Bringing together two sets of constraints

Focusing on the learners:

Distinguishing–eliciting–connecting. How to:

• distinguish between stars, planets and other astronomical objects
• make the scales of distances and times meaningful
• avoid getting buried in the history
• enliven the physics using the history

Teacher Tip: These are all related to findings about children's ideas from research. The teaching activities will provide some suggestions. So will colleagues, near and far.

Focusing on the physics:

Representing–noticing–recording. How to:

• make older theories intelligible and credible
• link how we know to what we know
• present a credible, yet lightweight, philosophy of physics

Teacher Tip: Connecting what is experienced with what is written and drawn is essential to making sense of the connections between the theoretical world of physics and the lived-in world of the children. Don't forget to exemplify this action.

Confusing the locations of: Earth and Moon; the planets in our solar system; other stars

Wrong Track: You can see the stars at night and the planets like Jupiter and Saturn are just there with them.

Right Lines: You can often see the planets in the night sky, but these are much closer to us than the stars. For example the Plough constellation (Ursa Major) is about 64–95 light years away (the different stars which make up the Plough are at different distances from the Earth, they just look as though they are in the same plane), while Jupiter is only about 600 × 10⁶ kilometre away when the Earth and Jupiter are aligned on the same side of the Sun.

Exploring ideas about science

This whole topic area of the solar system and beyond offers lots of opportunities for considering various ideas about science:

• Seeing how scientific ideas can undergo change with time. The classic example here is the total acceptance of the Earth-at-centre models of the solar system for hundreds of years before giving way to the Sun-at-centre view.
• Appreciating that science values simplicity and elegance in its theories and explanations. An obvious shortcoming of the Ptolemaic system (as compared with the Sun-at-centre view) was that it was so complicated.
• Recognising that scientists work within a broad social setting and that this can influence the manner and extent to which new scientific ideas are accepted. The opposition to Galileo and his Sun-at-centre ideas by the Church provides a powerful example of the way in which this can work out in practice.
• Appreciating that the science from ancient times is not necessarily simplistic and crude in its content and approach. For example, Ptolemy's model of the solar system was extremely sophisticated (and complicated) and lasted for so long simply because it generated predictions of the positions of bodies within the solar system with great precision.

A teaching story for Mars, as an example of a resource for ideas about science

Giovanni Schiaparelli was an Italian astronomer who, in 1877, first observed the canals on Mars. The features he observed included straight lines that joined in a complicated pattern. He called these lines canali, which means channels. However, the Italian word was mistranslated into the English word canals.

That, combined with the suspicious straightness of the lines, suggested artificial structures, and this created a furore. Much speculation concerning the possibility of intelligent life on Mars sprang up in the popular press.

This idea was given further substance by the American Percival Lowell's suggestion in 1892 that the reason for the existence of the canals is that they were constructed for the express purpose of fertilising the oases and that their geometric nature must have been due to some form of intelligent life. Not surprisingly, Lowell's theory caused an immediate sensation. It was highly controversial as others claimed that they could not even observe the lines suggested by Lowell and Schiaparelli. Nevertheless, the idea took root in the popular imagination reaching its height with H.G. Wells' epic tale The War of the Worlds.

Two scales in use at once

Teacher Tip: In solar system modelling activities it is important to be aware that the scale for the size of the planets is not the same scale as that used for the distance between them (although both the relative sizes and relative distances are to scale). If the scales were the same, the object representing Pluto would be about 1.2 kilometre from the Sun.

Forests are similar to galaxies

On walking into a mature forest you see a snapshot of tree development. There are seedlings, saplings, young trees, mature trees, dying trees, and decaying trees. From this collection, visible all at once, you can infer an ordered development, the life cycle of a tree. The auxiliary data that you need are a means of ordering the specimens, putting them in a time order: this is provided by the tree rings.

Looking out at the stars is just like walking into a forest. We can see a huge variety of luminous sources. Here the time-ordering principle is rather simple. As it takes time for light to travel from the stars to us, in any one snapshot the oldest stars at that moment are those which are furthest away.

The parallel reasoning goes like this:

Those trees with the greatest number of rings are oldest: the photons that have been in transit longest, and so have travelled furthest, come from stars that are oldest.

The conversion rate for trees is one ring every year. The conversion rate for light is one light-year of travel per year.

That makes getting the scale of distance, and the particular distance to any star, important, as our construction of the life-cycle of stars depends on it.

Avoiding embarrassment

Teacher Tip: The name of this planet always causes a great deal of mirth. Among the astronomical community the officially agreed pronunciation is your-un-us and not your-anus.

There's a good chance you could improve your teaching if you were to:

Try these

• explicitly modelling the scales of objects
• drawing attentions to the different scales used in representations
• engaging the imagination so that the very real difficulties in matching the phenomena to the models are evident

Teacher Tip: Work through the Physics Narrative to find these lines of thinking worked out and then look in the Teaching Approaches for some examples of activities.

Avoid these

• using diagrams where multiple scales are represented uncritically
• providing too much detailed history
• treating the different models as facts
• not separating what is observed from the explanations for those observations

Teacher Tip: These difficulties are distilled from: the research findings; the practice of well-connected teachers with expertise; issues intrinsic to representing the physics well.