Planetarium

1 The Armillary sphere

The journey of the stars around the Earth, setting in parallel and rising again, the relative progression of the bodies elucidated in the Timaeus - these concepts were part of the great Celestial Sphere suitably offset from the Sun to account for the geocentric position of the observer. The Armillary sphere proposed one such theoretical centre of the celestial sphere. According to Ptolemy this basic instrument consisted of several concentric circles - that is, two metal rings of equal size, joined at right angles, one of which was an ecliptic/zodiac ring marked into degrees. An outer mounted ring, set in the meridian with diametrically opposite pivots, supported these longitudinal rings. The plane-vertical shadow cast by two inner movable rings, in conjunction with the ecliptic ring, allowed the sun's longitude to be determined. The innermost graduated [slip]ring was fitted with pinholes and could be aligned with a celestial object.

2 The Astrolabe

The astrolabe was based on the planisphere, a stereographic projection where the celestial sphere is clipped against the plane of the meridian. Astrolabes were circular in shape, comprising:  i.the Mater;  ii.the Rete framework;  iii.the Alidade, or moving rule; and  iv.a set of interchangeable metal Tablets engraved with coordinate lines. The Mater which is graduated on one side in degrees and minutes was held in a vertical position and the centrally pivoted Alidade with diametrically opposite pinholes aligned with a guide star to obtain its current altitude. Showing the zodiac and coordinates of several guide stars, the Rete would be adjusted in combination with a latitude-specific Tablet to measure the time. At this point a number of meaningful celestial observations could be made.

3.1 Prague Astronomical Clock

The clockwork universe of concentric spheres, planetary epicycles, and deferents, that "imitates eternity and revolves according to a law of number" is based upon a cyclical perception of time. A variety of calendrical notations resulted from this measure of time typically delimited by 'geometric peaks' like summer solstice, the vernal equinox, and eclipse pairs.

A collaboration between the mathematician Jan Sindel and the clockmaker Mikulas of Kadan, the construction of the Prague Astronomical Clock began in 1410. A calendar disk was added in 1490 and the first puppet show in the 17th century. With a pinion of 24 teeth the axle produces 24 x 15 1/4 turns per day to mesh with three co-axial wheels. In one solar day the sun-wheel [366 teeth] rotates once, while the zodiac-wheel [365 teeth] advances by one tooth, or 4 minutes, and the moon-wheel [379] falls back by about 50 minutes.
Mounted on two hollow co-axial shafts in the centre of the dial (marked with a horizon boundary, twilight arc, and night circle) are the zodiac-ring of the rete and three pointers to indicate siderial time and the movements of the sun and the moon. The golden hand of the solar pointer moves over the outer ring marked with Bohemian hours - itself rotating around the fixed 3m-diameter dial to indicate the time of sunset. The pointer also carries a sun-emblem that moves radially in order to synchronize with the revolving rete. The moon-icon, half-silvered and half-black, rotates once in a synodic month.

3.2 The Clock Tower of Berne

The Zytglogge, or Clock Tower of Berne, dates back to the early fourteen hundreds. A new mechanism was constructed in 1527-30 for the dialwork of the west- and east elevations as well as a 3.6m-diameter astronomical dial. Designed by Caspar Brunner, the movement is powered by the pendulum, a combined drive-weight of 450kg, and a wheel-train of five gears.
The plate of the astrolabe is marked with gilded circles, i.e. the south horizon, the zone of night, the tropics, and the equinox. The revolving rete has the [Tierkreis] figures of the celestial signs described upon it. The pointing devices of this clock include a star-emblem for the day of the week and the sun-icon to tell the hour.

4.3 The Strasbourg Clock

An evolving cycle of dimensions, the projected revolutions of the philosopher's clock gave rise to a macrocosmic analogia, symmetrical harmony, and disposition of the parts, also explained in Descartes' mechanistic theory of the body. The tower-like structure of the Strasbourg Clock, a prototype located inside the red stone Cathedral, expressed the perfect circles of the universe using wheeltrains and a mechanically determined human form. It was replaced some two hundred years later with a design by Conrad Dasypodius, a professor of mathematics, who calculated the gears for a more precise movement. Constructed by Isaac Haabrecht, and measuring some 18m in height, the drive-weights of the clock turned various pointers, the armillae of a celestial sphere, and a carousel of allegorical figures.

4 The Orrery

The pursuit of mechanistic cosmological systems found their reflection in de'Dondi's Astrarium (1369) and the great astronomical clocks. Likewise, Stukeley's concept drawing (1705) for a three-dimensional model of the earth/moon/sun system, based on Copernican principles, and the sophisticated sphères mouvantes (1789-1801) developed by Antide Janvier rendered aspects of Cartesian and Newtonian space.

