Planetarium

A planetarium is an advanced educational device used to demonstrate the locations and movements of celestial objects. It is a sophisticated optical system that projects images of planets, the moon, and stars onto a domed ceiling, providing an accurate simulation of the night sky. The structure housing this instrument is also referred to as a planetarium.

Typically, a planetarium creates star images by focusing light through thousands of tiny holes in metal plates, arranged around two spherical structures: one representing the Northern Hemisphere stars and the other for the Southern Hemisphere. Separate projection devices display images of the moon and planets. Using motors and gears, the planetarium simulates the rising and setting of stars and the movement of celestial bodies along the ecliptic. It can also represent the sky as seen from any location on Earth at any time in the past or future. Additional projections depict phenomena such as eclipses, auroras, meteors, and other celestial events, as well as instructional tools like celestial coordinates and constellation outlines.

Another kind of planetarium utilizes a computer-controlled cathode-ray tube, similar to the picture tube in a television. The images of stars and planets are displayed on the screen of the tube and then projected onto a domed ceiling through a fish-eye lens.

Early Planetariums

In the past, early planetariums consisted of either portable painted depictions of the starry sky displayed inside a sphere or dome, or mechanical models of the solar system. In late 17th-century Europe, small planetariums designed to replicate the planetary movements around the sun were created inside clocks, some even showing the moons orbiting their planets.

One of the first planetariums of this period, the Gottorp Globe, featured a portable starry sky painting and was located in what is now Germany. The main component of the planetarium was a hollow copper sphere 10.2 feet (3.1 meters) in diameter, containing a table and curved bench seating for 12 people. The inner surface of the sphere was adorned with constellation images, with stars represented by gold-coated copper nail heads that glowed when illuminated by a central oil lamp. A copper globe representing Earth was placed on the table.

In the early 18th century, an orrery, a model planetarium named after the Earl of Orrery, an Irish nobleman who commissioned one in 1712, was constructed. These miniature models continue to be used by science educators today to help students grasp the concept of planetary movements.

With the advent of electric lighting and motors in the late 19th century, constructing large orreries became a possibility. The first of these was installed in the Deutsches Museum in Munich, Germany, in the early 1920s.

In the center of a circular room stood a large illuminated globe representing the sun. Smaller lit globes symbolized the planets, suspended from the ceiling by rods. These rods were attached to motorized cars that followed "orbital" tracks around the central globe. Beneath the globe representing Earth was a small motor-driven platform for a person to ride on. As the orrery operated, the rider could observe a simulation of the planets revolving around the sun from Earth's viewpoint. Similar orreries were later built at the Hayden Planetarium in New York City and at the University of North Carolina at Chapel Hill.

Mechanical Projectors

The first modern planetarium was constructed at the Zeiss optical works in Jena, Germany, around 1924. This device, known as Mark I, was installed at the Deutsches Museum in Munich by the German company Carl Zeiss, and was housed within a dome 32 feet (10 meters) in diameter. The Adler Planetarium, built in Chicago in 1930, was the first major planetarium in the United States.

A concave metal sphere, called a star ball, utilized 31 lenses to project images of 4,500 stars onto the dome. Seven additional projectors attached to the ball displayed images of the sun, moon, Mercury, Venus, Mars, Jupiter, and Saturn. These projectors moved to replicate the solar bodies' movement relative to the stars. The images were illuminated by a bright electric lamp placed in the center of the ball, surrounded by the 31 lenses. Behind each lens was a disk, known as a star plate, functioning as a photographic slide. Light from the lamp passed through holes in the plate, each corresponding to a star. The combined projection of the 31 projectors on the dome created an image of the entire sky.

The Munich Planetarium, however, had certain limitations. Its view was restricted to Munich and other regions at the same northern latitude, meaning it could only display stars that were visible above the horizon at Munich's latitude. But with technological progress, upgraded versions of the Munich planetarium could now simulate the sky from any location on Earth and for any time period, even extending up to 26,000 years into both the past and the future. In these enhanced planetariums, which use two large star balls with a planet projector in between, stars look the same regardless of where you are in the solar system. However, the planets appear differently, as the solar system's size is much smaller compared to the vast distances to the stars.

The success of the Zeiss projectors led to the establishment of thousands of planetariums worldwide during the 20th century. In the United States, the first Zeiss projectors were installed in the 1930s at several locations, including the Adler Planetarium in Chicago, the Hayden Planetarium in New York City, the Fels Planetarium at the Franklin Institute Science Museum in Philadelphia, and the Griffith Observatory in Los Angeles. Japanese companies, including Goto Optical Manufacturing Company and Minolta Company Limited, as well as the U.S. company Spitz Incorporated, also became major producers of planetarium projectors in the late 20th century.

Today, the advanced mechanical projectors are capable of displaying images as bright and clear as actual stars. One such projector, the Zeiss Mark IX at the Hayden Planetarium in New York City, is capable of projecting over 9,000 stars. It uses an ultra-thin strand of glass called optical fiber to shine light on the dome, so small that the projected stars appear as pinpoints, resembling real stars in the sky. Separate projectors controlled by computer motors create images of the sun, moon, and planets.

Digital Projectors

Computers process information using numerical codes, and thus planetariums with computerized projectors are referred to as digital planetariums. Unlike mechanical projectors, which can only show stars from one region of space—our solar system—due to the fixed positions of the holes in their star plates, digital projectors don't have this limitation. Instead of using star plates, digital projectors rely on computers to generate images displayed on video screens. These images are then projected onto the dome by a lens. To simulate the stars as seen from different locations beyond the solar system, the computer simply updates the images on the screens.

In the early 1980s, the Evans and Sutherland Corporation from the United States launched the first digital planetarium at the Science Museum in Richmond, Virginia. Since then, digital planetariums have continued to advance and now offer far more realistic depictions of planets. These sophisticated planetariums are also capable of displaying images relevant to fields beyond astronomy. For example, they can simulate a journey through an extraordinarily magnified image of a living cell. Furthermore, astronomy-focused computer programs that can replicate sky conditions at any time of the day are also considered a form of digital planetarium.

Digital planetariums also provide entertainment shows. For example, they can be used to display animated, computer-generated art, enhanced with a musical soundtrack.

Portable Planetariums

While technologically advanced planetariums can be prohibitively expensive for most schools and communities, many have opted for affordable portable planetariums. For example, one model features an inflatable dome that can accommodate 35 children. The projector consists of a small light bulb encased in a plastic cylinder, along with a black cylinder equipped with clear spots that project stars. Additional affordable cylinders can project images of galaxies, constellations, and even the interior of a living cell.