--- title: "The Moon in the Sky" book: "Primary & Middle School Physics" subject: physics language: en chapter: 11 exercises: 10 source: https://one-course.com/books/physics/1/en/chapter/11-the-moon-in-the-sky --- # Chapter 11 — The Moon in the Sky Some evenings a thin silver smile hangs over the rooftops; two weeks later, a full round lantern floods the garden with pale [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source). It is the same [Moon](#def-g2-moon-in-the-sky-moon) — our closest neighbor in the sky, and the only other world people have ever walked on. Time to get properly acquainted. ## 11.1 Our neighbor in the sky **Definition 11.1 (The Moon).** The *Moon* is a huge ball of gray rock that travels around the Earth, our faithful companion in space. It is much smaller than the Earth, and very far away — farther than any airplane flies, so far that the astronauts who visited it traveled for three days to get there. **Example 11.2 (Looking closely).** To the plain eye the [Moon](#def-g2-moon-in-the-sky-moon) shows pale patches and bright zones — some people see a face in them. Through binoculars (a wonderful thing to try on a full-moon night) the patches sharpen into plains and round *craters*, bowl-shaped scars left by rocks from space that crashed there long ago. The [Moon](#def-g2-moon-in-the-sky-moon) keeps its scars forever: it has no [wind](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-wind) and no rain to wipe them away — indeed it has no [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air) at all. No [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air) also means a silent world: on the [Moon](#def-g2-moon-in-the-sky-moon), even a shout makes no sound. ![The Moon’s face, photographed from orbit: bowl-shaped craters everywhere, wide dark plains — and no air, no wind, no sound. Photo: NASA/GSFC/Arizona State University.](https://one-course.com/images/onecourse/chapters/physics-1/g2-moon-in-the-sky/img-ce53ac82ea4e.jpg) *The [Moon](#def-g2-moon-in-the-sky-moon)’s face, photographed from orbit: bowl-shaped craters everywhere, wide dark plains — and no [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air), no [wind](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-wind), no sound. *Photo: NASA/GSFC/Arizona State University.** **Example 11.3 (The Moon that follows you).** Walk down the street on a moonlit evening: houses and trees slide past, but the [Moon](#def-g2-moon-in-the-sky-moon) seems to follow you, always at your shoulder. It is not really chasing you — it is so far away that walking a whole street changes your view of it not at all, while the nearby houses change all the time. Very distant things seem to follow every traveler; the [Moon](#def-g2-moon-in-the-sky-moon) does it best because it is the champion of far. ## 11.2 Reflected light **Example 11.4 (Moonlight is reflected sunlight).** Is the [Moon](#def-g2-moon-in-the-sky-moon) a [light source](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source)? Remember the test from last year: does it make its own [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source)? It does not — the [Moon](#def-g2-moon-in-the-sky-moon) is a ball of dark rock, like a huge pebble in the sky. What we call moonlight is sunlight *reflected* by that rock — bounced back toward us, the way a white wall [lights](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) a room without being a lamp. The Sun [lights](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) the [Moon](#def-g2-moon-in-the-sky-moon) bright on one side; we see whatever part of that bright side faces us. ![The Sun lights one half of the Moon, always the half that faces it. The other half is in its own night.](https://one-course.com/images/onecourse/chapters/physics-1/g2-moon-in-the-sky/fig-4bc53ccef552.svg) *The Sun [lights](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) one half of the [Moon](#def-g2-moon-in-the-sky-moon), always the half that faces it. The other half is in its own night.* **Remark 11.5 (A daytime Moon).** Many people believe the [Moon](#def-g2-moon-in-the-sky-moon) is a creature of the night — but look up on some afternoons and there it is, a pale ghost in the blue sky! The [Moon](#def-g2-moon-in-the-sky-moon) travels on its own schedule around the Earth and does not care whether it is our daytime or our night. It simply shows less proudly by day, when the Sun’s glare fills the sky. ## 11.3 The Moon changes shape **Example 11.6 (The Moon’s changing shapes).** Watch the [Moon](#def-g2-moon-in-the-sky-moon) over many evenings and it changes shape: a thin *crescent* like a fingernail clipping, then a half-circle, then a fat almost-round shape, then the perfect disc of the *full moon* — and then back down again, to a [crescent](#ex-g2-moon-in-the-sky-shapes) facing the other way, until for a night or two the [Moon](#def-g2-moon-in-the-sky-moon) seems to vanish entirely. The whole show takes about a month, and then it repeats, as reliably as day and night. ![Two weeks of Moon-watching: the lit shape grows night after night from nothing to the full disc — then it shrinks back.](https://one-course.com/images/onecourse/chapters/physics-1/g2-moon-in-the-sky/fig-6f4bdefce4ec.svg) *Two weeks of Moon-watching: the lit shape grows night after night from nothing to the full disc — then it shrinks back.* ![The real thing: a thin crescent (left — look closely: the dark part of the disc glows faintly too) and an almost-full Moon in a daytime sky (right). Photos: G. Donatiello, CC0; Cbaile19, CC0.](https://one-course.com/images/onecourse/chapters/physics-1/g2-moon-in-the-sky/img-0ef4e12d2de4.jpg) ![The real thing: a thin crescent (left — look closely: the dark part of the disc glows faintly too) and an almost-full Moon in a daytime sky (right). Photos: G. Donatiello, CC0; Cbaile19, CC0.](https://one-course.com/images/onecourse/chapters/physics-1/g2-moon-in-the-sky/img-b6275bdcf499.jpg) *The real thing: a thin [crescent](#ex-g2-moon-in-the-sky-shapes) (left — look closely: the dark part of the disc glows faintly too) and an almost-full [Moon](#def-g2-moon-in-the-sky-moon) in a daytime sky (right). *Photos: G. Donatiello, CC0; Cbaile19, CC0.