A Sky by Optical Projection

A Sky by Optical Projection

The idea of using optical projection to form a moving artificial sky came first to Walther Bauersfeld of Carl Zeiss Jena shortly after the end of World War I. “The great sphere (the planetarium dome),” he wrote, “shall be fixed; its inner white surface shall serve as the projection surface for many small projectors which shall be placed in the centre of the sphere. The reciprocal positions and motions of the little projectors shall be interconnected by suitable driving gears in such a manner that the little images of the heavenly bodies, thrown upon the fixed hemisphere, shall represent the stars visible to the naked eye, in position and in motion, just as we are accustomed to see them in the natural clear sky”.

In August 1923, after five years of unflagging toil, the new planetarium became a reality. The complex but compact projection instrument first underwent a long series of tests beneath a hemisphere 40 feet in diameter, erected on the roof of the Zeiss Works. So remarkable was its performance that everyone who heard about it was eager to see the Wonder of Jena. Many thousands had an opportunity of seeing it before May, 1925, when it was transferred to the Deutsches Museum, Munich, and where it can still be seen, although only as a Museum exhibit.

Astronomical Power. Illustration: Megan Jorgensen

A second planetarium instrument, similar to the first, was sent to Dusseldorf in 1926, and is now in the Planetarium of the Haagsche Courant, The Hague. The stars formed by each of the two Zeiss planetarium instruments are projected by 31 optical systems, each projecting a pentagonal or hexagonal shaped portion of the artificial sky. Ideally there should be 32 projectors, but one of the fields is cut up by the planetary projector framework at the southern end of the star globe. Some 4,500 stars are projected. They obtain their light from a single 500-watt light bulb mounted at the center of the globe, and before projection consist of small holes pierced in plates of copper foil. Connected to the star globe is a lattice-work cylinder containing mechanisms and projectors for the sun, moon, and five naked-eye planets. These projectors are actuated through suitable gearing by an “annual” drive which can make the planetarium sun circle the ecliptic in periods which range from a few seconds to several minutes. Once the sun’s image is set in motion, the images of the moon and planets go through their own individual and proportionate movements. Finally the star globe and planetary framework can be rotated slowly on a common axis (for precession of the equinoxes), and also on a polar axis (for the daily of diurnal motion).

The first Zeiss projectors had the disadvantage of showing the sky for only one particular latitude. In 1924 Walter Villiger of Zeiss introduced an instrument which could reproduce the sky as seen from any latitude. This “universal” as distinct from “fixed latitude” model had the general shape of a large movable dumb-bell, and after numerous modifications and additions, became the remarkable versatile Zeiss planetarium instrument of modern times.

Star Globes

Star Globes

Some historians regard early star globes or celestial spheres as forerunners of the modern planetarium. This is reasonable enough, since the motion of the seven wanderers (the Greek name for the Sun, the Moon and the five planets we can see naked-eye from the ground) can best be studied in relation to the background of “fixed” stars. The earliest extant star globe is that of the Atlante Farnesiano, a marble statue in the National Museum, Naples, Italy. The statue shows a kneeling figure of Atlas, some six feet high, whose broad shoulders support a celestial globe 26 inches in diameter.

Instead of showing stars, the globe has 42 constellation figures carved on its surface and raised lines for the celestial equator and the ecliptic, or apparent path of the sun. The arrangement of the figures relative to the lines suggests that the globe was made about 300 B.C.

A similar emphasis on constellation figures is found on later star globes, and also on star charts. To most early astronomers, constellation figures were an essential part of cosmography, or the charting of the heavens. The great Danish astronomer Tycho Brahe spared no expense to have them drawn on his large star globe, completed about 1595. They decorated the famous star atlas which John Flamsteed prepared in the following century, and almost overwhelm the stars in Bode’s large atlas of 1801. Indeed, they appeared on most popular star globes and charts right up to modern times.

Gottorp Globe. Gottorp Celestial and Terrestrial Globe, reconstructed. The Atwood Star Globe in the Museum of the Chicago Academy of Sciences

The Gottorp Globe

A bid disadvantage of a celestial globe is that any stars marked on it are seen from the outside and therefore in mirror-reverse to those seen in the real sky. Adam Oeschlager, court mathematician and librarian to Duke Frederick of Holstein-Gottorp, designed a hollow globe large enough for several people to sit inside and see objects painted on the inner surface. The globe made of copper and eleven feet in diameter, was prepared between 1654 and 1664 by Andreas Busch of Limberg, and assembled in the Duke’s castle of Gottorp.

