Tom Thumb Planetarium Easily Built from Odds and Ends (Oct, 1937)
Tom Thumb Planetarium Easily Built from Odds and Ends
By GAYLORD JOHNSON
SIMPLE PROJECTOR MAKES THE CONSTELLATIONS MARCH ACROSS A SCREEN IN YOUR LIVING ROOM
IF YOU ever attended a performance in a large public planetarium, you probably envied the lecturer’s ability to rehearse any part of the drama of the skies at will. Or perhaps you never have witnessed the march of the stars across a giant dome, and are anxious to see a man-made sky in action. At a cost of less than a dollar, you can assemble, from odds and ends, a midget planetarium that will put on a performance right in your own parlor.
When you have completed the simple projector described in this article, you can entertain your friends with an illustrated lecture that reproduces many of the big, $100,000 instrument’s astronomical stunts. You can show the nightly procession of the constellations from east to west across the sky. Then, as dawn begins to come, you can make the stars pale and fade out. Across the sky the sun travels along the path of the ecliptic, to set in the west, and in the twilight you can make a young moon appear, just as it does in nature. Your homemade planetarium will enable you to repeat this daily round for your audience as many times as they wish.
Then, simply by changing a little circular projection slide, you can take your audience upon a voyage from your latitude on the earth to the north pole and back to the equator, thence returning to your home location.
You also will be able to use your planetarium to show the seasonal changes in the zodiacal constellations, and how to use the Big Dipper in telling time as both a clock and a calendar.
The images of the night sky are projected on a screen or a light-colored wall from special lantern slides that you can produce photographically, and making these should be the first step in constructing the lilliputian planetarium.
The slides consist of celluloid camera-film negatives glued between one-inch microscope cover glasses. To make them, you will need a camera capable of photographing the circular drawings that accompany this article one-half full size. The films, when developed, will show white lines on a black field, and this is exactly what is desired for projecting pictures of bright stars on a dark sky.
The best kind of camera to use for this purpose is the kind with a ground-glass focusing back, and a long bellows extension. However, with one or two improvisations, you can use an ordinary roll-film camera.
Set the camera up on a rigid tripod or other stand so you can make adjustments without disturbing its position, then stretch a piece of thin tissue or tracing paper in the position normally occupied by the film. With an ordinary camera you will not be able to focus closely enough to get a one-inch image, so tape a reading glass, or other inexpensive magnifying lens, in front of the regular camera lens. This will act as a powerful portrait attachment. Adjust the bellows and move the black-and-white star drawings nearer to or farther from the camera until you get a sharp, one-inch image, then remove the tissue paper, insert a film, and photograph the set of drawings.
It is advisable to use ordinary “colorblind” film rather than panchromatic, because the black-and-white image then will be more contrasty. The ideal negative material is the so-called process cut film; it will give clear, black-and-white star maps. However, you will not be able to use it unless you have a camera equipped with plate holders. In photographing, it is well to cover up everything in front of the lens except the star map itself with black cloth to keep stray light out of the camera, where it would tend to fog the image.
It is not necessary to make the slides exactly one inch in diameter. This dimension is suggested because it is the largest size in which circular cover glasses are readily available. If you cannot get these circular covers, use square pieces of thin glass, and make your film negatives to suit. You will then have to alter the projection apparatus to fit the square slides.
The planetarium apparatus, which rotates the circular slides and projects the pictures upon a white sheet, is in two partsâ€”the slide rotator and the lamp house. The slide rotator can be built easily, as shown in the detailed construction diagram, from two pieces of plywood and a piece of wooden curtain pole.
The curtain pole is bored lengthwise with a one-inch auger, and smoothed up into a hollow cylinder with a rat-tail file and sandpaper. The disk is sawed out of quarter-inch plywood. Its center hole is fitted to the end of the hollow cylinder and glued in place.
The cylinder then is arranged to rotate very easily and smoothly in a felt-lined hole bored through the upright board. The driving crank is built as illustrated. It turns a small wood cylinder covered with a bit of rubber tubing, and this engages a flat ring of fine sandpaper glued to the back of the disk.
A spring made of hard brass wire keeps the slide carrier in contact with the rubber-covered cylinder on the driving crank. It also insures contact with three smooth nail heads, which act as bearings for the disk.
The construction of the lamp house offers no difficulty. I used a child’s lunch box bought for twenty cents. A porcelain lamp socket was mounted inside, and supplied with an ordinary frosted thirty-watt bulb. You can, of course, solder up a complete lamp house from tin or sheet brass, and make it large enough to accommodate a more powerful bulb. A brighter light would enable the apparatus to project a larger picture on the screen.
It is advisable to defer the mounting of the lamp socket in the lamp house until the slides and slide-rotating mechanism are complete, as you then can center the brightest spot in the lamp directly in the axis of the lens system, insuring an evenly illuminated screen.
