Science, Maths & Technology

### Become an OU student

An introduction to exoplanets

Start this free course now. Just create an account and sign in. Enrol and complete the course for a free statement of participation or digital badge if available.

# 2.4  Circles and spheres

Once the size of a star is known, it is possible to work out the size of any planet that transits in front of it. This is done simply by measuring how much light from the star is blocked by the planet.

But how much light is that? To work it out, you need to think about the cross-sectional area of a sphere.

## Activity _unit5.2.1 Activity 1  Cross-sectional shapes

The cross-section of an object is the shape you would see if you took a slice through that object. Can you work out the cross-sectional shapes of these solids?

1. What shape is the cross-section of this cube?

Figure _unit5.2.6 Figure 8  Cube with cross-section cut marked

a.

Square

b.

Diamond

c.

Rectangle

2. What shape is the cross-section of this cone?

Figure _unit5.2.7 Figure 9  Cone with cross-section cut marked

a.

Triangle

b.

Circle

c.

Square

3. What shape is the cross-section of this solid?

Figure _unit5.2.8 Figure 10  Prism with cross-section cut marked

a.

Square

b.

Diamond

c.

Hexagon