1.7 Sizing moons

Figure 12 The Moon and Tethys, shown to correct relative scale

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This is an image of the Moon and Tethys, shown to correct relative scale.

Figure 12 The Moon and Tethys, shown to correct relative scale

To give you a feel for the sizes of some moons, have a go at Activity 2 below. There you will see we have provided images of eight moons, but not shown them to scale. All you have to do is guess the correct diameter for each moon (for markedly non-spherical moons we require the length of their long axis). If you get stuck, try consulting the moons data table from Week 1.

These are the moons we have chosen.

• Enceladus: the sixth-largest moon of Saturn and named after a mythological giant of ancient Greece.
• Deimos: the outer moon of Mars and named for the Greek personification of terror.
• Phoebe: thought to have been captured by Saturn’s gravity. It is named after a Titan of Greek mythology who was grandmother to Apollo.
• Iapetus: the third-largest satellite of Saturn, named after the Titan who fathered Atlas and Prometheus.
• Tethys: a medium-sized satellite of Saturn, named after a Titan and aquatic sea goddess.
• Pandora: a satellite of Saturn named after the Greek mythological figure who inadvertently unleashed all the evils of humanity from a jar.
• Prometheus: a moon of Saturn, named after the Titan who stole fire from the gods. Zeus then condemned him to have his liver eaten daily by an eagle for all time.
• The Moon: which goes by many names in different cultures; our own name derives from a Proto-Germanic word, mǣnōn.

Activity 2 Sizing moons – what do you know?

Timing: Allow approximately 20 minutes.

Select the answer for

Question 1a

here

Figure 13 Enceladus

What is the diameter of this moon?

504 kmWell done.

1062 kmNo. Enceladus is smaller than this.

1470 kmNo. Enceladus is smaller than this.

12.4 kmNo. Enceladus is larger than this.

 

a. 

504 km

b. 

1062 km

c. 

1470 km

d. 

12.4 km

The correct answer is a.

a. 

Well done.

Select the answer for

Question 1b

here

Figure 14 Deimos

What is the diameter of this moon?

3474 kmNo. Deimos is smaller than this.

104 kmNo. Deimos is smaller than this.

12.4 kmWell done.

213 kmNo. Deimos is smaller than this.

 

a. 

3474 km

b. 

104 km

c. 

12.4 km

d. 

213 km

The correct answer is c.

c. 

Well done.

Select the answer for

Question 1c

here

Figure 15 Phoebe

What is the diameter of this moon?

213 kmWell done.

1470 kmNo. Phoebe is smaller than this.

271 kmNo. Phoebe is smaller than this.

208 kmNo. Phoebe is larger than this.

 

a. 

213 km

b. 

1470 km

c. 

271 km

d. 

208 km

The correct answer is a.

a. 

Well done.

Select the answer for

Question 1d

here

Figure 16 Iapetus

What is the diameter of this moon?

1062 kmNo. Iapetus is larger than this.

3474 kmNo. Iapetus is smaller than this.

104 kmNo. Iapetus is larger than this.

1470 kmWell done.

 

a. 

1062 km

b. 

3474 km

c. 

104 km

d. 

1470 km

The correct answer is d.

d. 

Well done.

Select the answer for

Question 1e

here

Figure 17 Tethys

What is the diameter of this moon?

136 kmNo. Tethys is larger than this.

3474 kmNo. Tethys is smaller than this.

1062 kmWell done.

504 kmNo. Tethys is larger than this. As you’ve just seen, this is the diameter of Enceladus.

 

a. 

136 km

b. 

3474 km

c. 

1062 km

d. 

504 km

The correct answer is c.

c. 

Well done.

Select the answer for

Question 1f

here

Figure 18 Pandora

What is the diameter of this moon?

12.4 kmNo. Pandora is larger than this. As you’ve just seen, this is the diameter of Deimos.

104 kmWell done.

271 kmNo. Pandora is smaller than this.

3474 kmNo. Pandora is smaller than this.

 

a. 

12.4 km

b. 

104 km

c. 

271 km

d. 

3474 km

The correct answer is b.

b. 

Well done.

Select the answer for

Question 1g

here

Figure 19 Prometheus

What is the diameter of this moon?

136 kmWell done.

1062 kmNo. Prometheus is smaller than this. As you’ve just seen, this is the diameter of Tethys.

3474 kmNo. Prometheus is smaller than this.

213 kmNo. Prometheus is smaller than this. As you’ve just seen, this is the diameter of Phoebe.

 

a. 

136 km

b. 

1062 km

c. 

3474 km

d. 

213 km

The correct answer is a.

a. 

Well done.

Select the answer for

Question 1h

here

Figure 20 The Moon

What is the diameter of the Moon?

1470 kmNo. The Moon is larger than this. As you’ve just seen, this is the diameter of Iapetus.

1062 kmNo. The Moon is larger than this. As you’ve just seen, this is the diameter of Tethys

3474 kmWell done.

504 kmNo. The Moon is larger than this. As you’ve just seen, this is the diameter of Enceladus.

 

a. 

1470 km

b. 

1062 km

c. 

3474 km

d. 

504 km

The correct answer is c.

c. 

Well done.