INTRODUCTION

A typical science course at the high school level includes some information on planets and their moons. For example, it is well-known that Jupiter has 16 moons and Saturn has 18 moons. Add to this the enthusiasm of the public in the collision of comet Shoemaker-Levy 9 with Jupiter in July 1994. This immediately raises the possibility of a collision of a comet with a moon of Jupiter. Due to this possibility a strange fact about these moons comes into the picture, that is some of them are prograde in nature and some are retrograde. Can these two types of moons pose any problems in teaching? The present situation in education leads us to believe that they can pose some problems. It is described below, in support of this answer.

Educators from many countries have observed that the Aristotelian ideas continue to persist among graduates, in spite of learning Newtonian mechanics in colleges also. This is evident, for example, in the fact that many students think that a tangential force acts on a body performing circular motion, instead of the centripetal force. So the greatest and global problem is how to get rid of the tangential force from the minds of students and how to impregnate the centripetal force instead.

Recent history of science education reform in the USA

In 1981, in response to growing concerns that the United States was falling behind the rest of the world educationally, the federal Secretary of Education created a national commission on excellence in education. This commission was charged with gathering data about the status of U.S. education compared to the rest of the developed world and to define the problems which would have to be faced to successfully pursue the course of excellence in education.

In 1983 this commission issued its report, A Nation at Risk, (Secretary of Education, 1983). The release of this book produced a flurry of activity by schools, political entities and professional groups representing various educational disciplines. These groups included, the National Council of Teachers of Mathematics, the National Governors Association and the National Science Teachers Association and others. By 1989, the American Association for the Advancement of Science (AAAS), a major American organization representing a broad spectrum of the sciences, produced its own call for an improved educational climate for science and engineering. Their book, Science for All Americans, attempted to produce a comprehensive expression of the scientific community as to what constitutes literacy in science, mathematics and technology (Rutherford and Ahlgren, 1990). The release of this report, coming from a credible, broad-based and nationally recognized organization of scientists and engineers produced a great deal of interest in the American press and calls came for developing strategies for action.

The Edinburgh Teaching Packages.

For many years, copies on film of photographs, both direct and through objective prisms, taken with the 1.2 m United Kingdom Schmidt Telescope, have provided teaching material suitable for universities and colleges (Brück and Tritton, 1988). Table 1 outlines the various types of application to which the photographs may be put. With additional data, some real physics can be injected into the exercises, allowing students to perform quite elaborate projects.

Uses for UK Schmidt Telescope Film Copies

Direct photographs

1. 1. Recognition of objects:

2. galaxies

3. minor planets

4. HII regions, SNRs (in external galaxies)

5. globular clusters (in the Magellanic Clouds)

6. 2. Statistics

7. star-counts, for various purposes

8. number-magnitude counts

9. star-galaxy counts

10. galaxies in clusters

11. 3. Changes in position (from more than one photograph)

12. precession

13. comet

Objective prism photographs

1. 1. Spectral classification:

2. coarse classification (of about 100 stars per film)

3. 2. Search for unusual objects:

4. emission-line stars

5. carbon stars

6. planetary nebulae

7. quasars

A limitation to such purely visual observations is in regard to photometry, where we have to make do with rather rough estimates of magnitude. Measuring the brightnesses or magnitudes of objects is a basic necessity in astronomy, but one that is, ironically, less easy to perform with students than it was ten or twenty years ago. Instruments that were once standard equipment and could be employed on the films – photographic photometers and microphotometers – have fallen into disuse as astronomers receive their data ready processed. For the brighter stars, down to magnitude 13 or 14, magnitudes may be estimated visually to about a fifth a magnitude. This is adequate, however, for our stellar statistics problems (e.g. Fig. 1).

The Dilemma of the Introductory Astronomy Laboratory

Were we meeting a century ago to discuss the state of astronomy education, we might have noted that remarkable changes were taking place in our field. The discipline, then regarded as a branch of geometry or mechanics, concerned itself primarily with the determination of positions in the heavens and the mapping of places on the earth. But with the advent of spectroscopy and the construction of large telescopes, astronomy was beginning to probe the how and the why of the heavens as well as the where and when. It was, in short, transforming itself into astrophysics, the study of the physical nature of the universe.

A century ago, we would have called for a change in the things we teach; and in fact there was such a change. When we look at the astronomy of the succeeding century, the material we now offer to introductory astronomy students at most universities and colleges, we see only a vestige of the earlier preoccupation with place and time. Judging by most textbooks, and by the course syllabi I have seen, most of us devote only a small fraction of our courses to astronomical coordinate systems, timekeeping, geodesy, and celestial mechanics. When we teach the solar system, we teach comparative planetology. When we teach the stars, we teach about main sequence and giant branch, about hydrostatic equilibrium and neutron degeneracy, about pulsars and supernovae. When we discuss the universe at large, we teach about the physics of the early universe, the dynamics of galaxies, and the fundamentals of general relativistic cosmology.

