Planetarium

3751

Plantain

Primary planets are further classified as lin-
ferio' when they revolve inside the earth's
orbit. Such are visible only in morning or
evening twilight; they are bound to the vi-
cinity of the sun; the angle of maximum elon-
gation can in no case reach 90°. Their appar-
ent motions are direct from w. to e.—at su-
perior, retrograde at inferior conjunction;
while at elongations, their velocities being
radially directed to or from the earth, they
seem stationary. The superior planets show
retrogradations only when nearly opposite
to the sun, while stationary periods mark
the limits of each 'arc of regression.' Owing
to the inclinations of their orbits to the

—an artificial inverted hemispherical dome—
in which are prajected the sun, moon, planets,
and stars. It can be so operated that any and
all apparent motions connected with ordinary
or extraordinary astronomical phenomena of
the heavens can be shown. In 1937, due to
the tremendous cost of these planetaria, the
projector alone selling at about $150,000, only
four cities in the United States support one—
Chicago, Philadelphia, Los Angeles, and New
York.

Plankton. A term applied collectively to
all those animals which swim about near the
surface of any body of water, as the sea or a
lake.

Planetarium, New York City. General view of building. Inset—Giant Projector

ecliptic, the planets do not simply return
upon their own tracks in executing their os-
cillations in longitude, but pursue looped
paths, representing their simultaneous devia-
tions in latitude. Mercury, Venus, and Mars
rank with the Earth as 'terrestrial planets.'
They are bodies of the same order of magni-
tude of not very dissimilar density, and ad-
vanced geological age. The exterior planets,
on the contrary, are giants in she, and bear
the stamp of inchoate globes. They are of
slight consistence, possess profound and tur-
bulent atmospheres, and rotate swiftly, but
unequally, in drifts and zones.

Planetarium, a machine for representing
the motions and orbits of the planets. The
name is given specifically to an arrangement,
for creating mechanically the illusion of a sky

Plantagenet, Family of. The name first
appears in the rolls of Parliament in 1460,
having been adopted by Richardj Duke of
York, to express the superiority of his house
over that of Lancaster. The name, however,
is sometimes applied to the whole Angevin
dynasty, which occupied the throne from
1154 till 1485, and included Henry n., Rich-
ard i., John, Henry m., Edward i., Edward
n., Edward m., Richard n., Henry iv., Henry
v., Henry VL, Edward rv., Edward v., and
Richard m.

Plantain (Plantago), a genus of herba-
ceous plants belonging to the order Plantag-
inacese. The only species grown in gardens is
P. brasyliensis, which bears whitish flowers
in a dense cylindrical spike. Several species,
however, are common weeds.