No. XVII. 



On the Construction of Eclipses of the Sun. By John Hum- 

 mer e. Read March 20, 1829. 



WE may, without diminishing the accuracy of the re- 

 sults, dispense with the description and division of an 

 ellipse, which are necessary in the usual method of project- 

 ing eclipses of the sun. and which render it so troublesome. 

 This is most conveniently done, hy supposing the sun's cen- 

 tre to remain fixed in the centre of the circle of projec- 

 tion, and giving to the moon a parallax in right ascension. 

 equal in magnitude, but opposite in direction, to the distance 

 of the projection of the sun's centre from the universal meri- 

 dian, at the time; and a parallax, parallel to the uniYersa] 

 meridian, or parallax in declination*, equal in magnitude, 

 but opposite in direction, to the distance of the sun's centre 

 from a plane passing through the centres of the Bun and earth; 

 perpendicular to the universal meridian. The figure to 

 which I shall refer, i< the construction of an eclipse of the 

 sun. that w ill occur on the l 2th of February 1831. It is 

 adapted to the meridian and latitude of Philadelphia. 



The semicircle ACI) represents the northern half of the 

 circle of projection. AC is a parallel to the equator; SI' 

 is the universal meridian; SL a circle of latitude; |'(J the 



* These quantities arc not, rigorously speaking, the moon's parallax in right as- 

 cension and declination, but it is convenient to call them o, 

 VOL. III. <) C 



