Planetarium
A planetarium (plural planetaria or planetariums)
is a theatre built primarily for presenting educational and
entertaining shows about astronomy and the night sky, or for training in celestial navigation.
A dominant feature of most planetaria is the large dome-shaped projection screen onto which scenes of stars, planets, and other celestial objects can be made to appear and move realistically to simulate
the complex 'motions of the heavens'. The celestial scenes can be created using
a wide variety of technologies, for example precision-engineered 'star balls'
that combine optical and electro-mechanical technology, slide projector, video and full dome projector systems, and lasers. Whatever
technologies are used, the objective is normally to link them together to
simulate an accurate relative motion of the sky. Typical systems can be set to
simulate the sky at any point in time, past or present, and often to depict the
night sky as it would appear from any point of latitude on Earth
The term planetarium is sometimes used
generically to describe other devices which illustrate the solar system, such
as a computer simulation or an ornery. Planetarium software refers to a software application that renders a
three-dimensional image of the sky onto a two-dimensional computer screen. The
term planetarian is used to describe a member of the
professional staff of a planetarium.
HISTORY
The ancient Greek polymath Archimedes is attributed with creating a primitive planetarium
device that could predict the movements of the Sun and the Moon and the planets. The discovery of the Antikythera mechanism proved that such devices already existed during antiquity,
though likely after Archimedes' lifetime. Campanus of
Novara (1220–1296) described a
planetary equatorium in his Theorica Planetarum, and
included instructions on how to build one. The Globe of Gottorf built around 1650 had constellations painted on the
inside. These devices would today usually be referred to
as orreries (named for the Earl of Orrery, an Irish peer: an 18th-century Earl of Orrery had one
built). In fact, many planetaria today have what are called projection
orreries, which project onto the dome a Sun with planets (usually limited to
Mercury up to Saturn) going around it in something close to their correct
relative periods.
Domes
Planetarium domes range
in size from 3 to 35 m in diameter, accommodating from 1 to 500 people. They can be permanent
or portable, depending on the application
Traditional electromechanical/optical projectors
Traditional planetarium projection apparatus uses a hollow ball with a light inside, and a pinhole
for each star, hence the name "star ball". With some of the brightest
stars (e.g. Sirius, Canopus, Vega), the hole must be so big to let enough light through that
there must be a small lens in the hole to focus the light to a sharp point on
the dome. In later and modern planetarium star balls, the individual bright
stars often have individual projectors, shaped like small hand-held torches,
with focusing lenses for individual bright stars. Contact breakers prevent the
projectors from projecting below the 'horizon'
However, the new breed of
Optical-Mechanical projectors using fiber-optic technology to display the stars
shows a much more realistic view of the sky.
Digital projectors
An increasing number of
planetaria are using digital technology to replace the entire system of interlinked
projectors traditionally employed around a star ball to address some of their
limitations. Digital planetarium manufacturers claim reduced maintenance costs
and increased reliability from such systems compared with traditional
"star balls" on the grounds that they employ few moving parts and do
not generally require synchronisation of movement across the dome between
several separate systems. Some planetaria mix both traditional opto-mechanical
projection and digital technologies on the same dome.
In a fully
digital planetarium, the dome image is generated by a computer and then projected onto the dome using a variety
of technologies including cathode ray tube, LCD, DLP or laser projectors.
Sometimes a single projector mounted near the centre of the dome is employed
with a fisheye lens to spread the light over the whole dome surface,
while in other configurations several projectors around the horizon of the dome
are arranged to blend together seamlessly.
Digital
projection systems all work by creating the image of the night sky as a large
array of pixels. Generally speaking, the more pixels a system can
display, the better the viewing experience. While the first generation of
digital projectors were unable to generate enough pixels to match the image
quality of the best traditional "star ball" projectors, high-end
systems now offer a resolution that approaches the limit of human visual acuity.
REF : Wikipedia