The primordial Earth

Four and a half billion years ago, the proto-Earth was completing its formation. During the last accretion phases, its growing gravity had increased the impact velocities, so that their energies had been transformed into more and more heat. Hence the proto-Earth became progressively covered with a thick layer of molten lava, possibly to a very great depth.

The large-scale differentiation that separated the denser iron core from the mantle of lighter silicates was triggered by this intense heat. The last large impact occurred somewhat later, notably the one which, by a tangential grazing collision, caused the appearance of a transient ring around the Earth that rapidly became the Moon. The smaller cometary impacts, however, persisted and ended by establishing, not only the atmosphere and the oceans, but also the minor differentiation that separated the terrestrial crust from the underlying mantle.

The chemical and isotopic evidence that the terrestrial crust formed so early on was an enigma for geologists. It seems to be resolved by the cometary bombardment, when chondritic silicates were plowed deeply into the surface of the Earth after the separation of the core from the mantle.

To understand cosmic evolution, it was necessary first to evaluate the immense times involved. It began with geology. To find the age of a rock, one method came out on top: that of measuring the time elapsed from the moment when a radioactive element was confined in the rock. Uranium-238 (238U) suits this particularly well, because it decays into lead-206 (206Pb) with a half-life of 4.5 billion years. This half-life is the time needed for half of the radioactive substance to decay. After two half-lives, there is only ¼ left; after three half-lives, ⅛, etc. This is what is called an exponential decay.

The ratio of 238U to 206Pb present in a rock is a direct measure of the age of solidification of this rock. When a rock solidifies, the radioactive clock is reset to start at zero, because there is no 206Pb in the uranium oxide crystal just formed (lead remains in the liquid state in the original magma or lava). The rate of radioactive decay is extraordinarily constant, and nothing short of destroying the rock can influence it. This stems from the fact that radioactive reactions call for much higher energies than do chemical reactions.

The oldest terrestrial rocks are 3.8 billion years old. NASA astronauts have brought back lunar rocks; the oldest of them are 4.1 billion years old. Most of the carbonaceous chondrites (coming from the asteroid belt) are all of the same age: 4.6 billion years to within 0.1 billion years.

Chirality is the property of those molecules that can exist into two symmetrical forms corresponding to mirror reflections, but cannot be superimposed on each other by a mere rotation in space. Left-hand and right-hand gloves are an example of chirality. Chiral objects must be three-dimensional, since two symmetrical plane objects can always be superimposed by a reversal in space.

Many of the molecules used by life are chiral. However, when they exist in non-living matter, most of the time one half is in the right-hand form and the other half is in the left-hand form. This is what is called a racemic mixture. In contrast, life nearly always chooses only one of these two forms. For instance, all proteins consist of left-hand amino acids, whereas RNA and DNA are always built up from right-handed sugars. When a living organism dies and decays, thermal fluctuations change molecular shapes at random, so that, in the long run, there is racemization. Since the opposite process does not exist, a mechanism was needed to trigger the emergence of life by selecting preferentially one of the two chiral forms. The continuity of life then becomes only a mere copying process.

Was the choice random? Two forms of life of different chirality could have emerged. Left-handed proteins could have eliminated righthanded proteins by a random evolutionary process. This matter does not seem fundamental for elucidating the origins of life, because all biochemical processes depend on chemistry; that is, on the electromagnetic interaction which is mirror-symmetric.

The plurality of inhabited worlds

Are there other worlds in the Universe that are inhabited by intelligent beings? This question has always fascinated thinkers and philosophers. In the absence of serious observational data, dreams and wishes nearly always prevail, and most answer yes to the question. The recurrent argument centers by and large on teleology: since the ‘reason for the existence of the Earth’ is to shelter the human race, the other planets would ‘serve no purpose’ if they were uninhabited.

In antiquity Lucretius said: ‘We have to believe that there are in other regions of space, other beings and other men’. In the sixteenth century, the Italian monk Giordano Bruno ‘explained’ the plurality of inhabited worlds as God's design and as the purpose of the infinite Universe; for Bruno had read Copernicus and rejected the ‘crystalline spheres’ of antiquity. He was burned at the stake only ten years before Galileo Galilei discovered the phases of Venus with his new telescope, establishing that planets are not stars, but are ‘worlds’ like the Earth since they reflect solar light like the Moon does.

When knowledge about the planets became less uncertain, the French man of letters Fontenelle published the famous Entretiens surla Pluralité des Mondes, in 1686. Later, the Dutch astronomer Christiaan Huyghens wrote Cosmotheoros on the same subject, published post humously in 1698.

