---
title: "The Sun–Earth–Moon System"
book: "Primary & Middle School Physics"
subject: physics
language: en
chapter: 43
exercises: 12
source: https://one-course.com/books/physics/1/en/chapter/43-the-sunearthmoon-system
---

# Chapter 43 — The Sun–Earth–Moon System

Three worlds run our sky: the Sun that gives the day, the Earth that carries us, the [Moon](https://one-course.com/books/physics/1/en/chapter/11-the-moon-in-the-sky#def-g2-moon-in-the-sky-moon) that keeps us company. You know each one; now we chart the *system* — who circles whom, how fast, how far — and pay an old debt: the promised secret of the [seasons](https://one-course.com/books/physics/1/en/chapter/18-the-sun-in-the-sky-days-and-seasons#def-g3-sun-days-seasons-year) is in this chapter.

## 43.1 Three motions, three clocks

**Definition 43.1 (Axis and orbit).**

The Earth’s *axis* is the imaginary line through its poles around which it spins, like a top’s spindle. An *orbit* is the closed path one heavenly body travels around another — very nearly a circle for the Earth around the Sun, and for the [Moon](https://one-course.com/books/physics/1/en/chapter/11-the-moon-in-the-sky#def-g2-moon-in-the-sky-moon) around the Earth.

**Proposition 43.2 (The three motions).**

The whole calendar hangs on three steady motions:

1. the Earth *spins* on its [axis](#def-g6-sun-earth-moon-axisorbit) : one turn in $24$ [hours](https://one-course.com/books/physics/1/en/chapter/8-measuring-time#ex-g2-measuring-time-clock) — the day, parading Sun and [stars](https://one-course.com/books/physics/1/en/chapter/26-the-solar-system#def-g4-solar-system-star) across our sky;
2. the Earth *[orbits](#def-g6-sun-earth-moon-axisorbit)* the Sun: one lap in about $365$ days and a quarter — the year, sliding the Sun’s arc up and down through the [seasons](https://one-course.com/books/physics/1/en/chapter/18-the-sun-in-the-sky-days-and-seasons#def-g3-sun-days-seasons-year) ;
3. the [Moon](https://one-course.com/books/physics/1/en/chapter/11-the-moon-in-the-sky#def-g2-moon-in-the-sky-moon) *[orbits](#def-g6-sun-earth-moon-axisorbit)* the Earth: one lap in about a month — and, as its sunlit half faces us by turns, parading its phases through your old [Moon](https://one-course.com/books/physics/1/en/chapter/11-the-moon-in-the-sky#def-g2-moon-in-the-sky-moon) diary.

Three motions, three clocks: day, month, year — humankind’s oldest timekeepers, all still running.

![The system’s plan (distances squeezed as always): the Earth spins while orbiting the Sun; the Moon orbits the Earth.](https://one-course.com/images/onecourse/chapters/physics-1/g6-sun-earth-moon/fig-03c257c02421.svg)

*The system’s plan (distances squeezed as always): the Earth spins while orbiting the Sun; the [Moon](https://one-course.com/books/physics/1/en/chapter/11-the-moon-in-the-sky#def-g2-moon-in-the-sky-moon) [orbits](#def-g6-sun-earth-moon-axisorbit) the Earth.*

**Example 43.3 (The quarter day and the leap year).**

The Earth’s lap takes $365$ days *and about a quarter* — nature owes no loyalty to round numbers. Calendars pay the debt by saving the quarters: four years of quarter-days make one whole day, added to the calendar every fourth year — the $366$-day *leap year*, with its extra day at February’s end. Miss this bookkeeping for a century and the calendar drifts weeks away from the [seasons](https://one-course.com/books/physics/1/en/chapter/18-the-sun-in-the-sky-days-and-seasons#def-g3-sun-days-seasons-year) — as older calendars, painfully, discovered.