** **Method 11.7 (Keeping a Moon diary).** 1. Each clear evening, at about the same hour, look for the [Moon](#def-g2-moon-in-the-sky-moon) ; 2. draw the shape you see in a little calendar square — a circle, and shade the dark part; 3. if clouds hide it, write “clouds” and keep going; 4. after two weeks, lay your drawings in a row. Your own drawings will show the shape changing, a little more each night. *Why* the [Moon](#def-g2-moon-in-the-sky-moon) does this is a delicious puzzle — it will be solved properly in a later year of this course, and your diary is the first step of the proof. ## 11.4 Exercises **Exercise 11.1 ★.** Is the [Moon](#def-g2-moon-in-the-sky-moon) bigger or smaller than the Earth? Nearer or farther than the clouds? How do we know people can visit it? **Solution of Exercise 11.1.** Much smaller than the Earth, and much farther than the clouds — farther than any airplane flies. We know people can visit because astronauts did: the trip took them three days each way. **Exercise 11.2 ★.** What are the round bowl-shaped marks on the [Moon](#def-g2-moon-in-the-sky-moon) called, and what made them? Why are they still there after so long? **Solution of Exercise 11.2.** Craters — bowl-shaped scars made by rocks from space that crashed into the [Moon](#def-g2-moon-in-the-sky-moon). They remain because the [Moon](#def-g2-moon-in-the-sky-moon) has no [wind](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-wind) and no rain to wear them away. **Exercise 11.3 ★.** Is the [Moon](#def-g2-moon-in-the-sky-moon) a [light source](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source), like a lamp, or a light-catcher, like a white wall? Where does moonlight really come from? **Solution of Exercise 11.3.** A light-catcher. Moonlight is sunlight bouncing off the [Moon](#def-g2-moon-in-the-sky-moon)’s gray rock, the way a white wall [lights](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) a room without being a lamp. **Exercise 11.4 ★.** Which half of the [Moon](#def-g2-moon-in-the-sky-moon) does the Sun [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source): always the half facing the Sun, always the same half, or a different half at random? **Solution of Exercise 11.4.** Always the half facing the Sun — the sunny side is bright, the far side is in its own night. **Exercise 11.5 ★.** Can the [Moon](#def-g2-moon-in-the-sky-moon) ever be seen in the daytime? If a friend says no, what would convince them? **Solution of Exercise 11.5.** Yes — on many afternoons a pale [Moon](#def-g2-moon-in-the-sky-moon) hangs in the blue sky. The best proof for a doubting friend: go outside on such an afternoon and point at it. **Exercise 11.6 ★.** Name the [Moon](#def-g2-moon-in-the-sky-moon)’s shapes in order, starting from the thin fingernail clipping and ending at the round lantern. About how long does the whole show take before it repeats? **Solution of Exercise 11.6.** [Crescent](#ex-g2-moon-in-the-sky-shapes), half, almost full, [full moon](#ex-g2-moon-in-the-sky-shapes) — then back down through almost full, half, [crescent](#ex-g2-moon-in-the-sky-shapes), to the hidden “new” [Moon](#def-g2-moon-in-the-sky-moon). The whole show takes about a month. **Exercise 11.7 ★.** On the [Moon](#def-g2-moon-in-the-sky-moon) there is no [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air). What would happen to a shout there? And to a kite? **Solution of Exercise 11.7.** A shout would make no sound — sound needs [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air). A kite could not fly: no [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air) means no [wind](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-wind), and nothing for the kite to lean on. **Exercise 11.8 ★.** Start the [Moon](#def-g2-moon-in-the-sky-moon) diary of [Method 11.7](#met-g2-moon-in-the-sky-diary) tonight. What two things must stay the same each evening to make the diary fair? **Solution of Exercise 11.8.** The same hour and the same honest drawing each evening (same place helps too): a fair diary changes only one thing at a time — the night. **Exercise 11.9 ★★.** “The [Moon](#def-g2-moon-in-the-sky-moon) followed our car all the way home!” says little Rosa. Explain to her, with the street-walk of [Example 11.3](#ex-g2-moon-in-the-sky-follows), why very far things seem to follow, while nearby things slide past. **Solution of Exercise 11.9.** The houses are near, so driving past them changes how Rosa sees them — they slide by and are gone. The [Moon](#def-g2-moon-in-the-sky-moon) is so far that the whole drive changes her view of it not at all: it stays at her shoulder, seeming to follow. The champion of far always “follows”. **Exercise 11.10 ★★.** On a [crescent](#ex-g2-moon-in-the-sky-shapes) night, look carefully: sometimes the *dark* part of the [Moon](#def-g2-moon-in-the-sky-moon) glows very faintly, so the whole circle can just be guessed. The Sun does not shine on that part — but something else shines nearby. What big, bright, sunlit ball could be lighting the [Moon](#def-g2-moon-in-the-sky-moon)’s dark side, the way the [Moon](#def-g2-moon-in-the-sky-moon) [lights](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) our nights? (Hint: you are standing on it.) **Solution of Exercise 11.10.** The Earth itself. Sunlight bounces off the bright Earth onto the [Moon](#def-g2-moon-in-the-sky-moon)’s dark side and faintly [lights](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) it — earthshine: our planet returning the [Moon](#def-g2-moon-in-the-sky-moon)’s favor.