The outer surface of this globe showed a map of the then-known world, and a wide horizontal circle enabled visitors to walk around the globe and examine the map in detail. The inner surface, lit by two oil lamps, showed gilded stars and constellation figures. Inside, a circular platform suspended from the axis of rotation held as many as ten people, and as the globe rotated, many stars and constellations drifted across the artificial sky in a way similar to that of the real sky.

In 1715 the globe was sent as a present to Czar Peter the Great, who in turn presented it to the Academy of Sciences of St. Petersburg. In 1747 it was so badly damaged by fire that only the axis and a few bars of a framework remained. It was rebuilt in 1778, given more up-to-grade features on both surfaces, and is now on display at the Lomonosov Museum, St. Petersburg.

The globes of Weigel, Long and Atwood

The next globes of this type were made by Erhard Weigel who, from 1653 until his death in 1699, was professor of mathematics in the University of Jena. One of them is said to have had a diameter of about eleven feet and to have been made of iron sheets. Weigel was proud of the fact that his artificial sky could be seen at all hours of the day and night, in sunshine and in rain. In the middle of the globe, above the observing platform, a small model earth added a nice touch of realism. The model earth contained working models of Aetna and Vesuvius which gave out steam, flames and “pleasant odours”. Meteors, rain, hail wind, thunder, and lighting could also be reproduced. If spectators experienced all these in succession they must have emerged with a greater sense of appreciation of the world outside.

One of Weigel’s smaller globes is now in the Franklin Institute, Philadelphia. It is about 18 inches in diameter and its stars are formed by little holes pierced in the globe. To see them the observer looks into the dark interior through one of several larger holes cut in comparatively starless areas.

Another similar globe was designed by Roger Long, Lowndes’ professor of astronomy at Cambridge, and erected in 1758 at Pembroke College, Cambridge, England. It had a diameter of 18 feet, could carry about thirty people on its platform, and had star represented by holes of various appropriate sizes. The whole device could be rotated by which and rackwork.

Long hoped that his “Uranium”, as he called it, would encourage popular interest in astronomy but attendances were poor and although a keeper was paid six pounds a year to keep the apparatus in good running order, it gradually fell into a state of disrepair. In 1874, no longer in use, it was broken up and sold as scrap metal.

A far more successful project was the globe constructed in 1911 for the Chicago Academy of Sciences after a design by W. W. Atwood, president of Clark University. This has a diameter of 15 feet and is still in a good state of preservation in the Academy building in Lincoln Park. Made of thin galvanized sheet-iron, it weighs only 500 pounds and rests at its equator on electrically driven rollers. The stars are formed by numerous holes, all carefully graded in size and properly positioned, and the sun is represented by a small electric light, movable along the ecliptic.

Orreries in the Ceiling

Orreries in the Ceiling

One of the most unusual orreries was that built by Eise Eisinga, a woolcomber of Franeker, near Leeuwarde, in West Friesland. Its construction, begun about 1774, took seven years. The mechanism, weight-driven and regulated by a pendulum, consists largely of cog-wheels in the form of oak disks and hoops fitted with iron pegs. It is all concealed. The pendulum moves inside a cupboard-bed, the weight hangs in an adjoining wardrobe, the main cog-wheel assembly is built into an attic, and the working parts of the model solar system are mounted in the double ceiling of a living room. Visitors in the living room therefore see the face of the orrery, about 12 feet in diameter, above their heads. The model planets move in eccentric slots in periods equal to those of their natural counter-parts. Each slot or orbit carries the symbols of the zodiacal signs, and each sign is subdivided, thus enabling the zodiacal positions of the earth and any naked-eye planet to be read off quite easily. Outside the orbit of Saturn a pointer, moving in another slot, indicates the date and the sun`s position in the zodiac.

On the south side of the ceiling Eisinga arranged dials for showing the day, the months, and year, the moon`s phase and position, and the times of its rising and setting. In a panel over the cupboard-bed is the face of an ingenious moving sky-chart from which one can see at a glance what stars are above the horizon at Franeker at the time of observation.

Ceiling Orrery. Source: object.com

In 1787, only six years after completing the planetarium, Eisinga had to leave his home and family and seek safety abroad. Against his will he had become involved in political differences and civil strife. His exile lasted eight years, during which time his wife died, his house was rented to strangers, and the planetarium became neglected. In 1796, however, he had it going again and was able to remain with it until his death in 1828. After his death the house was presented to the community of Franeker by King William III and the planetarium has been kept in working order ever since.