**Example 43.4 (The system, to scale).**

Numbers for the notebook: the Earth is about $12\,800\,\mathrm{km}$ across; the [Moon](https://one-course.com/books/physics/1/en/chapter/11-the-moon-in-the-sky#def-g2-moon-in-the-sky-moon), about a quarter of that, [orbits](#def-g6-sun-earth-moon-axisorbit) about $384\,000\,\mathrm{km}$ away — thirty Earths laid side by side. The Sun is about $109$ Earths across and sits $150$ million kilometres away. Shrink the Earth to a [centimetre](https://one-course.com/books/physics/1/en/chapter/7-measuring-length#def-g2-measuring-length-units) marble and the model says: [Moon](https://one-course.com/books/physics/1/en/chapter/11-the-moon-in-the-sky#def-g2-moon-in-the-sky-moon), a peppercorn $30$ [centimetres](https://one-course.com/books/physics/1/en/chapter/7-measuring-length#def-g2-measuring-length-units) away; Sun, a ball over a [metre](https://one-course.com/books/physics/1/en/chapter/7-measuring-length#def-g2-measuring-length-units) wide, standing more than a hundred [metres](https://one-course.com/books/physics/1/en/chapter/7-measuring-length#def-g2-measuring-length-units) down the street. Our whole neighborhood is mostly emptiness — as the park scale model of the [Solar System](https://one-course.com/books/physics/1/en/chapter/26-the-solar-system#def-g4-solar-system-family) already whispered.

## 43.2 The debt repaid: why seasons

**Proposition 43.5 (The tilt makes the seasons).**

The Earth’s [axis](#def-g6-sun-earth-moon-axisorbit) is not upright on its [orbit](#def-g6-sun-earth-moon-axisorbit): it leans, by about a quarter of a right angle — and it keeps leaning *the same way* all around the year’s lap, like a faithful [compass](https://one-course.com/books/physics/1/en/chapter/25-magnets-and-the-compass#def-g4-magnets-and-compass-compass) of the [stars](https://one-course.com/books/physics/1/en/chapter/26-the-solar-system#def-g4-solar-system-star). So for half the lap, our half of the world leans *toward* the Sun: the Sun rides high, days run long — summer. Half a lap later, the same unchanged lean points us *away*: low Sun, short days — winter. The [seasons](https://one-course.com/books/physics/1/en/chapter/18-the-sun-in-the-sky-days-and-seasons#def-g3-sun-days-seasons-year) are not the Sun changing, nor the distance: they are a tilted [planet](https://one-course.com/books/physics/1/en/chapter/26-the-solar-system#def-g4-solar-system-planet) carrying its unchanged lean around its [star](https://one-course.com/books/physics/1/en/chapter/26-the-solar-system#def-g4-solar-system-star).

![One tilted Earth at two stations of its lap. The axis (blue) leans the same way in both — toward the Sun in June, away from it in December, for the marked northern town.](https://one-course.com/images/onecourse/chapters/physics-1/g6-sun-earth-moon/fig-6fa63b48ba89.svg)

*One tilted Earth at two stations of its lap. The [axis](#def-g6-sun-earth-moon-axisorbit) (blue) leans the same way in both — toward the Sun in June, away from it in December, for the marked northern town.*

**Method 43.6 (The globe and the lamp).**

See the whole machine on a table:

1. set a lamp mid-table as the Sun, and carry a tilted globe (or a ball on a slanted skewer) slowly around it, *keeping the skewer pointing at the same corner of the room* the whole way;
2. at each quarter of the lap, stop and look at your home’s latitude: watch how much of its daily spin-circle lies in the [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) ;
3. on one side of the lamp, your hemisphere basks tilted-in — long lit arcs, high lamp: summer; opposite, tilted-out — short arcs, low lamp: winter;
4. at the two stations between, neither pole leans in: day and night share equally — spring and autumn.

The one discipline that makes it work: never let the skewer swing — the [axis](#def-g6-sun-earth-moon-axisorbit)’ faithfulness *is* the [seasons](https://one-course.com/books/physics/1/en/chapter/18-the-sun-in-the-sky-days-and-seasons#def-g3-sun-days-seasons-year).

