The word "
planetarium" has two
meanings.
1. An
optical instrument designed to
project an image of the
night sky onto the inside of a
hemisphere so that it may be
studied. Usually has the appearance of a
see-saw with a ball on each end. Each ball has a
plethora of
lenses on it which
projects the
artificial star field on to a surface.
2. A
building or
room containing said
instrument, usually containing many seats, as in a
theater so that an
audience may view the
star field.