Spinning in Step同步自轉
The real Moon. The near side always faces Earth, like a dancer who always faces her partner.
真實的月亮。正面一直朝著地球，就像一直面對舞伴的舞者。
Astronomy · Lesson 13 · 天文教育 第13課
Why Do We Always See the Same Side of the Moon?月亮為什麼總是同一面對著我們？
The Moon does spin: exactly once for every trip around Earth. Switch its spin off or speed it up and see what Earth would see, then find out how the real Moon is tipped today.
月亮其實會自轉：繞地球一圈，正好自轉一圈。把它的自轉關掉或加快，看看地球會看到什麼；再看看今天真實的月亮微微轉向哪一邊。
Big idea · 一句話看懂
The Moon spins exactly once for every trip around Earth, so the same side always faces us. Long ago, Earth's tides slowed the Moon's spin until the two times matched.
月亮繞地球一圈，正好自轉一圈，所以永遠同一面朝向我們。很久以前，地球的潮汐作用讓月亮越轉越慢，直到兩個時間一模一樣。
3D Model · 3D 模型
One Spin for Every Trip Around Earth 繞一圈，轉一圈
The red arrow sticks out of the middle of the Moon's near side. Press Play: in the real Moon, the arrow keeps pointing at Earth, and the small window on the right, the view from Earth, always shows the same face. Now switch the spin to No spin or Too fast and watch the far side come around. Turn on Exaggerate rocking to see how the real Moon sways a few degrees each month.
紅色箭頭從月亮正面的中央伸出來。按「播放」：真實的月亮，箭頭一直指著地球；右邊「從地球看」的小窗，永遠是同一張臉。再把自轉切成「不自轉」或「轉太快」，看背面怎麼轉過來。打開「放大搖晃」，看真實的月亮每個月怎麼左右搖幾度。
Drag to turn · 拖曳旋轉 Scroll or pinch to zoom · 滾輪／雙指縮放
This 3D model needs WebGL, which this browser does not support. Today's Moon and the cards below still work.
這個瀏覽器不支援 WebGL，無法顯示 3D 模型；下方「今天的月亮」與卡片一樣能用。
- Red arrow: points out of the center of the near side
紅色箭頭：從月亮正面的中心伸出 - Dashed line: the line from Earth to the Moon
虛線：地球到月亮的連線 - Gray loop: the Moon's slightly oval orbit
灰色圈：月亮略呈橢圓的軌道 - The Moon is drawn about three times its real size, and the distance is squeezed
月亮畫成真實大小的三倍左右，距離也壓縮了
The real Moon is about 60 Earth radii away and about a quarter of Earth's width; here the distance is squeezed and the Moon is enlarged. The orbit's oval shape (eccentricity 0.055) and the 6.7° tilt of the Moon's spin axis are real, and can be exaggerated three times. The view from Earth uses a model of the real near side.
真實的月亮在約 60 個地球半徑外，寬度約是地球的四分之一；這裡壓縮了距離、放大了月亮。軌道的橢圓（偏心率 0.055）和月球自轉軸 6.7° 的傾斜是真實的，可以放大三倍。「從地球看」用的是真實正面花紋的模型。
Libration: optical libration formulas from J. Meeus, Astronomical Algorithms, chapter 53; Moon position: Meeus, chapter 47. Earth map: Natural Earth (public domain). The Moon's surface is a drawn model with the main maria in their real places.
天平動：Meeus《天文演算法》第 53 章光學天平動公式；月球位置：同書第 47 章。地球地圖：Natural Earth（公有領域）。月面是依真實月海位置繪製的模型。
Reading · 英文閱讀
13
Unit 13: Why Do We Always See the Same Side of the Moon?
Look at the full Moon, and you will see the same dark patches your great-grandparents saw. In Chinese stories, they form a jade rabbit pounding medicine; in the West, people see a face, the Man in the Moon. Poets thousands of years ago saw exactly the same picture. The Moon always keeps one face toward Earth, and its other side stays hidden. Why?
看看滿月，你會看到和曾祖父母看過的一模一樣的暗色斑紋。在中國的故事裡，它們是一隻正在搗藥的玉兔；在西方，人們看到一張臉，叫「月亮上的人」。幾千年前的詩人看到的，也是同一幅圖。月亮永遠用同一面朝向地球，另一面一直藏著。為什麼？
It is not because the Moon does not spin. The Moon does rotate, exactly once each time it goes around Earth, about every 27.3 days. Try it yourself: walk in a circle around a chair while always facing the chair. When you get back to where you started, you have also turned around once, facing every wall of the room. The Moon does the same dance, so its near side always faces us.