**Example 43.7 (The old clues, accounted for).**

Every observation of your [sundial](https://one-course.com/books/physics/1/en/chapter/18-the-sun-in-the-sky-days-and-seasons#def-g3-sun-days-seasons-sundial) years now reports to the tilt. Summer’s high arc: our hemisphere leans in, so the Sun stands taller at noon. Winter’s stubby arc and long [shadows](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-shadow): leaning out, the Sun skims low. Opposite [seasons](https://one-course.com/books/physics/1/en/chapter/18-the-sun-in-the-sky-days-and-seasons#def-g3-sun-days-seasons-year) across the equator — December beach holidays in the south while the north shovels snow: when one hemisphere leans in, the other must lean out. Even the equal days of spring and autumn fall out of the lamp-and-globe walk. One quarter-of-a-right-angle lean runs the whole show.

**Remark 43.8 (Not distance — and the proof).**

The Earth’s [orbit](#def-g6-sun-earth-moon-axisorbit) is so nearly circular that sunward distance changes little — and, delightfully, we ride slightly *closer* to the Sun in the northern winter. The [seasons](https://one-course.com/books/physics/1/en/chapter/18-the-sun-in-the-sky-days-and-seasons#def-g3-sun-days-seasons-year) care about the *slant* of the [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source), not the length of the trip: leaning in, sunlight strikes steep and concentrated; leaning out, the same [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) spreads thin across the ground. A torch held straight-on versus slanted against a wall shows the difference in one second — steep [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) bites, grazing [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) barely warms.

## 43.3 Exercises

**Exercise 43.1 ★.**

Name the three motions of the system and the clock each one [winds](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-wind).

**Solution of Exercise 43.1.**

The Earth spins on its [axis](#def-g6-sun-earth-moon-axisorbit) — the day. The Earth [orbits](#def-g6-sun-earth-moon-axisorbit) the Sun — the year. The [Moon](https://one-course.com/books/physics/1/en/chapter/11-the-moon-in-the-sky#def-g2-moon-in-the-sky-moon) [orbits](#def-g6-sun-earth-moon-axisorbit) the Earth — the month.

**Exercise 43.2 ★.**

What is the Earth’s [axis](#def-g6-sun-earth-moon-axisorbit)? What quarter-of-a-right-angle fact about it, plus what faithfulness, makes the [seasons](https://one-course.com/books/physics/1/en/chapter/18-the-sun-in-the-sky-days-and-seasons#def-g3-sun-days-seasons-year)?

**Solution of Exercise 43.2.**

The imaginary line through the poles around which the Earth spins. It leans by about a quarter of a right angle — and keeps leaning the same way all around the [orbit](#def-g6-sun-earth-moon-axisorbit): lean plus faithfulness make the [seasons](https://one-course.com/books/physics/1/en/chapter/18-the-sun-in-the-sky-days-and-seasons#def-g3-sun-days-seasons-year).

**Exercise 43.3 ★.**

Why does the calendar add a day every fourth year? What would slip if it never did?

**Solution of Exercise 43.3.**

The Earth’s lap takes $365$ days and a quarter; four saved quarters make the leap year’s extra day. Without it the calendar would drift through the [seasons](https://one-course.com/books/physics/1/en/chapter/18-the-sun-in-the-sky-days-and-seasons#def-g3-sun-days-seasons-year) — about a day every four years, weeks in a century.

**Exercise 43.4 ★.**

About how many Earths, laid side by side, reach the [Moon](https://one-course.com/books/physics/1/en/chapter/11-the-moon-in-the-sky#def-g2-moon-in-the-sky-moon)? How many Earths span the Sun’s face?

**Solution of Exercise 43.4.**

About $30$ Earths to the [Moon](https://one-course.com/books/physics/1/en/chapter/11-the-moon-in-the-sky#def-g2-moon-in-the-sky-moon); about $109$ Earths across the Sun’s face.

**Exercise 43.5 ★.**

In the globe-and-lamp walk, why must the skewer keep pointing at the same corner of the room? What breaks if it swings?

**Solution of Exercise 43.5.**

The skewer plays the [axis](#def-g6-sun-earth-moon-axisorbit), and the real [axis](#def-g6-sun-earth-moon-axisorbit) keeps its lean fixed against the [stars](https://one-course.com/books/physics/1/en/chapter/26-the-solar-system#def-g4-solar-system-star). Let it swing — always leaning toward the lamp, say — and the model shows an endless summer: the [seasons](https://one-course.com/books/physics/1/en/chapter/18-the-sun-in-the-sky-days-and-seasons#def-g3-sun-days-seasons-year) vanish with the faithfulness.

