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## 'Perfect Eclipse' printed from http://nrich.maths.org/

### Why do this problem?

This problem gives a good opportunity to practise some trigonometry in context. Students have the opportunity to make decisions about how to model the situation and what diagrams to draw in order to address the questions raised. Students will need to take into account considerations of lower and upper bounds, and the effects of changing
measurements.

### Possible approach

Perhaps start with some discussion about the apparent sizes of the sun and the moon, as viewed from Earth, and how the very similar apparent sizes makes solar eclipses possible.

Explain that the task is to work out the apparent sizes of the sun and the moon. In order to display this, students could cut out circles to represent the sun and the moon.

Ask students to consider in pairs what information they think they would need in order to work this out, and then provide any information they ask for from the data in the problem or using

these information cards.

It may not be obvious to students at first that they can work out angles to the edge of the sun or moon, or how to use this to compare the apparent sizes of the sun and moon. The diagram in the Hint may help.

### Key questions

What assumptions do you need to make to model the situation?

Can you draw a diagram to represent the appropriate lengths and angles?

What difference does it make that the orbits involved are elliptical?

### Possible extension

Once students have had a chance to work out the necessary data for our moon, and drawn a diagram to show their findings, the problem provides

data for other moons so that students can investigate how rare a perfect eclipse is within our solar system. This task could be divided up between groups with each group looking at the moons of a different
planet, with everyone presenting their findings at the end of the lesson.

### Possible support

The problem can be scaffolded by discussing modelling assumptions together as a class and working out what sort of diagrams will be helpful - the image in the Hint could be used as a starting point for discussion.