不是因為月亮不自轉。月亮確實會自轉，而且每繞地球一圈，正好自轉一圈，大約 27.3 天。自己試試看：繞著一張椅子走一圈，臉一直朝著椅子。回到起點時，你也轉了一整圈，面對過房間的每一面牆。月亮跳的就是這支舞，所以它的正面永遠朝著我們。
Why are the two times exactly the same? The answer is tides. Earth's gravity stretches the Moon slightly, just as the Moon raises tides on Earth. Long ago, the young Moon spun faster. Earth kept tugging on its stretched shape like a brake, slowing the spin down until the bulge pointed straight at Earth and the Moon was locked in step. Most of the large moons in the solar system are locked to their planets in the same way.
為什麼兩個時間剛好一樣？答案是潮汐。地球的引力把月亮微微拉長，就像月亮在地球上造成潮汐一樣。很久以前，年輕的月亮轉得比較快。地球不斷拉著它被拉長的部分，像煞車一樣讓它越轉越慢，直到那個鼓起正對著地球，月亮就被「鎖」住了。太陽系裡大部分的大衛星，都用同樣的方式被它們的行星鎖住。
The lock is not perfect. The Moon's orbit is a slight oval, so it moves faster when it is closer to Earth and slower when it is farther away, while its spin stays steady. Its spin axis is also tilted a little. As a result, the Moon seems to rock from side to side and nod up and down every month, letting us peek around its edges. Over time, we can see about 59 percent of its surface.
這把鎖並不完美。月亮的軌道是稍扁的橢圓，離地球近時走得快、遠時走得慢，自轉卻一直等速；它的自轉軸也有一點傾斜。結果月亮每個月好像左右搖一搖、上下點點頭，讓我們從邊緣偷看到一點點背面。日積月累，我們看得到月面的大約 59%。
No one saw the far side until 1959, when the Soviet spacecraft Luna 3 photographed it. People sometimes call it the dark side, but it gets just as much sunlight as the near side; at new Moon, the far side is fully lit. In 2019, China's Chang'e 4 made the first landing on the far side and sent out a rover named Yutu-2, or Jade Rabbit 2. In 2024, Chang'e 6 brought back the first rocks from the far side.
直到 1959 年，蘇聯的太空船月球三號拍下照片，人類才第一次看見月球背面。有人叫它「暗面」，但它照到的陽光和正面一樣多；新月的時候，背面整個是亮的。2019 年，中國的嫦娥四號第一次在月球背面著陸，放出一輛叫「玉兔二號」的月球車。2024 年，嫦娥六號第一次從月球背面帶回岩石。
The same face has looked down on all of human history. More than 2,000 years ago, the poet Qu Yuan asked why a rabbit sits in the Moon's belly. Tides are still at work today: they push the Moon about 3.8 centimeters farther away each year and slow down Earth's spin, making our day longer by about two thousandths of a second every hundred years. Look below to see how the real Moon is tipped today, and watch it rock over the coming weeks.
同一張臉，看著人類的全部歷史。兩千多年前，詩人屈原就在〈天問〉裡問：月亮的肚子裡為什麼有一隻兔子？潮汐到今天仍在作用：它每年把月亮推遠大約 3.8 公分，也讓地球越轉越慢，每一百年白天大約變長千分之二秒。看看下面今天真實的月亮微微轉向哪裡，接下來幾週再觀察它怎麼搖。
Read aloud · 課文朗讀
Questions · 閱讀理解
Does the Moon spin? How long does one spin take?
Yes. It spins once every 27.3 days, exactly the time it takes to go around Earth once.
What slowed down the Moon's spin long ago?
Tides: Earth's gravity tugged on the Moon's stretched shape like a brake until its spin matched its trip around Earth.
Why can we see about 59 percent of the Moon's surface instead of just half?
Because the Moon seems to rock and nod each month. Its orbit is an oval, so its speed changes while its spin stays steady, and its spin axis is a little tilted, so we can peek around its edges.
