Roland Szostak
"EAAE Summerschools" Working Group
University of Münster - Germany
Abstract
A new technique for overhead projector is presented, which offers great advantages for teaching astronomy in schools and for a good cooperation between planetariums and schools. It shows projected bright stars within the dark background of the sky and allows to study many details of motions in the sky. By working with polarized light and interference effects the projected sky can be converted into twilight and even into colors. One can also insert illuminated coordinates or movable celestial bodies into the dark sky and study the retrograde motion of a planet.
Introduction
The motivation for paying a visit to a planetarium is clearly enhanced by the knowledge of the phenomena, which are presented in a planetarium show. Especially for teachers it is desirable to come with well prepared classes, because their pupils will notice more details and understand better certain complex phenomena.
In order to enable the teacher to prepare such a visit in a very effective and attractive way, we developed a new kind of a mini-planetarium, which is extremely cheap and simple to use in the classroom. It displays the stellar sky by means of an overhead projector, generating a surprising touch 0f the atmosphere to be expected in a planetarium. It shows selected constellations, the diurnal rotation of the stellar sky, the rising and setting of the stars at the local horizon and the seasonal change of the night sky.
In a somewhat advanced version, which works with polarized light, teachers and pupils can simulate, for instance, the break of the dusk and the disappearance of the stars in the twilight. By simple additional means they can even convert the projected sky smoothly into satured blue or other colors by virtue of interference effects. They can also insert a bright moon and move it through the stellar sky. [n the same way they may insert a brilliant planet and guide it on a route typical for its retrograde movement. By simple supplements one can insert illuminated coordinates or the ecliptic line. [t is also possible to simulate the scenario of a lunar eclipse, having the additional opportunity of observing the shadow of the earth in space artificially. Star occultations are also available for manipulation.
The great advantage of these presentations is the fact, that the phenomena can be discussed in the classroom with all details and be repeated or modified due to the individual questions of the pupils. This facilitates a proper understanding and enhances the quality of the planned visit. It is expected that this tool for the teachers might be offered by planet aria to schools as a help for preparing the visit with remarkable advantages in understanding and motivation.
A simple display of stars by overhead projector
Usually a visit to a planetarium may be prepared in the classroom by talking about some astronomical phenomena, showing some slides or regarding a star map. The stellar sky may be also discussed by regarding an overhead projection. But due to the techniques, which are available, the stars are presented then as black dots within a white background for the sky at night, which 15 rather curious and irritating. 50 we reversed this presentation in a way, that the stars are bright spots on a dark background like in nature. These stars are little holes. made by a needle in a cardboard, which is put quite normally onto an overhead projector. In this case the cardboard prevents all light from being projected except for the light of these stars. This simple technique generates a surprising atmosphere, when it is presented in a perfectly darkened class room. The auditory will feel being very close to the magnificent spectacle in nature.
In the beginning one may reduce the number of the projected stars to a few popular and well known constellations, for instance the Big Dipper, the Orion, the Cassiopeia and the Lion. can be done by covering the stellar cardboard with a mask, which is simply a sheet or with holes for certain areas, which allow only selected stars to be projected (Fig. 3). These few constellations and their relative positions are registered by the eye very easily with high attention. Everybody feels familiar with this view at once. So the pupils learn how to certain stars in the night without any difficulty. When the selecting mask has been removed. the eyes recognize the constellations easily among all the other stars. Tests with pupils even down to an age of below 10 years have shown an excellent memory.
If this stellar cardboard is made as a relatable disc, one can show how the stellar sky rotates around the polar star due to the rotation of the earth. This stellar disc can be combined with the local horizon as known from planispheres. For this purpose the stellar disc is mounted retractably with a snap on top of a cardboard, which covers the light-emitting desk of the overhead projector and has a suitable transparent window for the local horizon. Then the auditory can persue the rising and setting of stars at the horizon (Fig. 4).
1 = backing platform, 2 = stellar disk, 3 = transparent window, 4 = rotatable axis
One also learns how the part of the stellar sky, which appears above horizon changes with the rotation of earth. One can also make these stars twinkle simply by putting a normal transparent sheet. which has an arbitrary pattern of lines. on top of the stellar disc. These lines randomly cover a part of the stellar holes. When this sheet is moved, the changing occultations induce a fluctuating scenery. This generates the impression of twinkling stars.
