McLaughlin Planetarium

The McLaughlin Planetarium

(the planetarium was closed in 1995, this text has an historical value)

Royal Ontario Museum, Toronto

The planetarium at the Royal Ontario Museum is only one of Robert Samuel McLaughlin’s many philanthropic gifts. A McLaughlin Foundation finances postgraduate study overseas for about 40 Canadian doctors yearly; he has given generously to hospitals, the Red Cross, the Community Chest, the Boy Scouts, and to numerous civic projects in Oshawa, Ontario.

Mr. McLaughlin was born September 8, 1971 at Enniskillen, Ontario. In 1876 his father’s carriage-building plant was moved to Oshawa and the young McLaughlin served his apprenticeship in it as an upholsterer. When automobiles began to appear, Samuel persuaded his father to switch to auto production. The McLaughlin Motor Company was formed in 1907 with R.S. McLaughlin as president. In 1918 the company was sold to General Motors Corporation and Mr. McLaughlin was named president of General Motors of Canada. In 1945 he became company chairman. Mr. McLaughlin was a director of several corporations and he was Life Honorary Colonel of the Ontario County Regiment (Tanks). His gift of the planetarium contained only one stipulation: that it be the best. Visitors will agree this request was fulfilled.

Seeing Stars

Just over 200 years ago, James Ferguson, a humble Scots farm lad, made his first attempts to know the stars. “I went into a field with a blanket over me”, he wrote, “lay down on my back, and stretched a thread with small beads upon it at arm`s length between my eye and the stars.” He moved the beads so they covered various bright stars and then transferred their positions to paper. In this way he came to know and to love the stars so well that astronomy became his life-long interest.

In those old days star maps and books were few in number and known only to the educated. People who, like Ferguson, wished to learn something about the stars, had to study and interpret the night sky for themselves. Today, in contrast, several hundred people at a time can seat in comfort beneath an artificial sky and have its wonders described to them in clear straightforward terms.

This approach to an age-old science is provided by the planetarium, where on a great dome the night sky can be reproduced with such remarkable realism that spectators feel they are sitting outdoors on the clearest of nights. Some 3,000 stars shine overhead and the Milky Way spans the sky like a giant bridge of pearly light. At the touch of a switch, the sky turns slowly to bring into view more stars while others dip below the horizon. Perhaps the moon and one of the planets appear, perhaps a “shooting star” traces a lonely path, or polar lights flicker in the north. For the visitor, the auditorium is transformed into a “theater of the skies” in which the sun, moon, planets and stars are the actors and the vault of heaven is the stage.

The Zeiss Instrument

Most visitors to any planetarium are slightly overawed when they first see the Zeiss projector. This is not surprising. The massive dumb-bell looks like some double-headed monster of science fiction, especially when the lighting is low. This feeling is maintained even during a show, for there is still little to suggest that the stars and other objects on the dome are formed by the instrument at the centre. Yet, the performance of the instrument depends on the application of comparatively simple principles.

The Zeiss projector used in planetariums consists basically of a long dumb-bell supported by slender but extremely rigid steel struts. The two “balls” of the dumb-bell are hollow globes from which the stars are projected. The connecting lattice-work “bar” contains projectors for the sun, moon, and five naked-eye planets.

McLaughlin Planetarium building

Star Globes

Between them the two star globes project well over 8,900 stars, although only half of this number can be seen at any one time, the rest being hidden below the horizon. These stars are positioned relative to each other and graded in size to correspond to the stars in the real sky for our epoch. They include all the stars which can be seen by a keen eye on an exceptionally clear and dark night, a range which extends from the brightest to faint stars of magnitude 6.5. A visitor to the star theatre can see about 4,500 stars at any one time; in the real sky under good conditions he would see only 2,500 to 3,000.

The planetarium stars are the images of tiny holes punched in copper foil 0.015mm thick. The foil is the sandwiched between two glass plates to form what is known as a “star field plate”. To reproduce the different brightness of the stars in the real sky the holes range from 0.023 to 0.452 mm in 52 different sizes. Some of the star field planets also have small stippled areas to represent star clusters and nebulae visible to the unaided eye (e.g. the globular star cluster in Hercules, the Great Nebula in Orion, and the Clouds of Magellan). Illumination is provided by a 1500-watt lamp mounted at the center of each globe. Light from this is collected by 16 radially placed aspherical condenser lenses and passed through the same number of star plates with their associated f/4.5 Zeiss projection lenses.

The 16 projection lenses of each star globe together form a mosaic of 16 separate but completely integrated star fields. Each projector forms a pentagonal or hexagonal-shaped portion of the sky. Ideally the star field plates should be at the center of curvature of the dome. The fact that they are several feet away from the center means that their tiny holes have to be displaced by calculated amounts. Each star field plate has a built-in distortion, so to speak, which compensates for the effect of its distance from the center of the dome.

In earlier Zeiss Jena instruments only Sirius, the brightest star in the night sky had its own projector. This was partly because the projector was designed to illustrate the apparent displacement of a nearby star due to the diameter of the earth’s orbit (an effect known as annual parallax), and to the velocity of the earth in its orbit in relation to the finite velocity of light (ab effect known as the aberration of light). These effects can be illustrated with different stars. The brightest stars have comparatively small but bright disks and therefore look highly realistic. They possess their characteristic colours. Betelgeuse and Antares, for example, shine with a reddish light, Capella looks yellow, and Rigel and Spica have a blue colour.

