---
title: "The Solar System"
book: "Primary & Middle School Physics"
subject: physics
language: en
chapter: 26
exercises: 11
source: https://one-course.com/books/physics/1/en/chapter/26-the-solar-system
---

# Chapter 26 — The Solar System

The Earth, you know, makes a yearly lap around the Sun. But is it traveling alone? Look up on a clear evening: among the fixed sparkles, a few bright [lights](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) slowly wander from week to week. The ancients called them the wanderers; we call them [planets](#def-g4-solar-system-planet) — worlds circling the same Sun as we do.

## 26.1 One star, eight planets

**Definition 26.1 (Star).**

A *star* is a gigantic ball of glowing-hot stuff that makes its own [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) — a true [light source](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source). The Sun is a star: our star, close enough to warm our faces. (The night’s other stars get a chapter of their own next year.)

**Definition 26.2 (Planet).**

A *planet* is a big, round world that travels around a [star](#def-g4-solar-system-star) and makes no [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) of its own — we see planets, as we see the [Moon](https://one-course.com/books/physics/1/en/chapter/11-the-moon-in-the-sky#def-g2-moon-in-the-sky-moon), by the sunlight they send back. The Earth is a planet.

**Definition 26.3 (The Solar System).**

The *Solar System* is the Sun together with everything that travels around it: the eight [planets](#def-g4-solar-system-planet) — in order: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune — plus their moons and a crowd of smaller rocks and ice balls.

![The Sun and its eight planets, in order — drawn shoulder to shoulder here, though in truth each orbit lies far, far beyond the last.](https://one-course.com/images/onecourse/chapters/physics-1/g4-solar-system/fig-6bc05bcefcb5.svg)

*The Sun and its eight [planets](#def-g4-solar-system-planet), in order — drawn shoulder to shoulder here, though in truth each orbit lies far, far beyond the last.*

**Proposition 26.4 (The planets’ habits).**

The [planets](#def-g4-solar-system-planet) keep tidy habits: each travels around the Sun on its own nearly-circular road, all of them circle the same way, and all the roads lie nearly in one flat plane — like rings drawn on one tabletop. And the farther a [planet](#def-g4-solar-system-planet)’s road from the Sun, the longer it takes for one lap: Mercury laps in $88$ days, the Earth in one year, far Neptune in $165$ years.

**Example 26.5 (Rocky planets and gas giants).**

The four inner [planets](#def-g4-solar-system-planet) — Mercury, Venus, Earth, Mars — are balls of rock, small and [solid](https://one-course.com/books/physics/1/en/chapter/14-solids-liquids-gases#def-g3-solids-liquids-gases-solid): worlds you could stand on. The four outer ones — Jupiter, Saturn, Uranus, Neptune — are the *giants*: enormous balls mostly of thick [gas](https://one-course.com/books/physics/1/en/chapter/14-solids-liquids-gases#def-g3-solids-liquids-gases-gas), with no ground to stand on at all. Jupiter, the biggest, could swallow more than a thousand Earths; Saturn wears its famous shining rings — countless little pieces of ice circling it like a crowd of tiny moons.

![The real faces of the Sun and its eight planets, photographed by space probes (sizes and distances not to scale; Venus is shown as a radar map — its clouds hide the ground). Photos: NASA (public domain).](https://one-course.com/images/onecourse/chapters/physics-1/g4-solar-system/img-d1ebe2a831a9.jpg)

![Mercury](https://one-course.com/images/onecourse/chapters/physics-1/g4-solar-system/img-de672d663076.jpg)

![Venus](https://one-course.com/images/onecourse/chapters/physics-1/g4-solar-system/img-5044cdf1be2a.jpg)

![Earth](https://one-course.com/images/onecourse/chapters/physics-1/g4-solar-system/img-6ea4b7215b88.jpg)

![Mars](https://one-course.com/images/onecourse/chapters/physics-1/g4-solar-system/img-21a9fd03c58d.jpg)

![Jupiter](https://one-course.com/images/onecourse/chapters/physics-1/g4-solar-system/img-444e41635e96.jpg)

![Saturn](https://one-course.com/images/onecourse/chapters/physics-1/g4-solar-system/img-c9824eb3c4ef.jpg)

![Uranus](https://one-course.com/images/onecourse/chapters/physics-1/g4-solar-system/img-25d440f850e2.jpg)

![Neptune](https://one-course.com/images/onecourse/chapters/physics-1/g4-solar-system/img-bd4eed5e7525.jpg)