The orrery was introduced by English mathematical instrument makers to represent the planetary systems and their inclination to the axis of the ecliptic. Rowley's orrery (1713) had a geared heliocentric mechanism set in motion by the turn of a crank. Orreries featured a stationary brass ball to represent the Sun, a revolving Earth globe with a moon that displayed its phase, and a horizontal calendar ring to indicate elapsed time. Subsequent versions of the orrery demonstrated the [temporal] mechanics of the known solar system, incorporating assemblies of Jupiter and Saturn based on tubular stems and actuating cogs to turn their moons. One revolution of the winch turned the earth once and 365 1/4 turns reproduced its annual motion.

5.1 The Jena-Planetarium

An optical planetarium was developed by Carl Zeiss Jena during the 1920's. This simulation involved a large domed surface and motor-driven projectors to orchestrate a celestial field of view. The geodesic dome of the Jena-Planetarium in Germany - 23 metres in diameter so as to match projector specifications - consisted of 8000 steel members and thin-shell concrete capped with sheetmetal.

The Zeiss Model II planetarium (1926) permitted the display of stars and astronomical objects in a theatre environment. Placed in the centre, the pivoted steel axis of this machine supported individual planetary projectors and one spherical cluster of star projectors at each end. Every star projector, assigned to a particular region of the celestial sphere, contained a metallic starplate accurately perforated to imitate v magnitudes.

5.2 The Adler Planetarium

The Adler Planetarium (1930) on the lakefront of Chicago was designed by architect Ernest Grunsfeld to house a Zeiss II projector and numerous astronomical artefacts including the Atwood Globe.
A polygonal granite-clad structure defined by the receding elements of the pendentive, telescope terraces, and stairway - this building dedicated to the sky and decorated with sculpted signs of the zodiac also refers to a Mayan temple. Approached through a vestibule and foyer, the 392-seat theatre was surrounded by exhibition spaces, a lecture room, and a library.

5.3 The Griffith Observatory & Planetarium

The Griffith Observatory of Los Angeles (1935) was designed in the Art Deco style by architect F.M. Ashley. A plan view delineates the Entrance and the Main Rotunda where a Foucault Pendulum has been installed. The East Hall leads to the East Rotunda [meteorite exhibit], the South Gallery to the copper-domed Planetarium, and the West Hall [Tesla Coil] to the West Rotunda.
The cultural programme of the Griffith planetarium began with presentations of basic celestial phenomena. It then advanced toward the didactic development of contemporary astronomical data obtained, for example, from Palomar Observatory's 200-inch telescope. The console-operated Zeiss IV (1964) offered the lecturer more possibilities: a celestial perspective [past, present, future] could be gained from any station-point on earth and diurnal motion increased [by turning the projector about its polar axis] in order to compress one day into 1 1/2 minutes.

6 Planetarium Design

a) The orb of the star projector, surrounded by rows of seats and surmounted by the domed auditorium, constitutes a structural core of the modern planetarium which attracts facilities like exhibition halls, a lecture theatre, computer labs, and the library. These components may form part of a Museum, Technology Park, or Science Centre. The design of planetariums has combined the inner dome with various geometric shapes - pyramid, sphere, truncated cone - or incorporated it during the renovation of a historic building. A typical hardware configuration of the star theatre (apart from the central projector) includes computer graphics- and slide projectors positioned about the dome periphery, a sound system, the communications network, and a control panel to synchronize events.

The Universarium IX planetarium has one projector aggregate for the solar system and a 2200mm-diameter starball. Inside the sphere, which thirty motors turn and orient upon three axes, light flows through fiber optics, chrome-coated starplates, and [Tessar] portholes to the dome. A console allows coordination with other devices, e.g. laser projection and computer graphics, as well as rapid transition between eye-points, moments in time, and space flight perspectives. The fiber projectors of the starball can emulate atmospheric twinkling, or scintillation, and recreate the hues of certain stars.

b) An exemplar of high-tech architecture, The Rose Center for Earth and Space in New York is dedicated to the science of astrophysics. The museum design by architect James Polshek suspends an enormous aluminium sphere, weighing 2000 tonnes, inside a transparent cube comprising 736 glass panels supported by steel columns and slender wall trusses. A custom Zeiss star projector is situated in the upper half of the Hayden Sphere. The performance space below leads to the Cosmic Pathway. The Digital Galaxy database stores theoretical and observed astronomical objects for scientific- and content research.

Concept, Text, Coding (c) Marcel Ritschel, Sydney 30.01.2005