Q19. In 1610, the Italian scientist, Galileo, observed four bright moons near Jupiter. Each night the moons had moved.

(a)

(i) The Sun and stars are light sources, and the planets are seen by reflected light. Explain how we can see the moons of Jupiter.

Light from the Sun  is reflected from the moons’ surfaces  and then travels through space to our eyes           

Do not accept ‘light from the Sun and stars’

The stars do not give out enough light to illuminate objects in our solar system that are a great distance from us - Under starlight you can bearly make out objects a few feet from you on Earth!

Do not accept ‘the Sun reflects off the surface’

Saying the Sun reflects off a surface means that the Sun bounced off it like a ball would! It is the LIGHT from the sun that is reflected!

3 marks

(ii) The four moons are approximately the same distance from the Earth. However, they do not have the same brightness. Suggest one reason for this.

• they reflect different amounts of light or some absorb more of the sunlight than others. Accept: 'they have different albedos' or a 'some are darker' or 'they are different colours'
• they are not the same size. The larger moons will reflect more light and look brighter.
• one could be partly in the shadow of Jupiter or another moon.

1 mark max

(b) The table below shows the distances of the four moons from the centre of Jupiter, and the times of their orbits. (Europa's distance has been left out).

name of moon	distance from Jupiter, in millions of km	time for one orbit, in Earth days
Io	0.42	1.8
Europa		3.6
Ganymede	1.07	7.2
Callisto	1.88	16.7

The graph below was plotted using the information in the table. Use the graph to estimate Europa's distance from Jupiter.

Europa is 0.68 million kilometres from Jupiter.

In questions like this be VERY precise! Carefully draw a dashed line from the axis to the graph line and then follow it through with a dashed line to the other axis.

Each little square on the y axis represents 0.4 Earth days and each little square on the x axis represents 0.04 millions of kilometres. Always double check your answer by checking that the number of squares to the next marked line add up correctly from your answer!

1 mark

(c) Galileo realised that Jupiter and its moons formed a model of our Solar System. In this model:

(i) what did Jupiter represent? the Sun (or our star - Sol)

(ii) what did the moons represent?the planets

2 marks

Maximum 7 marks