Question

Tom can see the Sun because it is a light source. It gives out its own light. Neptune and Pluto are not light sources but Tom can see them when he looks through his telescope. Explain why Tom can see Neptune and Pluto even though they are not light sources

Answer 1

(a) What force keeps planets in orbit around the Sun?

The gravitational pull of the Sun. 

1 mark

(b) Give two reasons why it takes Pluto more time than Neptune to orbit the Sun.

any two from: 

its average speed is lower for most of its orbit

the pull of the Sun is weaker because it has less mass (force of gravity is less)

its orbit is longer accept - it travels further' or 'the orbit is bigger'

for most of its orbit it is further accept 'it is further from the Sun' from the Sun or 'further away'

2 marks

(c)

(i) Tom can see the Sun because it is a light source. It gives out its own light. Neptune and Pluto are not light sources but Tom can see them when he looks through his telescope. Explain why Tom can see Neptune and Pluto even though they are not light sources.

Light from the Sun reflects off Pluto and Neptune 

2 marks

(ii) Between points A and B, Pluto is nearer than Neptune to the Earth. Tom noticed that Pluto is not as bright as Neptune, even when Pluto is closer than Neptune to the Earth. Give one reason why Pluto is not as bright as Neptune.

any one from 

it is smaller than Neptune

it reflects less light than Neptune

it absorbs more light than Neptune

This is another question where you need to compare - you need to say 'smaller' rather than 'small' etc.