E. Earth and Space Systems · Space

Shining or Reflecting? Light in Space

E2.4 A1.2 A1.5About 40 minutes

Learning goals

I can identify bodies in space that emit (give off) their own light.
I can identify bodies in space that reflect light from the Sun.
I can explain why we can see the Moon and planets even though they make no light.

Words to know

emit
To give off. A star emits its own light.

reflect
To bounce light off a surface. The Moon reflects sunlight.

luminous
Making its own light, like the Sun or another star.

non-luminous
Not making its own light. We see it only when light bounces off it, like the Moon or a planet.

light-year
The distance light travels in one year, about 9.5 million million km. It is used to measure the huge distances to stars.

Remember from Grade 4

In Grade 4 you learned that some objects are sources of light (they make their own light, like a flashlight or the Sun) and some objects reflect light (light bounces off them, like a mirror or a white wall). You also learned that the Moon reflects light from the Sun. Now you will use these ideas for every kind of body in space.

A puzzle about the Moon

On some nights the Moon is so bright that it makes shadows. But the Moon is just a big, dusty rock. As you learned in Grade 4, moonlight is sunlight that has bounced off the Moon.

What about the planets, comets and stars you can see at night? Which ones shine on their own, and which ones only reflect light?

A full Moon rising over a lake at night, with a silvery path of moonlight on the water. — The Moon does not make its own light. The moonlight shining on this lake is sunlight that has bounced off the Moon. AI-generated image

Bodies that emit light

Stars emit (give off) their own light. They are luminous.

The Sun is our star.
Every star you see at night is a distant sun. The nearest one after our Sun is about 4.2 light-years away. Its light has been travelling to us for more than 4 years!

A meteor also gives off light for a moment: the space rock and the air around it get so hot that they glow.

Bodies that reflect light

All the other bodies in the solar system make no light of their own. They are non-luminous. We see them only because sunlight reflects off them.

Planets, including Venus, Mars and Jupiter. Venus is often the brightest point in the evening or early morning sky. People sometimes call it the "evening star", but it is a planet reflecting sunlight.
Moons, including Earth's Moon.
Asteroids and comets. A comet's glowing tail is mostly sunlight reflecting off dust and light from gas that the Sun has energized.

Earth reflects sunlight too! Astronauts on the Moon can see Earth shining in their sky.

The Sun emits light. Sunlight travels to the Moon, and only the half of the Moon facing the Sun is lit. Some of that light reflects to Earth, so we can see the Moon.

Body	Emits its own light?	Reflects light?
the Sun	yes	(it is so bright that its own light is what we see)
other stars	yes	(same as the Sun)
planets	no	yes, sunlight
moons	no	yes, sunlight
asteroids	no	yes, sunlight
comets	no (the tail glows only because of sunlight)	yes, sunlight
meteors	yes, for a moment, because they are so hot	no

Emit or reflect?

Sort space bodies into two groups. Only stars (and a meteor, for a moment) make light; everything else is lit by the Sun on the side facing it.

Worked example

Let's figure out why planets look like stars

On a clear evening, Ravi sees a very bright dot in the western sky. His app says it is Venus. "How can a planet shine like a star?" he asks.

Step 1: Does Venus make its own light? No. Venus is a planet, so it is non-luminous.
Step 2: Where does the light come from? The Sun shines on Venus.
Step 3: What happens next? Venus is covered in bright clouds that reflect a lot of the sunlight. Some of it travels to Earth and into Ravi's eyes.
Step 4: Why does it look so bright? Venus is close to Earth compared to the stars, and its clouds reflect light well.

Venus shines because it reflects sunlight. Stars shine because they emit their own light.

Just for fun

Moonlight is really second-hand sunlight! It leaves the Sun, travels about 8 minutes to the Moon, bounces off, and then needs only about 1.3 seconds more to reach your eyes. Quick stopover!

Hands-on activity

Model sunlight, moonlight and planet light

You need: a flashlight (the Sun), a ball (the Moon or a planet), a small mirror or a piece of aluminium foil, and a dark room.

Make the room as dark as you can. Can you see the ball? (No, or only barely.)
Turn on the flashlight and shine it at the ball from the side. Which part of the ball is lit? (Only the side facing the light.)
Hold the ball in a dark corner and shine the light on it. Now you can see it, even though the ball makes no light. That is how we see the Moon and planets.
Investigate (A1.2): shine the flashlight on the ball, a white sheet of paper, a dark sock and the foil, one at a time, from the same distance. Which reflects the most light onto a wall? Which looks brightest?
Communicate (A1.5): draw a labelled diagram with arrows showing light going from the "Sun" to the "Moon" and then to your eye. Label which object emits and which reflects light.

What to look for: shiny and light-coloured surfaces reflect more light. Only the half of the ball facing the flashlight is lit, just like the real Moon. Your diagram should have arrows going from the light source to the object and then to the eye (not from the eye outward).

Safety: never shine the flashlight into anyone's eyes. Move carefully in the dark.

Your turn

Name a body in space that emits its own light.

Name three kinds of bodies in space that reflect light.

Where does moonlight come from?

Venus is sometimes called the "evening star". Is it a star? Explain.

What does luminous mean?

Why do we only see half of a ball lit up when we shine a flashlight on it?

More practice: warm-up

Luminous or non-luminous? The Moon.

Luminous or non-luminous? A star.

Luminous or non-luminous? Jupiter.

Luminous or non-luminous? The Sun.

Luminous or non-luminous? An asteroid.

More practice: on your own

If the Sun suddenly stopped shining, could we still see the Moon? Explain.

A meteor glows. Does that make it a star? Explain.

Astronauts on the Moon see Earth shining in the sky. Why does Earth shine?

Which would reflect more sunlight: a planet covered with bright white clouds or one covered in dark rock? Why?

The nearest star after the Sun is about 4.2 light-years away. What does that tell you about the light we see from it tonight?

Challenge

Explain, step by step, how light from the Sun lets you see Mars at night, even though it is night where you are.

Why might a comet be impossible to see far from the Sun, but easy to see near the Sun?

Sort into two groups and give each group a title: Saturn, Sun, Moon, a star in the Big Dipper, comet, Mercury.

Check your understanding

Quiz

Which body in space emits its own light?

Why can we see the Moon?

Venus shines brightly in the evening sky because it...

Which word means "making its own light"?

Which group contains only bodies that reflect light?

A meteor glows because...

For parents and teachers

Big idea (E2.4): identify bodies in space that emit light (stars, including the Sun) and those that reflect light (planets, moons, asteroids, comets). The flashlight investigation practises experimentation (A1.2) and communicating with a labelled diagram (A1.5).

How to help: on a clear evening, use a free sky app or a printed star chart to find a bright planet with your child. Ask: "Is that making its own light?" Look at the Moon together; it is always safe to look at.

Common difficulties:

Thinking the Moon glows on its own. Moonlight is reflected sunlight.
Arrows in the wrong direction in diagrams. Light travels from the source to the object to the eye.
Calling Venus a star. It is a planet; the old name "evening star" is misleading.

Safety: never look at the Sun, and never shine flashlights into eyes.

Source: the distance to the nearest star after the Sun (about 4.2 light-years, Proxima Centauri) is a standard NASA figure.