urray); Royal Geographical
Society's _Hints to Travellers_, 5s. (R. G. S., Savile Row). &c.]

CHAPTER XXI.

TRANSITS AND OCCULTATIONS.

No book professing to deal with eclipses would be complete without a few
words of mention of "transits" and "occultations." A transit is the
passing of a primary planet across the Sun, or of a secondary planet
(_i.e._ satellite) across its primary, whilst an occultation is the
concealment of a star by the Moon, or of a secondary planet (_i.e._
satellite) by its primary. A little thought given to this definition
will make it clear that a transit is essentially the same in principle
as an eclipse of the Sun by the Moon--one body comes in front of another,
and the former conceals in succession parts of the latter.

Practically the word "transit" in this connection is more especially
applied to passages of the inferior planets, Mercury and Venus, across
the Sun, or of the satellites of Jupiter across the disc of Jupiter,
whilst the word "occultation" more particularly calls to mind the
concealment of a star (apparently a little body) by the Moon (apparently
a big body) or of a satellite of Jupiter (a little body) by Jupiter (a
big body), the star and the satellite in each respective case passing
behind the occulting body and being concealed for a shorter or longer
time. Commonly the occulted body will remain hidden for an hour or two,
more or less. In the case of Jupiter the satellites of that planet may
also, on occasions, be seen to undergo eclipse in the shadow cast by
Jupiter itself. An eclipse of a Jovian satellite is therefore on all
fours in principle the same as an eclipse of the Moon, caused, as we
know, by the Moon passing for a short time into the dark shadow cast by
the Earth. The conditions just laid down in respect of Jupiter and its
satellites also find a counterpart in the case of the satellites of
Saturn, but whilst these phenomena are incessantly occurring and visible
in the case of Jupiter, they are exceedingly rare in the case of Saturn
owing to its greater distance and the difficulty of seeing most of its
satellites because of their small apparent size.

Having regard to the circumstance that transits of Mercury and Venus
only happen at intervals of many years, it is not worth while for the
purposes of this work to devote any great amount of space to them. In
point of fact, whilst the next three transits