**Exercise 43.6 ★.**

When it is summer here, what [season](https://one-course.com/books/physics/1/en/chapter/18-the-sun-in-the-sky-days-and-seasons#def-g3-sun-days-seasons-year) is it for the hemisphere across the equator, and why must it be so?

**Solution of Exercise 43.6.**

The opposite one: when our hemisphere leans toward the Sun, the other must lean away — one tilt cannot favor both ends at once.

**Exercise 43.7 ★.**

Use the torch-against-the-wall picture of [Remark 43.8](#rem-g6-sun-earth-moon-notdistance) to explain why leaning toward the Sun warms us more.

**Solution of Exercise 43.7.**

Straight-on, the torch’s [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) lands steep and concentrated — a small, bright, biting patch. Slanted, the same [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) smears across a long stretch of wall, thin and feeble. Leaning in gives us the steep, concentrated kind.

**Exercise 43.8 ★.**

An old friend returns: why does the [Moon](https://one-course.com/books/physics/1/en/chapter/11-the-moon-in-the-sky#def-g2-moon-in-the-sky-moon) show phases through the month? Which of the three motions runs that show?

**Solution of Exercise 43.8.**

The [Moon](https://one-course.com/books/physics/1/en/chapter/11-the-moon-in-the-sky#def-g2-moon-in-the-sky-moon)’s own [orbit](#def-g6-sun-earth-moon-axisorbit) around the Earth: as it circles, we view its one sunlit half from changing angles — [crescent](https://one-course.com/books/physics/1/en/chapter/11-the-moon-in-the-sky#ex-g2-moon-in-the-sky-shapes) to full and back. Motion three runs the phase show.

**Exercise 43.9 ★★.**

A stubborn uncle insists: “Winter is when the Earth is farthest from the Sun.” Marshal two facts from this chapter that together defeat him.

**Solution of Exercise 43.9.**

First: the northern winter happens while the Earth rides slightly *closer* to the Sun — distance points the wrong way. Second: the two hemispheres have opposite [seasons](https://one-course.com/books/physics/1/en/chapter/18-the-sun-in-the-sky-days-and-seasons#def-g3-sun-days-seasons-year) at the same moment and the same distance — no shared distance could do that; only the tilt, favoring one end and disfavoring the other, can.

**Exercise 43.10 ★★.**

Suppose the Earth’s [axis](#def-g6-sun-earth-moon-axisorbit) stood perfectly upright on its [orbit](#def-g6-sun-earth-moon-axisorbit) — no lean at all. Describe a year: what happens to [seasons](https://one-course.com/books/physics/1/en/chapter/18-the-sun-in-the-sky-days-and-seasons#def-g3-sun-days-seasons-year), to the Sun’s noon height, to the length of days? (Walk the upright globe around the lamp in your head.)

**Solution of Exercise 43.10.**

With no lean, every station of the lap looks alike: the Sun climbs to the same noon height all year, days and nights stay equal everywhere, and the year loses its [seasons](https://one-course.com/books/physics/1/en/chapter/18-the-sun-in-the-sky-days-and-seasons#def-g3-sun-days-seasons-year) — one long eternal spring, the [sundial](https://one-course.com/books/physics/1/en/chapter/18-the-sun-in-the-sky-days-and-seasons#def-g3-sun-days-seasons-sundial)’s arc frozen, the calendar’s chief drama canceled.

**Exercise 43.11 ★★.**

The [Moon](https://one-course.com/books/physics/1/en/chapter/11-the-moon-in-the-sky#def-g2-moon-in-the-sky-moon)’s lap takes about $27$ days, yet [full moon](https://one-course.com/books/physics/1/en/chapter/11-the-moon-in-the-sky#ex-g2-moon-in-the-sky-shapes) to [full moon](https://one-course.com/books/physics/1/en/chapter/11-the-moon-in-the-sky#ex-g2-moon-in-the-sky-shapes) takes about $29.5$. Propose the resolution — what else moved while the [Moon](https://one-course.com/books/physics/1/en/chapter/11-the-moon-in-the-sky#def-g2-moon-in-the-sky-moon) lapped? (Sketch the Earth advancing along its [orbit](#def-g6-sun-earth-moon-axisorbit) during one Moon-lap.)