*the Sun, our [star](#def-g4-solar-system-star)*

*Mercury*

*Venus*

*Earth*

*Mars*

*Jupiter*

*Saturn*

*Uranus*

*Neptune*

*The real faces of the Sun and its eight [planets](#def-g4-solar-system-planet), photographed by space probes (sizes and distances not to scale; Venus is shown as a radar map — its clouds hide the ground). *Photos: NASA (public domain).**

**Example 26.6 (Moons are not planets).**

Our [Moon](https://one-course.com/books/physics/1/en/chapter/11-the-moon-in-the-sky#def-g2-moon-in-the-sky-moon) circles the *Earth*, not the Sun directly — so it is not a [planet](#def-g4-solar-system-planet) but a *moon*, a companion of a [planet](#def-g4-solar-system-planet). Other [planets](#def-g4-solar-system-planet) have moons of their own: Mars has two small ones, Jupiter and Saturn dozens each. A moon belongs to its [planet](#def-g4-solar-system-planet); the [planet](#def-g4-solar-system-planet) belongs to the Sun.

## 26.2 How far is far

**Method 26.7 (A scale model of the Solar System).**

The distances between [planets](#def-g4-solar-system-planet) are enormous — millions of kilometres. To feel them, build a scale model of the [Solar System](#def-g4-solar-system-family) in a park, shrinking every size and distance by the same factor:

1. let the Sun be a football placed at the gate;
2. then the Earth is a peppercorn — and to place it fairly, walk about $30$ big steps from the football;
3. Mercury, a pinhead, stands about $12$ steps from the football; Jupiter, a marble, about $150$ steps; Neptune, a small bead, about $900$ steps — most of a kilometre away!

Walk it, and feel what no drawing shows: the [Solar System](#def-g4-solar-system-family) is almost entirely empty space, with tiny worlds very far apart. In this model one big step stands for about five million kilometres of real space.

![The scale model: shrink the Sun to a football, and the whole Solar System fits in a park — barely.](https://one-course.com/images/onecourse/chapters/physics-1/g4-solar-system/fig-e9491d62dce8.svg)

*The scale model: shrink the Sun to a football, and the whole [Solar System](#def-g4-solar-system-family) fits in a park — barely.*

**Example 26.8 (The wanderers in our sky).**

Venus is often the first “[star](#def-g4-solar-system-star)” of the evening, blazing in the west after [sunset](https://one-course.com/books/physics/1/en/chapter/4-day-and-night-the-sun#def-g1-day-and-night-daynight) — so bright because it is our nearest neighbor, wrapped in white clouds that send sunlight back generously. Mars glows faintly orange; Jupiter and Saturn shine steady and bright. Watch one of them for a few weeks against the fixed [stars](#def-g4-solar-system-star): it drifts — the very wandering that gave [planets](#def-g4-solar-system-planet) their ancient name.

**Remark 26.9 (An old picture, redrawn).**

For most of history, people placed the Earth at the center of everything, with Sun, [Moon](https://one-course.com/books/physics/1/en/chapter/11-the-moon-in-the-sky#def-g2-moon-in-the-sky-moon) and [planets](#def-g4-solar-system-planet) circling us. Careful watchers of the wanderers found that picture creaking — [planets](#def-g4-solar-system-planet) sometimes drift *backward* for a while, hard to explain with Earth enthroned in the middle. The Sun-centered picture you have just learned explained the wanderings simply, and it stands among the boldest ideas humans ever accepted: our home is not the center — it is the third [planet](#def-g4-solar-system-planet) from the Sun.

## 26.3 Exercises

**Exercise 26.1 ★.**

What is the difference between a [star](#def-g4-solar-system-star) and a [planet](#def-g4-solar-system-planet)? Which one is the Sun? The Earth?

**Solution of Exercise 26.1.**

A [star](#def-g4-solar-system-star) makes its own [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source); a [planet](#def-g4-solar-system-planet) makes none and travels around a [star](#def-g4-solar-system-star), shining only by returned sunlight. The Sun is a [star](#def-g4-solar-system-star); the Earth is a [planet](#def-g4-solar-system-planet).

**Exercise 26.2 ★.**

Recite the eight [planets](#def-g4-solar-system-planet) in order from the Sun. Which is ours?

**Solution of Exercise 26.2.**

Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune — ours is the third, Earth.

**Exercise 26.3 ★.**

Which [planets](#def-g4-solar-system-planet) are small and rocky? Which are the [gas](https://one-course.com/books/physics/1/en/chapter/14-solids-liquids-gases#def-g3-solids-liquids-gases-gas) giants? Name the biggest of all, and the one with the famous rings.

