AstronomySorting the Sun, the Moon, and the Planets
Nova the astronomer sits cross-legged inside a cozy observatory dome at night, holding up a glowing model of the solar system and pointing excitedly at the big yellow Sun, the small grey Moon, and several colorful planet balls arranged on the floor around her.
- Identify the Sun, the Moon, and at least two planets by name.
- Explain one way the Sun is different from the Moon.
- Sort space objects into the groups Sun, Moon, and planet.
- Compare the sizes of the Sun, the Moon, and a planet using words like bigger and smaller.
Key terms
- Sun
- Our closest star, a giant ball of hot glowing gas that gives light and warmth.
- Moon
- Earth's neighbor that travels around Earth and shines by reflecting sunlight.
- planet
- A big round ball of rock or gas that travels around the Sun.
- reflect
- To bounce light off a surface, the way a mirror sends light back.
- orbit
- The curved path one space object follows as it travels around another.
Three Kinds of Space Objects
Today we meet three kinds of space objects. The Sun is a star, a giant glowing ball of hot gas that makes its own light. The Moon is a rocky world that travels around Earth and shines only by reflecting the Sun's light. Planets are large round balls of rock or gas, like Earth, Mars, Saturn, and Jupiter, that all travel around the Sun on their own paths.
A Simple Sorting Trick
You can sort space objects with three easy questions. First ask, does it make its own light and glow like a giant ball of fire? Then it is the Sun, our star. Next ask, does it travel around a planet? Then it is a moon. Finally ask, does it travel around the Sun, the way Earth and Mars do? Then it is a planet. These questions tell every object apart.
Worked examples
Sort Saturn into the right group.
- Ask if Saturn makes its own light like the Sun — no, it does not.
- Ask if Saturn travels around a planet — no, it travels around the Sun.
- Since it orbits the Sun, Saturn is a planet.
Answer: Saturn is a planet, because it travels around the Sun.
Explain why the Moon glows in the sky.
- Remember the Moon does not make its own light.
- The Sun shines its light onto the Moon.
- The Moon reflects, or bounces back, that sunlight to our eyes, so it looks like it glows.
Answer: The Moon glows because it reflects the Sun's light, like a mirror.
Activity
Drag each space object into the correct group: Sun, Moon, or Planet.
Practice
Decide whether Mars belongs in the Sun, Moon, or planet group.
Tell one way the Sun is different from the Moon.
Common mistakes to avoid
- The Moon makes its own light.The Moon makes no light of its own; it only reflects sunlight back to Earth.
- The Sun is a planet like Earth.The Sun is a star that makes its own light, not a planet that orbits the Sun.
Check your understanding
Which space object gives us heat and light every day?
The Moon glows when we see it in the sky because it —
Which of these is a planet?
Recap
The Sun is our star, a giant glowing ball of hot gas, the Moon is Earth's neighbor that shines by reflecting sunlight, and planets like Earth, Mars, Saturn, and Jupiter travel around the Sun. To sort them, ask whether an object glows on its own, orbits a planet, or orbits the Sun.
Reflect
Which space object would you most want to visit, the Sun, the Moon, or a planet?