Lesson 1 of 6 · Our place & scale
What belongs to the Solar System?
7 minute lesson · 3 checks along the way
Your goal for this lessonDescribe the Solar System as a moving system and identify what a picture does—and does not—tell you about its scale.
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Words with a dotted underline open a short definition right here in the lesson.
Start with a star, then build the family
The Sun is a star: a great concentration of hot matter that releases energy through nuclear fusion in its core. Our Solar System includes that star and the worlds and smaller material bound to it by gravity. Earth is one of its eight planets. Moons, , asteroids, comets, and dust belong to the same larger story.
Moving outward from the Sun, the planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. This order is a useful starting map, not a complete description. Each world has its own size, composition, movement, and history. The stars that form patterns in our night sky are other distant suns, outside our Solar System.
The Solar Dynamics Observatory recorded this view on March 28, 2013. Orange represents extreme-ultraviolet light our eyes cannot see; the dark area is a coronal hole in the Sun’s outer atmosphere.
Look closely
Find one bright region and one dark region. Why should you read the caption before describing the Sun’s color?
See what to notice
The instrument measured extreme-ultraviolet light, and the image assigns it a visible color. The patterns tell us about the solar atmosphere, but orange is a display choice. Scientific images can reveal things our eyes would miss.
Check 1 of 3
10 pointsUse the reading above. You can try again without losing points.
Gravity keeps the picture moving
An is a path followed under gravity. A planet moves forward while the Sun's gravity continually changes its direction, producing a curved path around the Sun. The planet has not reached a place where gravity switches off. Moons also move around their planets while traveling with them around the Sun.
A familiar example is an astronaut floating inside an orbiting spacecraft. The astronaut and spacecraft are falling together around Earth, so the floor does not support the astronaut as it would in a room on the ground. Floating is evidence of that shared free fall, not evidence that Earth has stopped attracting them.
Size and distance answer different questions
A planet's diameter tells you its width. Its distance from the Sun tells you where its orbit lies. Those are different measurements. One , or AU, is approximately Earth's average distance from the Sun. Neptune's average distance is about 30 AU, so a distance model with Earth one step from the Sun would put Neptune about 30 steps away.
Planet portraits are often enlarged and placed close together so you can see them. Even a picture that compares their sizes accurately may compress the gaps or arrange them in a line they do not actually occupy. Read the caption before measuring the picture. A scale for size does not automatically provide a scale for distance or show positions on a particular date.
Explore the evidence
Bigger does not always mean farther away
Choose two planets. Compare their widths, then switch to their distances from the Sun. Try Jupiter and Neptune: does the larger planet also orbit farther out?
Earth: 12,756 km across. Jupiter: 142,984 km across.
Width means diameter across the equator, excluding rings. Both bars share one scale, starting at zero.
What does the comparison show?
Jupiter is much wider than Neptune, but Neptune orbits much farther from the Sun. A world’s size and its orbit describe different things. The scale resets when you change the pair or the measurement, so compare the two bars within each view.
Check 2 of 3
10 pointsUse the reading above. You can try again without losing points.
Work through a decision
Work the evidence: read a planet poster
Imagine a poster with eight equal-sized planet pictures in a neat row. It can help you learn names and order. It cannot establish equal diameters, equal gaps, or a permanent lineup. To compare sizes, request diameter measurements. To compare distances, request a labeled distance scale. Choose the evidence to match the question.
Carry that habit into the next lessons. We will compare small rocky worlds, then much larger planets with deep , then moons and smaller bodies. Instead of memorizing a list of winners and records, ask what each observation helps explain—and what additional evidence you would need before reaching a conclusion.
Check 3 of 3
10 pointsUse the reading above. You can try again without losing points.
Put it together
Your lesson progress
Each check connects to a part of the story. Complete all 3 to earn your 25-point lesson bonus.
Go to the source
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