Using demonstrations during lectures

Julieta Fierro, PASA, 18 (1), in press.

Using Demonstrations During Lectures

Julieta Fierro

Instituto de Astronomía, UNAM, Ap. P. 70-264, C.P. 04510, D.F. México.



I shall present a series of easy demonstrations that can be carried out during formal astronomy lectures. Since astronomy is physics we have developed on Earth applied to celestial objects, there are many useful experiments that can be carried out with relative ease by physics teachers in order to help pupils grasp the workings of the Universe and to relate them with every day experiences. One of the difficulties in teaching science is that we use bidimensional pictures to explain a three dimensional reality. Using models helps student understand some of the properties of celestial objects. Although it is important to have pupils experiment on their own some demonstrations are so simple that it is enough to have the teacher carry them out during the lecture to have pupils grasp their importance or just to keep their attention focused on the topic at hand.

The demonstrations can be used in elementary education as well as for introductory astronomy courses and teacher workshops


Keywords:astronomy education, demonstrations


1. Introduction

In general students have difficulty learning science. This is due to several reasons, amongst them is that science is a new language they have to learn how to cope with, sometimes they cannot make sense of what is being conveyed to them. Added to this is a general prejudice about its difficulty. Astronomy can be a mind broadening experience; it can convey scientific knowledge in a way that can be attractive to basic school level students. We should train teachers to help them teach science, in the references we point out several publications on teaching of astronomy.

One way in which we can help our students understand astronomy is by doing demonstrations during our formal lectures. This helps them: focus on the topic, recall what is fundamental, understand the basic physical principles, observe three dimensional models and consequently grasp better the idea to be explained. In other words demonstrations help make scientific language meaningful in a familiar way by presenting a simple concept which can later be increased in complexity and generality.

In the following paper, several very simple experiments that can be carried out during a formal astronomy lecture are presented; they must serve as a complement, each must be accompanied with the physical ideas and mathematical development pertaining to the subject, according to the level at hand. Many astronomy teachers have physics laboratory facilities available, so they can use equipment that already exists to carry out demonstrations during their lectures. Actually the idea is to repeat standard physics experiments but in the astronomical context, one must keep in mind that astronomy is the physics we have developed on Earth applied to the rest of the Universe.

A conclusion I have drawn from my lectures is, that one tends to believe our students have understood physics during basic courses and this is not necessarily the case, if one explains basic principles so they can understand them, they will internalize physics in a way that makes sense to them.

2. Demonstrations

2.1 The Earth is round


One can begin a lecture on our planet’s shape by pointing out that 2 000 years ago several Near East cultures thought the Earth was flat and held by elephants on a turtle surrounded by a snake. The Babylonians realized that during a lunar eclipse the Earth’s shadow is cast on the moon and they could compare their model with observations.

After this introduction one should encourage students to experiment with shadows by presenting them with different objects, including a small elephant, a sheet of paper and a ball so that they can realize that the only body that always projects a circular shadow is a circumference. One can show pictures of lunar eclipses to students and have them place a sheet in front of the figure so they observe that it is round and consequently understand how humans used this observation 2 000 years ago to discover our planet’s geometry.

One can proceed with the lecture by explaining how to measure the Earth’s circumference by using Eratosthenes’ method. A way to approach this is employing a long foam rubber (about 1 m long, 10 cm wide and 4 cm thick) and a few sticks (6 in total, 20 cm tall, 2 mm wide). One must insert the sticks along the foam rubber and place it on a flat surface to show students how the shadows of each of them would be equal if the Earth were flat. Now one must proceed to bend the foam rubber along its length so students realize that at noon since our planet is round the different sticks cast different shadows the larger the difference the smaller the circumference. If one has access to projection facilities one can show a slide of the Earth seen from space where half of it is in the dark and bend the foam rubber along its lit edge, to make the qualitative concept of the different shadows cast by obelisks easier to grasp.

One can subsequently mention that Mesoamerican cultures realized that the zenithal pass of the sun did not happen on the same date in their cities at different latitudes. They dug long narrow holes in several sites to measure this phenomenon. When the sunlight penetrated these deep holes the zenithal pass occurred. If one takes the sticks out from the foam rubber one can explain the procedure and mention how this is an alternative for measuring the Earth’s curvature.

Finally one can mention that Eratosthenes used an obelisk and a well to carry out his experiment that is equivalent to using a rod and a hole, a combination of the explanations given before.

The rest of the lecture can be carried out in the standard fashion using the equivalence of internal alernal angles and the distance between Sienna and Alexandria to quantitatively measure the Earth’s circumference. (Figure 1)


2.2 Translational periods of planets

When one lectures on the properties of the solar system one is tempted to present a table. To pupils this makes no sense, they do not realize that by studying a table similitudes and differences pop out immediately.

Students have trouble trying to relate to the different entries, this is a great chance to use analogies. For instance when it comes to describing the translation period of planets one can use a series of dolls, or pictures of peoples that have ages from 3 months, 7 months, etc. till 80 years old and have students try to imagine how they were and be at those ages. Then one can mention these are precisely the translation periods of the planets. (For periods of 150 and 247 years one can used dolls (or pictures) of people dressed in the fashion of the time.)