Introduction

Even more than the Renaissance period, the twentieth century will be remembered in human memory as an extraordinary era in every regard. The awareness of our true position, and of our isolation, in an immense and mysterious Universe began nearly 400 years ago, but recently it has expanded enormously (see Figure 1.1 A and B).

At the end of the nineteenth century, we did not know where we were, or where we came from; we did not even realize that we did not know it. The vastness of space and time had always been thought to be beyond any possible observation or experiment; in a word, their study was considered to be a part of metaphysics. Metaphysics concerns everything that might exist, but which we have no means of detecting. In contrast, the physical world is made up of what we can see, touch, hear, taste and smell, i.e. observe. In this sense it can be said that angels are a part of metaphysics, whereas a chair is part of the physical world.

Over the last 300 years, we have invented new means of detection that have extended our senses and give them ‘feelers’. We have rolled back the limits of metaphysics more and more.

At the end of Chapter 1, it was mentioned that asymmetries in all the forces of nature disappear at extraordinary short distances, and that all force constants converge toward a single value.

Since the nuclear forces (strong and electroweak) are confined inside the atomic nucleus, the symmetry breaking of the forces must be produced by a phenomenon that takes place within the size of the nucleus (incidentally, this is what sets the nuclear size). The symmetry breakdown causes a change of state.

Changes of state, including familiar ones among solids, liquids and gases, imply a change in the symmetry properties. Ice forms crystals whose symmetry differs from that of water. Microscopic symmetries in the positions of atoms are not the same in liquid water and in steam. On the other hand, ice that turns into water absorbs heat without changing its temperature; it is the latent heat of the change of state. This latent heat arises from the entropy change coming from the symmetry change from ice to water. Moreover, while cooling, water often reaches a temperature below its freezing point without immediately solidifying: this is called supercooling.

Breaking of the original symmetry of all the forces has begun between rows 7 and 8 of Table E.1. About 10-35 seconds after the Big Bang, it can be assumed that the gravitational force had begun to fall in strength, followed by the decoupling of the strong nuclear force from the electroweak force.

Symmetries are features which are preserved after a specified operation. For instance, two figures are mirror-symmetrical when we can turn one of them over the other and check by transparency that it coincides with the original. We call this property an invariance of the mirror reflection.

In this case, there is invariance in the drawing when it is turned over. Circular symmetry arises from the invariance of the length used as the circle radius. Symmetry of the equilateral triangle comes from the invariance of the length chosen for the three sides.

It is possible to extend the idea of symmetry to time. An invariance in time is called a ‘conservation’. A mass that remains the same in the future as in the past comes from mass conservation, which expresses a time invariance, that is a symmetry when time flows. All conservation laws (of mass, energy, angular momentum, electric charge, etc.) are thus time symmetries.

An antisymmetry is also an invariance in absolute value, but with a change of sign. For example, a negative electric charge is antisymmetric to the same positive charge.

The symmetry properties of elementary particles, as well as those of the four distinct forces of nature, appear through particle interactions. These numerous symmetries are usually described as if they were geometrical symmetries in an abstract space of multiple dimensions.

A symmetry group is a set of properties that remain symmetric or antisymmetric over a specified type of operation.

The Universe grows old

Enormous stretches of time have elapsed since the Big Bang. The twilight of the first million years has been transformed into near complete darkness. The fossil radiation, a relic of the Big Bang, that was still dimly lighting the large cooling masses of gas, has diluted and shifted to the infrared, because the expansion of space goes on. This radiation soon becomes completely invisible. In the opaque night of the first billion years, the masses of gas become more and more patchy. This is because density fluctuations increased, and the aggregates of gas were more and more separated, first into superclusters, then clusters of galaxies, and finally into galaxies.

The ‘timeless night’ probably ends during the second billion years, because the quasars light up the central clusters of many galaxies. Their dazzling light hides the simultaneous appearance of many small bright dots that studded the galactic halos. The stars have just lit up, in the globular clusters and in the large central cluster of many galaxies.

The galaxies each evolve somewhat differently. They display a large diversity in sizes, and in angular momenta, which comes from the turbulence in the gas masses.

The origin of the biosphere

The biosphere is the ensemble of the life-supporting regions of the terrestrial globe. It is made up of the oceans and fresh waters, plus the atmosphere and the layer of soil (spread over the continents) that contains organic matter. Water is the predominant component of the biosphere, in which the atmosphere plays an important role, and where the many compounds of carbon are essential because they are needed by life. In the outer crust of the Earth, however, inorganic carbon is by far the most abundant component, in the form of carbonates (limestone, dolomite, etc.). Heat easily decomposes carbonates and frees carbon in the form of carbon dioxide (CO2); thus this process is the principal source of volcanic CO2.

For a long time, the origin of the biosphere remained a mystery, because no fossil trace exists from the first billion years of the Earth's evolution.