Words & Phrases · 生字及片語
rotate(v.)旋轉；自轉The ceiling fan rotates slowly on hot days.熱天時，吊扇慢慢地轉。spin(n.)旋轉The ice skater finished with a fast spin.溜冰選手用一個快速的旋轉結束表演。face(v.)面向；朝著Our classroom windows face the mountains.我們教室的窗戶面向群山。hidden(adj.)隱藏的；看不見的We found a hidden path behind the temple.我們在廟後面發現一條隱密的小路。slow down(phr. v.)慢下來；減速Cars must slow down near the school.汽車在學校附近必須減速。lock(v.)鎖住；固定Please lock the door when you leave.離開時請把門鎖上。rock(v.)搖晃；輕輕擺動The boat rocked gently on the waves.小船在浪上輕輕搖晃。edge(n.)邊緣Don't stand too close to the edge of the cliff.不要站得太靠近懸崖邊。
Quick Check · 小測驗
1. How long does the Moon take to spin once?
2. Why do we always see the same side of the Moon?
3. What slowed the Moon's spin long ago?
4. About how much of the Moon's surface can we see over time?
5. Which spacecraft made the first landing on the Moon's far side?
Near Side, Far Side · 正面與背面
Four ways to think about the Moon's face 從四個角度看月亮的臉
Press See it in 3D to switch the Moon's spin in the model.
按「在模型中看」，就能切換模型裡月亮的自轉方式。
If It Did Not Spin如果不自轉
The Moon would keep pointing the same way in space, and Earth would see every side of it during a month.
月亮會一直指向太空中同一個方向，一個月裡地球會看到它的每一面。
Rocking and Nodding左右搖、上下點頭
The orbit's oval shape and the tilted spin axis let us peek around the edges: about 59% of the surface over time.
橢圓軌道和傾斜的自轉軸讓我們偷看到邊緣：日積月累看得到約 59% 的月面。
The Far Side月球背面
Not dark: it is fully lit at new Moon. It has fewer dark maria and many more craters than the near side.
不是暗的：新月時它整個被照亮。背面的暗色月海比較少，撞擊坑多得多。
Today · 今天
How the Moon Is Tipped Today 今天的月亮怎麼搖
Today's Moon with its real phase and its real tilt, and the best days this month to peek around each edge. Everything is calculated in your browser for today's date.
今天的月亮：真實的月相和真實的天平動，以及這個月最能偷看到各邊緣的日子。全部在瀏覽器裡照今天的日期現算。
Calculating… · 計算中…
East and West · 東西方的星空
The Face Everyone Has Seen 人人看過的那張臉
Because the Moon never turns away, every culture has looked at the same patterns and told stories about them.
因為月亮從不轉身，每個文化看到的都是同一組花紋，也各自說出了故事。
天問 · Qu Yuan
A Rabbit in the Moon's Belly顧菟在腹
夜光何德，死則又育？厥利維何，而顧菟在腹？——屈原〈天問〉"What power has the night light, to die and then be born again? What good does it gain, with a rabbit in its belly?" (Qu Yuan, Heavenly Questions)
More than 2,000 years ago, Qu Yuan asked about the waxing and waning Moon and the rabbit people saw in it. We still see that same rabbit, because the same face still looks at us.
兩千多年前，屈原問月亮為什麼會圓缺，又問月中那隻兔子。我們今天看到的還是同一隻兔子，因為月亮還是同一張臉朝著我們。
玉兔 · Yutu
A Jade Rabbit on the Far Side玉兔到了背面
China named its Moon program after Chang'e, the lady in the Moon, and its rovers after her jade rabbit. In 2019, Yutu-2 became the first rover to drive on the far side, where the rabbit's face can never be seen.
中國的探月計畫以月中仙子嫦娥命名，月球車則叫「玉兔」。2019 年，玉兔二號成為第一輛在月球背面行駛的月球車——那裡正好是永遠看不到兔子的一面。
1959 · Luna 3
The First Look Around the Back第一次繞到背後看
In October 1959, the Soviet spacecraft Luna 3 swung behind the Moon and photographed the far side. In 1968, the three astronauts of Apollo 8 became the first people to see it with their own eyes.
1959 年 10 月，蘇聯的月球三號繞到月球後方，拍下背面的照片。1968 年，阿波羅 8 號的三位太空人成為第一批親眼看到月球背面的人。
Pluto · Charon
Locked Face to Face面對面鎖住
Far from the Sun, Pluto and its large moon Charon are locked to each other: each always shows the other the same face. Earth's spin is also slowing, but it would take far longer than the Sun will last for Earth to lock to the Moon.
