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P. BIEFELD 
observation of the time of immersion by the students and myself. 
.No attempt was made to get first and second contacts noting 
only the time of immersion of the center of the planet. 
To have a check on the work Mr. Bannister, a last year’s 
student of Advanced Practical Astronomy, computed the occul- 
t at ion for Granville. A few statements concerning this are 
perhaps of interest. 
THEORY OF OCCULTATIONS 
An occupation of a fixed star, approximately an occupation 
of a planet, may be considered as a special case of an eclipse of 
the Sun, imagined so far away that its parallax and diameter 
may be taken equal to zero. Then the cone circumscribing the 
Sun and Moon in case of a solar eclipse becomes a cylinder in 
case of a fixed star or planet. The cylindrical shadow cast by 
the planet shining on the Moon is intercepted by the Earth, 
or better by a plane passed through the center of the Earth per- 
pendicular to a line passing through the center of the planet and 
the Moon, and of a linear diameter equal to that of the Moon. 
The intersection of the fundamental plane with the plane of the 
Earth’s equator plane forms the X-axis of a system of rectangu- 
lar coordinates of which the Y-axis lying in the same plane points 
to the north, x and y are then the coordinates of the point where 
the axis of the shadow pierces the fundamental plane, the center 
of the Earth being the origin; x and y being expressed in terms 
of the radius of the Earth as unity. 
The ‘^elements” for the prediction of the occupation as found 
in the American Ephemeris are then as follows: 
T, in G. M. T., is the time at which che planet is in geocentric 
conjunction in Right Ascension. 
H is the geocentric hour angle of the Moon and planet at this 
time. 
Y is the y-coordinate of the piercing point of the axis of the 
shadow cylinder with reference to the fundamental plane at 
that moment. 
x' and y' are the hourly variation in x and y. 
