OCCULTATION OF VENUS, 



709 



In the einersion, first came the internal contact of discs. See Diagram 

 5<ro. 4. This marks the close of the total phase. Occurring on the Moon's 



Fig 4 



bright Vimh, the time of its occurrence, or rather the instant of yzrsi( fi^At 

 from the rim of Yenus, could not be so distinctl}^ noted as the moment of 

 the preceding phases. I therefore used the time to change the position of 

 the dome and shutters, and noted neither the time of internal contact nor 

 the emersion of southern cusp. See Diagram ISTo. 5. The emersion of the 



northern cusp is shown in Diagram JSTo. 6. It was observed at 23/i. 58m. 

 15.27s. Gri. Sid. time, or at 6/i. 46m. 44.16s. Grl. mean time ; the duration of 

 the phenomenon, from first contact to the emersion of the northern cusp 

 being l/i. 9m. 59.75s. The clock error of the night of December 8, was 

 most carefully ascertained by meridian transits of seven standard stars — 

 the observations being reduced by the method of least squares. Hence, 

 any person so disposed, may confidently use phases two and three for com- 

 puting diff'erences of longitude from Washington or G-reenwich. For any 

 such I will add that the latitude of the dome of the Observatory has beoa 

 rigorously determined, and found to be 39° 16' 16.75" + 0.05". 

 Morrison Observatory, January 24, 1878. 



