52 Transit of the Shadovj of Titan. 



TRANSIT OF THE SHADOW OF TITAN— DOUBLE 

 STARS— THE MOON— OCCULTATIONS. 



BY THE EEV. T. W. WEBB, F.E.A.S. 



• TKANSIT OF THE SHADOW OF TITAN. 



A total eclipse of tlie sun to the inhabitants of the earth of 

 course infers the passage of the shadow of the moon over the 

 face of our globe. In consequence, however, of our comparative 

 nearness to the sun, and the resulting breadth of his disc, the 

 cone which this shadow forms tapers so rapidly that its point 

 frequently fails to reach the earth : in annular eclipses it falls 

 short of it ; and even in the largest total ones it is but of small 

 dimensions, its section at right angles to the axis seldom at- 

 taining a breadth of 180 miles, though it may be greatly drawn 

 out in length if it falls among the shadows cast by the rising 

 or setting sun. The case is very different with the projection 

 of the shadows of the satellites of Jupiter and Saturn. The 

 suit's diameter is so much lessened to them from their greater 

 distance, that the cone of shadow is always prolonged far 

 beyond the surface of the primary, and a dark spot is formed 

 there every time that a solar eclipse takes place ; and while the 

 shadow of our satellite upon the earth would be barely visible 

 from the nearest planets with powerful telescopes, and from the 

 remoter ones would be quite imperceptible, the corresponding 

 phenomenon in the system of Jupiter, and, as it now appears, 

 in that of Saturn also as far as the largest satellite is concerned, 

 is sufficiently conspicuous to be witnessed by us with com- 

 parative ease. 



In the case of Jupiter, these shadow-spots have been 

 familiarly known, since Campani, the celebrated Italian maker 

 of refractors of long focus, first observed one in 1658;* but as 

 regards Saturn, they have been hitherto little noticed. This 

 has been owing in part to the exceeding distance and minute- 

 ness of the object ; for it is certainly something extraordinary 

 to contemplate the effect of a solar eclipse at a distance never 

 less, often much more, than 760 millions of miles ; but it is 

 quite as much due to the different arrangement of that planet's 

 system. The general plane of the orbits of Jupiter's satellites 

 differs so little from that of his own path, that the shadow of 

 three of the satellites invariably, and that of the fourth for the 

 most part, falls on his globe once in every revolution, and every 

 " new moon" there brings a total solar eclipse ; but the inclina- 

 tion of the whole system of Saturn to the plane of his orbit is 



* Ci/ch of Celestial Objects, 1. 171. Arago erroneously gives the discovery to 

 Cassini I. in 1661. 



