she is thus seen be carefully noted. Again, 

 when the moon has reached the sensible horizon 

 at O, let her be viewed in the same manner 

 through the sight-holes, and the time be precisely 

 noted, making proper allowance for the refrac- 

 tion. Then, as the moon makes her apparent 

 revolution from the meridian to the meridian 

 again on an average in 4 hours and 43 minutes, 

 deduct the time in which she passes from E to 

 O, from 6 hours 12 minutes, and then you will 

 have the time in which she describes the arc OL, 

 and this will enable us to measure the moon's 

 horizontal parallax, or angle O A L. For as the 

 time of the moon's describing the arc E O is to 

 90 degrees, so is 6 hours 12 minutes to the de- 

 grees of the arc D d e, which measures the angle 

 Ji A L, from which subtract 90 degrees, and 



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