A. C. Twining on Occultations by the Moon and Planets. 17 
Now, as an illustration and confirmation of remark “third” in 
the preceding paragraph, it will be observed that if Mars is the 
occulting oe it presents an instance in which parallax of a 
terrestrial i 
the centre as to become small compared with the diameter, and 
shall therefore offer an unprecedented accuracy in the resulting 
oe: If Venus. were the planet the stellar transit as seen 
m one station might similarly occur (or be made to occur 
nearly at an extreme position, but then, as seen from the other 
semicircle of the disc, and at or near an extreme distance of 
th from the centre. 
Again, as an illustration and enforcement of the above “sec- 
ond” remark, it need only be observed that even so distant a 
| as Jupiter, by its slow apparent motion when nearing or 
eaving its stationary points in the heavens, might show a great 
' difference of times as the measure of a minute parallax. An 
7 this is equally true whether the terrestrial stations are so situated 
with respect to one another as to give a parallax, like that as- 
sumed in the diagram, normal to the planet’s line of orbital mo- 
tion, or situated relatively east and west in a manner to throw 
the parallax along that orbital line. Of the nearer planets, un- 
der a like favorable concurrence of circumstances, the same ma’ 
ing to which it would appear that Saturn, with his rings, oat 
be expected to occult some one star of not less than the nin: 
magnitude in each two years, on an average. Allowing this re- 
‘sult we find by a rough computation that Jupiter ought similar] 
ecult a star once in each year, Mars over four times in eac 
year, and Venus with a much greater frequency than either. 
3ut these numbers will be tly reduced if restricted to occul- 
_ tations which shall admit of observation in the hemisphere from 
which the sun is absent. For the present object it is sufficient 
that these phenomena ma expected at such moderate inter- 
vals as to justify the labor of exploring the planetary paths in 
advance and bestowing extraordinary attention upon those stars 
858. 
SECOND SERIES, Vor. XXVI, No. 76,—JULY, 1 
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