706 OCCULTATION OF VENUS. 



two occur in 1878, are but occultations of the Sun. The occultation of the 

 planet Saturn in August and September, 1876, was a rare astronomical 

 event. The immersion and emersion of the edge of the ball, and of the 

 ansa) of its ring, and the appearance ot the Moon's limb in the void space 

 between the ring and the great ball, were observed with marked interest 

 by astronomers. The reader may find an account of the Washington ob- 

 servations of this occultation as observed by Prof. Hall and others, in the 

 " Astronomiache Machrichten," No. 2107. He may also find accounts of the 

 occultation of Jupiter, January 2, 1857, in Month. Not. E. A. S., Yol. xviii ; 

 also of the occultation of Saturn, May 8th, 1859, Month. Not. R. A. S., Vol. 

 xrx. The occultation of the beautiful planet Uranus, was very carefully 

 observed at the Morrison Observatory, Glasgow, Mo., April 21, 1877. 



Moment of first contact or beginning of partial phase, 13A. 28m,. 43.5s. Gl. Sid. Time. 

 Moment of internal contact or beginning of total phase, 13/i. 28jn., 47.5s. Gl. Sid, Time. 



The external contact of emersion occurred at 14h. ISm. 50.5s., hence the 

 planet was entirely coneealed by the Moon, 50m. 3.5s. The occultation of 

 one planet by another planet than the Moon, is of exceedingly rare occur- 

 rence. Such was the occultation of Mercury by Venus, May 17th, 1737. 

 The occultation of a first magnitude star is an astronomical event nearly 

 as interesting as that of Venus. Four such stars lie near the Moon's path, 

 and are liable to occultation. They are Aldebaran, Eegulus, Spica and Au- 

 tares. The accurate observation of the immersion and emersion of all well 

 determined stars, furnishes to the astronomer the best purely theoretical 

 method of computing differences of longitude. In such observations the 

 notation of the precise instant of appearance or re-appearance, is of the 

 utmost importance. This notation is made by the Electro-Magnetic Chro- 

 mograph, and is instantaneous. The clock or chronometer error is ascer- 

 tained in the same way from njeridian transits of stars. Aside from the 

 error of personal ec[uation, the clock correction may tluis bo obtained to 

 the ino part of a second. 



Witli these preliminaries, I retuin to tlie occultation of Venus. The 

 evening at this Observatory was as favorable as possible. The sky was 

 cloudless, and temperature agreeable, being 37 Fahr; in the open air. Many 

 persons noted by watch the moment of disappearance. This was the only 

 phase distinctly cognizable by the unassisted eye. But in a large telescope 

 there were present several phases of very great interest. 1 will tr}^ to de- 

 scribe them, not for anstronomers, but for the young and the plain, practi- 

 cal people, who have shown so much interest iu the event. I shall speak 

 of the phenomena as they appeared in the field of a large refracting tele- 

 scope, under an amplifying power of 250. In such a field all the images 

 are inverted — up and down — right and left — east and west — north and south, 

 all exchange places. The diagrams, I send with this text, are intended to 

 show the phases, as seen in the field of the telescope. Again, since in a 

 large instrument, with even a moderately high eye-piece, only a very small 

 portion of the Moon's surface can be seen, while the whole risible surface of 