**Solution of Exercise 43.11.**

While the [Moon](https://one-course.com/books/physics/1/en/chapter/11-the-moon-in-the-sky#def-g2-moon-in-the-sky-moon) lapped its $27$ days, the Earth advanced along its own [orbit](#def-g6-sun-earth-moon-axisorbit) — so the Sun’s direction shifted, and the [Moon](https://one-course.com/books/physics/1/en/chapter/11-the-moon-in-the-sky#def-g2-moon-in-the-sky-moon) needs about two and a half extra days of travel to catch up to the same lineup with the Sun that makes a [full moon](https://one-course.com/books/physics/1/en/chapter/11-the-moon-in-the-sky#ex-g2-moon-in-the-sky-shapes). Lap and lineup are different finishing posts.

**Exercise 43.12 ★★★.**

In midsummer, regions near the North Pole keep the Sun above the horizon at midnight — the famous “midnight sun” — and in midwinter they wait weeks for a [sunrise](https://one-course.com/books/physics/1/en/chapter/4-day-and-night-the-sun#def-g1-day-and-night-daynight). Explain both marvels with the tilted spin-circles of [Method 43.6](#met-g6-sun-earth-moon-globe): what does the lean do to a polar town’s daily circle in June, and in December?

**Solution of Exercise 43.12.**

A polar town’s daily circle is small and, thanks to the June lean, can lie *entirely* in the lit half: the town spins around without ever leaving daylight — midnight sun. In December the same lean tips its little circle entirely into the dark half: whole spins without a [sunrise](https://one-course.com/books/physics/1/en/chapter/4-day-and-night-the-sun#def-g1-day-and-night-daynight). The tilt does gently to every latitude what it does absolutely at the poles.

## 43.4 Problem: The Sports-Field Model

**Problem 43.1.**

Weekend problem — the class builds the Sun–Earth–Moon system on the sports field; marbles, peppercorns and a very long walk; the tilt joins the model

The class builds the system to scale: the Earth is a marble $1\,\mathrm{cm}$ across. Handy true numbers: Earth $12\,800\,\mathrm{km}$ across; [Moon](https://one-course.com/books/physics/1/en/chapter/11-the-moon-in-the-sky#def-g2-moon-in-the-sky-moon) $3200\,\mathrm{km}$ across, orbiting $384\,000\,\mathrm{km}$ away; Sun $109$ Earths across, $150$ million kilometres away.

**Part I — Casting the parts.**

1. In the model, $1\,\mathrm{cm}$ stands for how many kilometres?
2. The [Moon](https://one-course.com/books/physics/1/en/chapter/11-the-moon-in-the-sky#def-g2-moon-in-the-sky-moon) is about a quarter of the Earth’s width. How wide is the model [Moon](https://one-course.com/books/physics/1/en/chapter/11-the-moon-in-the-sky#def-g2-moon-in-the-sky-moon) — and what kitchen item plays it well?
3. The true [Moon](https://one-course.com/books/physics/1/en/chapter/11-the-moon-in-the-sky#def-g2-moon-in-the-sky-moon) distance is $30$ Earth-widths. Where does the model [Moon](https://one-course.com/books/physics/1/en/chapter/11-the-moon-in-the-sky#def-g2-moon-in-the-sky-moon) stand from the marble?
4. The model Sun is $109$ marbles across. How wide is that, in [centimetres](https://one-course.com/books/physics/1/en/chapter/7-measuring-length#def-g2-measuring-length-units) — and what playground object is about that size?