When it comes to mentioning why planets turn around the Sun in the first place one can do the classical experiment of making water spin over one’s head so that pupils understand that satellites are continuously falling towards the Earth and in the same fashion planets are continuously attracted towards the sun. Since students have seen this demonstration done many times, and I must insist, have not necessarily understood the physics behind it, one should carry it out once more. One way to do it is by placing a crystal cup holding a liquid on a velvet-covered board held from its edges with strings. With a little practice one can easily spin it without dropping the cup. This way of doing amazes them in such a way that they can willingly follow the equations that explain why the crystal cup does not fall. (Figure 2)



2.3 Extraterrestrial creatures


It is my feeling that one must treat students as intelligent persons and must provide challenges for them, if they do not understand science it may be a question of semantics, if we let them express science in their own words we will achieve a lot.

If we are discussing the existence of extraterrestrial life one can have pupils build a model alien that has adapted to certain extreme life conditions, for instance by living on a planet that is totally gaseous or has only a liquid surface. Once they have completed their task one must ask them how they would communicate with their creature. In my experience when pupils create aliens (for homeless children I have them build puppets) they rarely include ears so it is straightforward to talk about communication difficulties. Needless to say this exercise is a great introduction to the topic of extraterrestrial life. (Figure 3)


2.4 Center of mass and binary stars

Sometimes during a lecture mention is made of the fact that binary stars spin around their common center of mass. Since not all pupils have an intuitive idea of what this means the teacher can carry out the following activity. He will need a rod (about 40 cm long, 3 mm wide), a loop of string about 40 cm long, several balls of play dough (preferably two small red ones of the same size, larger orange and green ones and a large blue one) and a large one made of light material.

The teacher will place two small balls on the extremities of the rod and insert the loop in the middle and make the system spin. After, the teacher will substitute consecutively a small ball with larger ones, as can be guessed, the outcome will be that he will have to move the location where he places the center of the loop towards the heavier balls. He should also place the large light ball to show the location of the center of mass does not depend on the volume but on the mass. Using a heavy and light ball pupils understand why sometimes the center of mass lies inside the more massive object.

This demonstration is also useful to explain eclipsing binaries. Pupils can see how one star can eclipse another total or partially. (Figures 4 and 5)


2.5 Interstellar absorption

Several candles can be used to explain how, by comparing their apparent luminosity, one can estimate their distance, assuming all objects are the same. These candles can be later employed to show how another method of distance determination is measuring interstellar absorption. The instructor can place tinted viewgraphs amongst the candles to show how light intensity decreases by absorption and dispersion and consequently it is possible to estimate distances to stars assuming extinction is proportional to distance.

The teacher should mention the difficulty of calculating distances to extended objects such as planetary nebulae whose envelope varies in size. He can explain how, by comparing the extinction of stars of known distance, that are in its vicinity to that of the planetary nebulae, its distance can be estimate. (A model planetary nebula, made out of cardboard, can be placed amongst the candles to make the point even clearer). (Figure 6)



2.6 Telescopes

Some students do not understand that the main purpose of a telescope is to gather light. One can explain that all the light that comes into our eye does so by entering our pupil and that if it could expand to several centimeters our eyes would have a much larger gathering power. If the teacher places some funnels in front of his eyes students immediately grasp the idea of a telescope being an instrument capable of intercepting a larger amount of photons than our eyes.

If the teacher is explaining charged coupled devices he can use an ice cube tray to represent the chip and colored buttons to simulate photons, as they fall on the tray they create the image. The instructor can explain how a filter works by only using buttons of a particular color, or having pupils look at the buttons through colored cellophane paper. (Figure 7)


2.7 Chaos

Students of all school levels are interested in modern physics and one can find ways to explain it to them so that students get a general feeling about the problems it wishes to study. Such is the case of chaos.

One can alternatively toss a candy and a balloon and show how different their trajectories are, one is totally predictable and the other unpredictable. One can mention that each balloon’s trajectory is different so the physics of chaos is that of diversity.


2.8 Tablecloth


The goal of this demonstration is to make students realize that one science’s strengths is predictability. This is a classic experiment, but one must realize our students have not necessarily understood it. One needs a smooth square of fabric, about 70 cm per side, upon which one places several objects, for instance a lit chandelier, a vase with flowers and a cup of wine. One withdraws the "tablecloth" by pulling it firmly, downwards, with both hands. Of course the objets stay put due to inertia. This experiment invariably works.

After the experiment has been carried out by several students the teacher should mention how dependable scientific knowledge is, the objets do not fall if the tablecloth is pulled out correctly. When an astronomer claims there will be a solar eclipse in the year 2002, or that the sun will live for another 4 500 million years our students should perceive this is based on strong foundations developed during hundreds of years of solid research.

This experiment can also be used to explain inertia that is the reason the objects on the tablecloth don’t fall. (Figures 8 and 9)


3. Conclusion


It is useful and easy to carry out simple demonstrations during formal lectures. Teachers can deliver them in almost all astrophysical classes. This helps students to focus on the topic, makes them understand by witnessing everyday experience with three dimensional objects that applies to the rest of the universe. I must emphasize the fact that not all high school and college level students will become scientists so it is important to help them cope with the new language of astronomy in such a way that they understand how science works and learn to enjoy it.



Fraknoi, A., 1996, Astronomy Education: Current Developments, Future Coordination, ASP Conference. Series, Vol. 89


Gouguenheim, D. McNally D. and Percy J. R. 1998, New trends in astronomy teaching, IAU Colloquium 162, Cambridge University Press


Isobe, S., 1994, Teaching of Astronomy in Asian-Pacific Region, Quarterly Bulletin.


Pasachoff, J.M. and Percy J. R. 1990, The Teaching of Astronomy, IAU Colloquium 105, Cambridge, Cambridge University Press.



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