A Moving Sky

The dumb-bell can be moved in three ways. The most important is rotation about a polar axis, or axis parallel to the axis of the earth. This reproduces the daily or diurnal motion of the stars and the regular succession of night and day. In nature the starry sky rotates once in about 24 hours. In the planetarium, the period is reduced to a maximum of 12 minutes, and is continuously variable down to about 30 seconds.

In another movement, generally referred to as the polar altitude motion, the dumb-bell is rotated on a horizontal east-west axis. This corresponds to a change in terrestrial latitude, and, like the diurnal motion, is continuously variable up to a maximum of 12 minutes for one complete turn of the dumb-bell.

The third movement takes care of the precession of the equinoxes. The term “precession” refers to a slow wobble of the earth as it rotates on its axis. The earth makes one complete wobble in 25,800 years, during which time a celestial pole makes one revolution about the corresponding pole of the ecliptic. One result of this is that the night sky will present a new face to people living in the far distant future. Different stars take turns in becoming the north pole star, and stars which for us rise slightly above the southern horizon will eventually disappear for thousands of years. Another result is that the verbal equinox moves along the ecliptic in a general east to west direction at a rate of about 1% degrees a century. To meet this requirement the dumb-bell is made to rotate on its long axis, and the precessional cycle can be reproduced in any period from one to 15 minutes.

Of course, during these movements the star images must be confined to the dome, otherwise they would run across the faces of spectators and spill themselves over the wall and floor. To achieve this each star field projector is fitted with a mechanical eyelid. As the projector approaches the level of the planetarium skyline a gravity-controlled shutter moves across the aperture of the lens and gradually reduces the brightness of the star images until they disappear completely. Similar shutters are also fitted to the approximately 20 individual star projectors.

Zeiss projector installed in Montreal Planetarium

Sun, Moon and Planets

Inside the lattice structure connecting the star globes are projectors for the sun, moon, and five naked-eye planets. The projectors for Saturn, the sun and the moon occupy the upper or northern cage; those for Mercury, Venus, Mars and Jupiter occupy the southern cage. The projectors are all doubles or paired, for the struts of the lattice, although thin, would otherwise cause appreciable dimming. Each pair of projectors is designed and aligned to produce a single, sizable disk of light. The image of Mars is red, Jupiter appears with cloud belts, and Saturn with a semblance of rings. Mercury and Venus do not show phases but are distinguishable in size and, along with Mars, have brightnessnes which are in relation to their distance from the earth and also to their phase angles.

The planetarium sun is formed by four projectors – two in tandem to produce the sun’s disk, and two to form a halo or aureole of diffuse light around the sun. The first two also contain glass prisms which reflect some of the light to two small supplementary projectors to produce the Gegenschein, or counter-glow, a nebulous patch of light diametrically opposite to the position of the sun.

The phases of the moon are formed by a projector which contains a concave mirror about which rotates a close-fitting hemispherical cap or shutter. As the cap rotates on its axis its projected edge gives rise to the moon terminator, or edge between light and darkness. In nature we see the sun from a moving earth. The sun therefore appears to move in a west to east direction against the background of stars, but the motion is by no means uniform owing to the slightly elliptical form of the earth’s orbit. 

The sun’s path, or the ecliptic, is inclined at about 23 ½ degrees to the celestial equator. This behaviour of the sun is well shown in the planetariums, for there the sun can be seen in a darkened sky along with the stars. The correct orientation of its path is assured by the design of the Zeiss instrument, the axis of the dumb-bell being at all times inclined to the polar axis at an angle of 23 ½ degrees.

In a planetarium the celestial drama is produced by a composite optical projector mounted at the center of the circular theatre. This giant instrument, made by the firm of Karl Zeiss, is shaped like a dumb-bell and contains nearly 150 individual projectors. The whole programme is operated by remote control from the lecturer’s console near the north wall of the theatre. When the dumb-bell with its projectors is made to move in one of several prescribed ways, the images on the dome move collectively to reproduce the general march of the stars and other objects across the face of the sky.

The projectors for the sun, moon and five naked-eye planets (Mercury, Mars, Venus, Jupiter and Saturn) can also be moved relative to the dumb-bell. When this is done, the corresponding optical images drift in a general west-to-east direction among the planetarium stars, and the spectator sees what is tantamount to a moving picture of the universe.

So great is the versatility of the Zeiss projector that it can faithfully reproduce the starry sky as seen from any place on earth and for any time – past, present, or future. In a matter of minutes the night sky for Toronto can be changed for that at the North Pole, where the stars neither rise nor set, or for one far south, where the Southern Cross and Clouds of Magellan rise high. With the earth as a footstool and time at one’s command, the effects of the passage of centuries can be squeezed into minutes. One can see the night sky of 14,000 A.D. when the bright star Vega will be the North Pole Star and the Big Dipper will no longer remain permanently above the skyline of New York. Or one can, in effect, go back 20 centuries to stand with the shepherds on the hills outside Bethlehem, and bridge even 40 centuries to see the stars familiar to Abraham as he gazed across the plains of Chaldea.

The instrument of the McLaughlin Planetarium (today closed) was the first Carl Zeiss Jena planetarium projector to have motion about a vertical axis. This was achieved by mounting its supports on a turntable which could be rotated either clockwise or anti-clockwise through 360 degrees by a variable speed motor. The turntable, and hence the entire instrument, runs on rubber wheels and has an associated sound-baffle system for noiseless operation. As the instrument rotates it can be made to project the letters of the four compass points on the base of the dome, so the problem of N.S.E.W. orientation never arises. By using this motion it is possible to demonstrate the aspect and apparent movement of the starry sky as seen from a spacecraft, a feature which enables spectators to escape more easily from the restrictions of the old earth-centered viewpoint.