在遙遠的外太陽系，冥王星和它的大衛星凱倫互相鎖住：兩邊永遠用同一面對著彼此。地球的自轉也在變慢，但要慢到被月亮鎖住，所需的時間遠比太陽的壽命還長。
Myth vs. Fact · 常見迷思
Four things people get wrong 四個常見的誤會
✗ Myth · 迷思The Moon does not spin.月亮不會自轉。
✓ Fact · 事實It spins once every 27.3 days, exactly as long as one trip around Earth. If it did not spin at all, we would see every side of it during a month.它每 27.3 天自轉一圈，和繞地球一圈一樣久。如果完全不自轉，一個月裡我們反而會看到它的每一面。
✗ Myth · 迷思The far side of the Moon is always dark.月球背面永遠是暗的。
✓ Fact · 事實The far side gets just as much sunlight as the near side. At new Moon, when the side facing us is dark, the far side is fully lit. It is far, not dark.背面照到的陽光和正面一樣多。新月時，朝著我們的一面是暗的，背面卻整個亮著。它是「遠」，不是「暗」。
✗ Myth · 迷思We see exactly half of the Moon.我們剛好看得到半個月亮。
✓ Fact · 事實Because the Moon rocks and nods a little each month, we can see about 59 percent of its surface over time.因為月亮每個月會微微搖晃、點頭，日積月累我們看得到大約 59% 的月面。
✗ Myth · 迷思The Moon has always shown us the same face.月亮從一開始就用同一面對著我們。
✓ Fact · 事實The young Moon spun faster. Tides from Earth slowed it down until it became locked, long before there were people to watch.年輕的月亮轉得比較快，是地球的潮汐讓它慢下來、最後被鎖住——那是遠在人類出現之前的事。
Remember It · 記憶口訣
Face the Earth 一句話記住月亮的臉
One spin, one orbit繞一圈，轉一圈
The Moon spins once for every trip around Earth: 27.3 days each.
月亮繞地球一圈，自轉一圈，都是 27.3 天。
Face the chair繞椅子走，臉朝椅子
Walk around a chair while facing it, and you turn around once without noticing.
繞著椅子走、臉朝著椅子，你不知不覺就自轉了一圈。
59, not 50看得到 59%，不是 50%
The Moon rocks and nods, so over time we see about 59% of it.
月亮會搖晃點頭，日積月累看得到約 59%。
Far side, not dark side背面不是暗面
The far side gets as much sunlight as the near side.
背面照到的陽光和正面一樣多。
Classroom Activity 1 · 課堂活動一
Walk Around the Earth 繞著地球走
You need · 準備材料
- A chair to be Earth, in the middle of an open space一張椅子當地球，放在空地中央
- A paper plate with a face drawn on it, held in front of the walker's chest (the Moon's near side)一個畫了臉的紙盤，拿在走路的人胸前（月亮的正面）
- Signs on the four walls: North, East, South, West四面牆貼上「北、東、南、西」
Steps · 步驟
- One student walks slowly in a circle around the chair, always keeping the plate's face toward the chair.一位同學慢慢繞著椅子走一圈，紙盤上的臉一直朝著椅子。
- The class calls out which wall sign the walker's back is facing. Did it change? (Yes: the walker turned once.)全班喊出走路的人背後對著哪一面牆。有變嗎？（有：走路的人轉了一圈。）
- Now walk around again, but keep facing the North wall the whole time. What does the chair see? (The back of the plate.)再走一圈，但這次一直面向北牆。椅子看到什麼？（紙盤的背面。）
- Discuss: which walk is like the real Moon?討論：哪一種走法像真實的月亮？
Teaching tip: then switch the 3D model between Real Moon and No spin, and compare it with the two walks.
教學提示：接著把 3D 模型在「真實」和「不自轉」之間切換，和兩種走法對照。
Classroom Activity 2 · 課堂活動二
Watch the Moon Rock 看月亮搖晃
You need · 準備材料
- The "How the Moon Is Tipped Today" section on this page本頁「今天的月亮怎麼搖」
- Binoculars, if you have them雙筒望遠鏡（有的話）
- A notebook with a circle drawn on each page一本每頁畫好一個圓圈的筆記本
Steps · 步驟
- Find Mare Crisium, the round dark patch near the Moon's right edge, on the page's picture.在頁面的月亮圖上找到危海：靠近右邊緣的一塊圓形暗斑。
- On several evenings over two weeks, sketch where Mare Crisium sits inside the circle. Note the date.兩週內挑幾個晚上，在圓圈裡畫出危海的位置，寫上日期。
- Compare your sketches. On some days it sits farther from the edge than on others.比較你的素描：有些日子它離邊緣比較遠。
- Check the page: were those the days when the east edge tipped toward us?對照頁面：那幾天是不是東緣轉過來的日子？
Tip: Mare Crisium is on the side lit by a waxing Moon in the evening, so the first two weeks after new Moon work best.
提示：危海在傍晚的盈月亮面上，所以新月後的頭兩週最好觀察。