To give the projected star pictures an illusion of realism, an artificial horizon is provided in the form of a mask. This is the sixth of the drawings that are provided for you to photograph. The negative, which will show a clear opening surrounded by a black area, is cemented to a square film of glass with the film side exposed. This piece of glass is fastened to a cardboard mask holder arranged to slide up and down in grooves.
The film side of the horizon mask should come into smooth contact with the surface of the circular slides; this close contact is necessary so that both the rotating slide and the mask can be brought into sharp focus on the screen.
From the mask a strip of stiff cardboard extends upward beyond the top of the lamp house. It acts as a handle, enabling you to frame the picture on the screen. It also is used to raise and lower the horizon during the imaginary voyage from the pole to the equator. A few pencil marks at the edge of the cardboard strip will enable you to put the slide in either of the two standard positions in which the picture is properly framed for the various slides.
The projection lens used in the model illustrated is made from two ten-cent magnifiers bought in the dime store. If you wish, you can use only one lens, but the picture on the screen will be smaller and not so brilliant. If you have any other projection lens (from a magic lantern or standard movie projector), by all means use it.
The lenses are glued into a cardboard tube, which slides smoothly inside of another tube attached to the front of the board in which the slide carrier rotates. You will, of course, have to adjust their exact position when you are ready to start the show, so as to focus the star images sharply in the screen.
In giving your lecture, remember that the stars on the screen must travel from right to left (counterclockwise) for slides showing the polestar and northern stars. With slides showing constellations in the zodiac, the stars on the screen must travel from left to right (clockwise). The star field, of course, travels east to west in both cases. The difference comes from the fact that the spectator views it looking north in one case and south in the other.
The writer has used the most ordinary, makeshift materials in order to demonstrate the ease of building a home planetarium which will duplicate some of the big instrument’s effects. However, if you have a turning lathe, and are used to working with metal, you can produce a much more finished job. Another possibility is to use an old magic lantern or toy movie projector as the basis for your planetarium. You may find it easier to alter such a machine so it will take the revolving slide holder than to build the complete projector yourself.
When you have completed the construction of the projector and have made the slides, you are ready to entertain your friends with a realistic pageant of star movements. Of course, unless they, too, are amateur star-gazers, they will be unfamiliar with the different constellations, and you will have to explain each slide to them as you show it. It is a good idea to prepare a little lecture like this, beginning it when you have placed slide No. 1 in position:
“Here, ladies and gentlemen, is the night sky on a clear, starlit evening (Start turning crank very slowly). The stars of the zodiacal constellations are traveling from east to west; new stars rise as others set.
“But now the night is over and the day is at hand. You see the dawn coming up in the east. It brightens, and here, peeping over the horizon, is the sun. The stars all fade out while it crosses the sky and finally sets in the west. Then the stars begin to appear again, and with them comes a crescent moon, which, in its turn, follows the sun over the horizon, leaving the stars to light the night again. (Change to slide No. 2).
“IF YOU were unable to recognize any I of the ‘star actors’ in the previous act of our little drama of the sky, this slide will enable you to do so (Turn cran slowly). Here you see the daily and yearly processions of the zodiacal constellations across the southern sky. Since the zodiac is the sun’s path, you were unable, in the previous slide, to see the constellations nearest to the solar disk, but here the sun is absent, so we view all the groups of stars in rotation. The month abbreviations at the top of the screen show when the constellations underneath them are directly south (on the meridian) at midnight. (Change to Slide 3 and frame picture with polestar halfway up the sky. Start turning crank slowly).
“Here you see the northern constellations revolving around the polestar. They do so because the earth’s axis points to it. In our latitude, about forty degrees north, the polestar is about halfway from horizon to zenith. But now let us get on the airplane of our imagination, travel to the north pole, and see what happens (Start sliding mask downward very slowly, still turning crank). As we go further north, the polestar rises higher and higher until, at the pole, it is right overhead.
“Now let us return to the equator (Start sliding mask upward, still turning crank). The polestar sinks toward the horizon as we go southward, until, at the equator, it rests directly upon it. (Change to Slide No. 4).
“YOU are now starting upon the 25,000-mile, round-trip journey which the polar axis of the earth makes among the stars. The present polestar no longer is the nearest to the earth’s axis. The axis finally points to Vega. That bright star will be polestar in 13,000 A. D. and will be succeeded by others, until Alpha Draconis will again occupy the position of honor that it did in 2800 B. C, when the great pyramid of Egypt was being so constructed that this star would shine down one of its passages. (Change to Slide No. 5).
“As I rotate this slide slowly, you can see that the line from the Big Dipper to the polestar really is the hand of a great twenty-four-hour star clock. Each quarter of its circuit is completed in six , hours. But you see that the Dipper also,. is a calendar, for, in addition, it revolves very slowly once every year.”