16 A.C. Twining on Occultations by the Moon and Planets. 
Occultations by planets available 8 ee the sun’s hori- 
zontal pi 
employed in a parallel manner for the same purpose. But the 
latter phenomena—supposing them ae mores with the 
former—possess several remarkable adva 
First, In their greater frequency (if offectually - brought within 
observation 
n)- 4 
Second, In the probability that the phenomena may occur, at 
times, in a position of the planet near one of its stationary points, 
—so giving, by the slow apparent motion, an exquisitely muicro- 
metric measure of parallax 
. Third, In a great change of duration becoming the measure 
of only a small lateral parallactic change in the line of the star's 
transit behind the planet. 
On the other hand, it is true, we may have to meet the disad- 
vantage of an increased distance from the object whose parallax 
is required,—also of a presumptive irregularity in the planet’s 
- outline,—and also of the possible existence of planetary atmos- 
pheres having no definite boundary. 
In the following diagram let AB represent a portion of the 
planet’s gies path or orbit; ad the apparent relative line of 
motion of the star s viewed from a terrestrial station, and a’ d’ 
the similar path of the same star s’ as seen froma station far 
north of the former (the planet’s motjon being from A to B). 
_ The two circles represent the planet’s visible outline. The 
: —- — fe 0 Sage ost 1s supposed to be known by so 
average I observed near and at measu 
pearance 
eu mot no hoe 
} measure of the 
ybserve red et of of the Nie rotation ;—we then compute the 
TOXI Fesistiine eae its central path; and the ratio of A 
olar to. its equatorial diameter is supposed to be given by the — 
eS 