**Part II — The long walk.**

5. The Sun’s distance is $150$ million km. In the model — divide by the answer to question 1 — how many [centimetres](https://one-course.com/books/physics/1/en/chapter/7-measuring-length#def-g2-measuring-length-units) is that, and how many [metres](https://one-course.com/books/physics/1/en/chapter/7-measuring-length#def-g2-measuring-length-units) ?
6. Will the model fit on a $100\,\mathrm{m}$ sports field? Where must the Sun-ball stand?
7. A pupil walks the Earth–Sun stretch at $1$ [metre](https://one-course.com/books/physics/1/en/chapter/7-measuring-length#def-g2-measuring-length-units) per second. About how many minutes does the walk take?
8. Standing at the marble and looking back at the metre-wide Sun-ball $117$ [metres](https://one-course.com/books/physics/1/en/chapter/7-measuring-length#def-g2-measuring-length-units) away, the class is surprised: it looks about the same size as the [quarter-centimetre](https://one-course.com/books/physics/1/en/chapter/7-measuring-length#def-g2-measuring-length-units) Moon-grain $30$ [centimetres](https://one-course.com/books/physics/1/en/chapter/7-measuring-length#def-g2-measuring-length-units) from the marble. Check the two ratios — and connect this coincidence to something that happens, rarely and gloriously, in the real sky.

**Part III — Making it run.**

9. To animate the model honestly, how long should the marble take to circle the Sun-ball, and the pea to circle the marble, if one real day is played by one second? (Count roughly: a year as $365$ days, a Moon-lap as $27$ days.)
10. The marble must also spin. At one real day per second, how fast is that in the model — and why will nobody see it on a $1\,\mathrm{cm}$ marble?
11. A skewer through the marble plays the [axis](#def-g6-sun-earth-moon-axisorbit) . At what slant, and pointing how, must the skewer be carried around the whole lap?
12. Write the model’s honest disclaimer: name one thing the sports-field model shows truly, and one thing no field-sized model can show at once (sizes and distances together? motions? [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) ?). Justify with your numbers.

**Solution of Problem 43.1.**

**1.** $12\,800\,\mathrm{km}$ per [centimetre](https://one-course.com/books/physics/1/en/chapter/7-measuring-length#def-g2-measuring-length-units). **2.** About $0.25\,\mathrm{cm}$ — a peppercorn (or a grain). **3.** $30 \times 1 = 30\,\mathrm{cm}$ from the marble. **4.** $109$ cm — about a [metre](https://one-course.com/books/physics/1/en/chapter/7-measuring-length#def-g2-measuring-length-units): a hula hoop or a large beach ball. **5.** $150000000 \div 12800 \approx 11\,700\,\mathrm{cm}$ — about $117\,\mathrm{m}$. **6.** Barely not: the Sun-ball must stand some $17\,\mathrm{m}$ beyond the far goal line of a $100\,\mathrm{m}$ field. **7.** About $117$ seconds — roughly two minutes. **8.** Sun: $109 \div 11700 \approx 0.009$; Moon-grain: $0.25 \div 30 \approx 0.008$ — nearly equal: the two look the same size from the marble. So too in our sky — and when the [Moon](https://one-course.com/books/physics/1/en/chapter/11-the-moon-in-the-sky#def-g2-moon-in-the-sky-moon) passes exactly before the Sun, it covers it almost exactly: a total solar eclipse, nature exploiting the coincidence. **9.** The marble’s lap: about $365$ seconds — six minutes of walking; the grain’s lap around the marble: about $27$ seconds. **10.** One full spin per second — and on a smooth [centimetre](https://one-course.com/books/physics/1/en/chapter/7-measuring-length#def-g2-measuring-length-units) marble with no markings, a spin leaves nothing for the eye to catch: painting a dot on it is the honest fix. **11.** Slanted by about a quarter of a right angle from upright — and carried around the whole lap pointing always at the same distant corner of the field, never turning to face the Sun-ball. **12.** For example: the model shows distances and sizes *together*, truly — the great emptiness is its triumph. What it cannot show at once is the motions at true pace (a six-minute “year” is already a wild speed-up; the real marble would creep) — nor, of course, the [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) and warmth: no metre-wide ball [lights](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) a sports field the way the Sun [lights](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) a [planet](https://one-course.com/books/physics/1/en/chapter/26-the-solar-system#def-g4-solar-system-planet).