**Solution of Exercise 26.3.**

Rocky: Mercury, Venus, Earth, Mars. Giants: Jupiter, Saturn, Uranus, Neptune. Biggest: Jupiter. The rings: Saturn.

**Exercise 26.4 ★.**

Why is the [Moon](https://one-course.com/books/physics/1/en/chapter/11-the-moon-in-the-sky#def-g2-moon-in-the-sky-moon) not counted among the [planets](#def-g4-solar-system-planet)? What does it circle?

**Solution of Exercise 26.4.**

It circles the Earth, not the Sun directly — so it is a moon, a [planet](#def-g4-solar-system-planet)’s companion, not a [planet](#def-g4-solar-system-planet).

**Exercise 26.5 ★.**

Mercury laps the Sun in $88$ days, the Earth in $365$, Neptune in $165$ years. What habit of the [planets](#def-g4-solar-system-planet) do these numbers show?

**Solution of Exercise 26.5.**

The farther the road from the Sun, the longer the lap: close Mercury races around quickly, far Neptune takes lifetimes.

**Exercise 26.6 ★.**

In the park scale model, what plays the Sun, and how many steps away does the Earth stand? What is nearly all of the [Solar System](#def-g4-solar-system-family) made of, by room?

**Solution of Exercise 26.6.**

A football is the Sun; the Earth (a peppercorn in the model) stands about $30$ big steps away. By room, the [Solar System](#def-g4-solar-system-family) is almost entirely empty space.

**Exercise 26.7 ★.**

Venus shines brilliantly, yet it is no [star](#def-g4-solar-system-star). Where does its [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) come from? (An old friend answers: think of the [Moon](https://one-course.com/books/physics/1/en/chapter/11-the-moon-in-the-sky#def-g2-moon-in-the-sky-moon).)

**Solution of Exercise 26.7.**

From the Sun: Venus, like the [Moon](https://one-course.com/books/physics/1/en/chapter/11-the-moon-in-the-sky#def-g2-moon-in-the-sky-moon), only sends sunlight back — its white clouds return it so generously that it outshines every true [star](#def-g4-solar-system-star) in our evening sky.

**Exercise 26.8 ★.**

How could you tell, watching the sky for a few weeks, that a bright [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) is a [planet](#def-g4-solar-system-planet) and not a [star](#def-g4-solar-system-star)?

**Solution of Exercise 26.8.**

Watch it against the fixed sparkles for a few weeks: a [planet](#def-g4-solar-system-planet) slowly drifts — wanders — among them, while true [stars](#def-g4-solar-system-star) keep their places in the patterns.

**Exercise 26.9 ★.**

In the park model, Jupiter stands at about $150$ steps and the Earth at about $30$. Roughly how many times farther from the Sun is Jupiter than the Earth?

**Solution of Exercise 26.9.**

$150 \div 30 = 5$: Jupiter is about five times farther from the Sun than the Earth.

**Exercise 26.10 ★★.**

A friend draws the [Solar System](#def-g4-solar-system-family) with all eight [planets](#def-g4-solar-system-planet) packed snugly around the Sun like the chapter’s first picture, and asks why probes take years to reach the outer [planets](#def-g4-solar-system-planet). Set them straight with the park scale model — what do almost all drawings of the [Solar System](#def-g4-solar-system-family) lie about, and why do they have to?

**Solution of Exercise 26.10.**

Almost all drawings lie about the distances: drawn to scale, a page showing Neptune’s orbit would shrink the [planets](#def-g4-solar-system-planet) to invisible specks — so artists squeeze the orbits together to show the worlds at all. In truth — as the walked model shows — the [planets](#def-g4-solar-system-planet) travel in vast emptiness: even [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) and fast probes must cross hundreds of “steps” of nothing, which takes years.

**Exercise 26.11 ★★.**

When the Earth was believed to sit at the center of everything, what sky-watching puzzle creaked against that picture? How did putting the Sun at the center dissolve the puzzle — and what did the new portrait cost us in pride?

**Solution of Exercise 26.11.**

The wanderers sometimes drift backward against the [stars](#def-g4-solar-system-star) for weeks — clumsy to explain with everything circling an enthroned Earth. With the Sun at the center, the backward drift is a simple overtaking: the Earth, on a faster inner road, passes a slower outer [planet](#def-g4-solar-system-planet), which then *seems* to slide backward. The cost: giving up the center — accepting that our home is [planet](#def-g4-solar-system-planet) number three.