Another large ceiling orrery was constructed in the early 1920`s for the Deutsches Museum, Munich. Designed by Franz Meyer, chief engineer of the firm of Carl Zeiss, it occupied a circular room 37 feet in diameter. In the centre a sun globe, nine inches in diameter and suspended from the ceiling, contained a 300-watt light-bulb which provided illumination for the entire room. Smaller globes for the planets, suspended from electrically driven carriages, moved on elliptical rails with speed proportional to their natural velocities. The spectator went round the model sun on a moving platform located directly beneath the earth globe, and by using a periscope could watch the planets as they moved against a background of constellations painted on the walls of the room. Unfortunately, the orrery was destroyed during World War II, but modern versions of the ceiling orrery can be seen at the American Museum-Hayden Planetarium, New York, and the Morehead Planetarium, Chapel Hill.

Orreries

Orreries

The next major development came from England where Thomas Tompion, the famous London clockmaker, and his colleague, George Graham, made a working model of the earth-moon-sun-system. It is now in view in the Museum of the History of Science, Oxford. Between 1704 and 1709 Graham made a similar machine for Prince Eugene of Savoy, and of this John Rowley, another instrument-maker, made several copies. One of them later went to Charles Boyle, fourth Earl of Orrery, an association which inspired Sir Richard Steel, editor and essayist to christen the machine an “orrery”.

The Rowley-made orrery sent to Boyle is owned by the present Earl of Cork and Orrery, and now is on loan to the Royal United Service Institution, London. The mechanism drives a model moon around an earth which in turn revolves about a model sun and is enclosed in a twelve-sided ebony and gilt case 30 inches in diameter and about nine inches deep.

Orrery made by John Rowley for Charles Boyle, fourth Earl of Orrery. Source of the photo : ScienceMuseum.org

The fame of the orrery soon spread far and wide and encouraged several London instrument makers to construct similar machines. Many were large, complicated and expensive. For example, the “Grand Orrery” made by Thomas Wright in 1733 showed the motions of the moon, earth, and five then-known planets about the sun, and is said to have cost 1,500 pounds. It was housed in the Royal Observatory, Richmond, and is now on loan to the Science Museum. London.

The first American-made orrery appears to have been a simple wooden model built in 1745 by Thomas Clap, president of Yale College. This was followed by a much more elaborate machine, constructed in 1770 by David Rittenhouse, astronomer and horologist. His aim was not to amuse or amaze the layman who, as he bluntly put it, was “ignorant of astronomy,” but to use the machine as a teaching aid for those who wished know more about the structure of the solar system. The model planets, unlike those of most earlier orreries, moved in elliptical orbits, and the entire mechanism and face plate were mounted vertically in a handsome cabinet. The orrery is now in the Library of the University of Pennsylvania while another, similar in design and construction, is in the Library of Princetown.

Early Models of the Solar System

Early Models of the Solar System

In the 17th century, with the rapid development of the mechanical sciences, the first attempts were made to demonstrate by models the relative motions of the moon and planets on a sun-centered or heliocentric basis. In Paris, the Danish astronomer Ole Römer, famous for his discovery of the finity velocity of light, devised machines which showed the relative motions of four of Jupiter`s satellites, and the satellites and ring of Saturn. Another of his machines, completed in 1680, showed the motions of the planets about the sun, and of the moon about the earth. The gearing, hand operated, lay behind a vertical plate in which elliptical slots represented the orbits of the planets, while in each slot moved a spindle surmounted by a small model planet. This planetarium made a great impression in Paris and copies were sent to Persia and China.

Solar System

In 1682, Johannes van Ceulen of The Hague made a small planetarium to a design provided by the Dutch scientist Christian Huygens. In his design Huygens solved the problem of using only a small number of simple gear-wheels to drive model planets at approximately the correct proportionate speeds. The machine, now in the National Museum of the History of Science, Leyden, takes the form of an octagonal box about 26 inches across and seven inches deep. Under the action of a clockwork mechanism, the model planets move with uniform motions in circular slots cut in the copper faceplate.

The actual planets, however, move with non-uniform motions in elliptical orbits. As a first approximation to this requirement, Huygens arranged the circular slots with their centres displaced from the central sun globe.

When will we announce that other worlds really exist? Image: Close Up © Megan Jorgensen (Elena)