3.2 · Why the Sun Looks Bigger

From Ohio 5th Grade Science Power Workbook · Chapter 3 · Lesson 3.2.

Big idea: Other stars look like tiny points because they are enormously farther away, not because they are small.

Learning goals: Explain why the sun looks larger than other stars

Use distance to explain how big a star appears

Compare the real sizes of stars

Distance changes how big something looks

Hold a coin close to your eye and it can block out a whole window. Put the same coin across the room and it looks tiny.

Nothing about the coin changed. Only the distance changed. Stars work the same way.

Distance changes how bright something looks, too. The same light looks dimmer the farther away it is, so a faraway star looks both smaller and fainter than it would up close.

Real size and apparent size are different

Some stars are really much larger than the sun. Some are really much smaller. The sun is a medium star.

You cannot tell which is which just by looking, because all of them are so far away that they look like points.

Worked examples

Question: Star W is really much larger than the sun, but it looks like a tiny point from Earth. Explain how both of those can be true.

Steps: Separate the two ideas. Real size is how big the star actually is. Apparent size is how big it looks from here.

Ask what changes apparent size. Distance does.

Star W is enormously farther away than the sun, so even though it is larger, it looks far smaller.

Answer: Real size and apparent size are not the same. Star W is larger than the sun but so much farther away that it appears as a tiny point.

Common mistakes

The sun is not the biggest or brightest star there is

The sun outshines everything in our sky, so it feels like it must be the biggest and brightest star anywhere.

It is a medium-sized star. Plenty of stars are larger and give off more light. They just are not next door.

Check your thinking: Ask whether the question is about how it looks from here or how it really is. Those give different answers.

Remember

Ask: looks, or really?

Every question in this lesson is one of two kinds. Looks questions are about distance. Really questions are about actual size. Decide which one you are being asked before you answer.

Practice tip

Remember: Watch for the words 'appears' and 'actually'.

Tip: Ohio uses those words carefully. 'Appears larger' is about distance. 'Actually larger' is about real size. Underline whichever one the question uses.

Written practice

Star W, from Example 3.2 in this lesson, is much larger than the sun but looks like a tiny point from Earth. Explain why, using the word distance.

Sample response: Star W is very much farther away from Earth than the sun is. The farther away something is, the smaller it looks, so even though Star W is really larger than the sun it appears as a tiny point.

Scoring guide: 1 point: says Star W is very much farther from Earth than the sun is.

1 point: connects that distance to how small it looks, or states that apparent size depends on distance rather than real size.

This is exactly the distance-to-apparent-size relationship Ohio lists as a response attribute.