As this technique is cheap in all its parts, this new type of display can be afforded by schools. By virtue of low costs it can be also used in developing countries. So there is a good chance, that really many people can be reached by this presentation of the stellar sky. is star projection in the classroom can provide further successful steps for a good cooperation between schools and planetariums. -We have prepared this stellar chart as a kit, which can be made ready for use by the teacher and his pupils. It can be offered to the teachers who make an appointment for a visit to the planetarium. We have these kits able for delivering them on request.
Advanced technique using polarized light
The described new display of the stellar sky with bright stars on the dark background provokes some additional insight: Because of the correct presentation of the Dark sky for the the auditory is led to consider file role of the sun. As one full turn of the stellar sky represents a period of 24 hours, the day-time with its bright sky must be included in it. By this becomes evident to everybody's eyes, that the stars are present in file sky above us during day-time and that they are not visible only by the fact, that they are submerged into the floodlight of the sun, which is filling the atmosphere.
This process, how the stars submerge in file raising floodlight of the atmosphere, can also be displayed on the overhead projector by an advanced version of the new technique. One needs polarizing sheets, which cover a great part of the light-emitting desk plane of the projector. The one of these sheets contains a pattern of small holes due to the positions of the stars. When these two filters are in a crossed orientation, the displayed background of the sky will be dark, meanwhile the holes of the stars stay bright. If one now starts turning file second filter, which has no holes, slowly into the orientation of the first filter, then file background of the sky becomes more and more light and the stars fade away in the growing twilight. This photo shows a star map of this kind ; the upper filter is fixed in a rotatable ring.
By a more advanced technique one can also insert moving celestial bodies into file background of the sky. Such a celestial body, the moon for instance, appears as a bright object in this projection. It consists of birefringent material, which is inserted between crossed two polarizing filters. This birefringent material rotates the plane of polarization that this light is able to pass through the crossed second filter. For instance, scotch a birefringent material which acts already suitably in this case. If it is fixed on a optically inert carrier, then file bright moon can be presented traveling through the dark with its stars (Fig. 7).
This kind of display again is very surprising to the auditory, because the relations of dark and light parts are reversed to the usual way 0f the overhead But for the astronomical objects these are just in best agreement with nature.
For fun one may let travel also an illuminated rocket through the sky by this technique. For teaching it is particularly interesting to insert a planet and to let it travel through the positions of the fixed stars. So its retrograde motion can be simulated by the teacher an the pupils. This activates the discussion of the ideas of Copernicus in the classroom. All these notions will be shown to the pupils in the planetarium with much more perfection. But by the fact, that they have generated these phenomena with their own hands, their eyes will see more and understand much better during the performance in the planetarium. - For a further level of discussion one can also insert illuminated coordinates or other data into the background, using the same technique of birefringent material (Fig. 8).
Similarly one can show illuminated lines, which combine the stars to constellations. In a further extension of the birefringent technique, one can also remove the light of these connecting continuously, meanwhile the stars stay bright and the background dark (Fig. 9).
One more remark about the simulation of the twilight with polarizing filters as described above: This process can be presented on the overhead even much more impressive by turning this twilight into colors. This can be achieved by inserting a birefringent sheet, which by its suitable thickness generates interference for a certain range of wavelengths. Then the light which passes through these filters looks colored. By turning these filters adequately, the projected Dark background of the sky can be changed continuously into a saturated blue and into a light white-blue 0f the daylight. During this conversion the stars stay unchanged white, they only submerge again in the raising light of the background. These bright stars look very nice in the blue sky, indeed. This presentation is very attractive to the auditory.
By inserting shadowed structures the possibilities of the polarized display can be still further extended. So, for instance, the bright silhouette of the growing moon is only part of the spherical body. Starting again with crossed polarizing filters, only the bright silhouette is visible. It becomes very instructive to the auditory, when by a slight turn of the upper filter the background is converted into a dimmed grey and the shadowed part of the moon becomes also visible (Fig. 10).
By the shadowed part of the moon one can also demonstrate the process of star occultations. - In a similar way also the shadow of the earth may be made visible at the sky artificially. This is very helpful for demonstrating the conditions of a lunar eclipse in a new way, by which the auditory becomes aware that during a lunar eclipse one can see the shadow picture of our globe in the sky (Fig. 11).
