A Flat Disk扁平的圓盤
100,000 × 1,000light-years wide × thick · 光年寬 × 厚
Flatter than a pancake. If the disk were the size of Taiwan, it would be only about 4 kilometers thick.
比煎餅還扁。如果盤面有台灣那麼大，厚度只有大約 4 公里。
Astronomy · Lesson 14 · 天文教育 第14課
What Is the Milky Way?銀河是什麼？
The glowing band across a dark summer sky is our own galaxy, seen from the inside. Fly out of the disk, then back to the Sun's place, and find where the band lies over Changhua tonight.
暗夜裡橫過天空的那條光帶，是我們從「裡面」看到的自己的星系。飛出星系盤，再回到太陽的位置，找找今晚彰化天上的銀河在哪裡。
Big idea · 一句話看懂
The Milky Way is our galaxy, a flat disk of hundreds of billions of stars. We live inside the disk, so when we look along it, the light of countless distant stars blends into a glowing band across the sky.
銀河就是我們所在的星系：一個由幾千億顆恆星組成的扁平圓盤。我們住在盤子裡面，順著盤面看出去，無數遙遠星星的光融成一條橫過天空的光帶。
3D Model · 3D 模型
Living Inside a Flat Disk 住在一個扁平的圓盤裡
Start outside the galaxy and find the Sun, marked in gold, out in the disk. Switch to Edge-on to see how flat the disk is. Then choose From the Sun: now you are inside the disk, and the stars along it pile up into a band that circles you, brightest toward the center in Sagittarius. The real bright stars are drawn around you, with Altair and Vega on opposite banks. On the right, the band is drawn over the real sky above Changhua.
先從星系外面找到太陽（金色記號），它在盤面靠外的地方。切換到「側面看」，看看這個盤子有多扁。再選「從太陽看」：現在你在盤子裡面，沿著盤面的星星疊成一條繞著你一圈的光帶，往人馬座的銀河中心最亮；周圍也畫上了真實的亮星，牛郎星和織女星正好在光帶兩岸。右邊把銀河畫在彰化真實的星空上。
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. The sky chart and tonight's guide still work.
這個瀏覽器不支援 WebGL，無法顯示 3D 模型；星圖與今晚的指南一樣能用。
- Gold ring: the Sun, about 26,000 light-years from the center
金色圈：太陽，離中心約 2.6 萬光年 - Blue-white: young stars in the spiral arms; yellow: older stars in the bulge and bar
藍白色：旋臂裡的年輕恆星；黃色：核球與棒裡的老恆星 - Right chart: the Milky Way band over Changhua, from real galactic coordinates
右側星圖：彰化天上的銀河光帶，照真實銀道座標畫 - One unit is 1,000 light-years; the whole disk is about 100,000 light-years across
1 單位＝1,000 光年；整個盤面寬約 10 萬光年
The galaxy here is a drawing based on what astronomers know: its size, thickness, bar, spiral arms, and the Sun's place. No one has ever photographed our galaxy from outside. The directions in the From the Sun view and the band on the sky chart use real galactic coordinates, and the stars are real.
這裡的銀河是依天文學家已知的大小、厚度、棒狀結構、旋臂與太陽位置畫出來的示意圖——從來沒有人從外面拍過我們的星系。「從太陽看」的方向與星圖上的光帶用的是真實銀道座標，星星也是真的。
Galactic coordinates: IAU definition (J2000). Distance to the center: GRAVITY Collaboration (2019). Stars: Yale Bright Star Catalogue, 5th ed. (public domain). The galaxy itself is a drawn model.
銀道座標：國際天文聯合會（IAU）定義（J2000）。到銀心的距離：GRAVITY 合作團隊（2019）。星表：耶魯亮星星表第五版（公有領域）。銀河本身是繪製的模型。
Reading · 英文閱讀
14
Unit 14: What Is the Milky Way?
Far from city lights, on a clear summer night, a pale, glowing band stretches across the sky like a river of mist. In Chinese, it is called the Silver River or the River of Heaven; in English, it is the Milky Way. For thousands of years, people wondered what it was. Was it a cloud? A road across the sky? A river of milk?
遠離城市燈光的晴朗夏夜，一條淡淡發光的帶子橫過天空，像一條霧氣做的河。中文叫它「銀河」或「天河」，英文叫 Milky Way（牛奶路）。幾千年來，人們一直在想它是什麼：一片雲？一條橫過天空的路？還是一條牛奶河？
In 1610, Galileo turned one of the first telescopes toward it and saw that the band is made of countless faint stars, too far away and too close together for our eyes to tell apart. Today we know that the Milky Way is our galaxy: a huge, flat disk of hundreds of billions of stars, along with gas and dust, all held together by gravity. Our Sun is just one of those stars.
1610 年，伽利略把最早的望遠鏡之一對準它，發現這條光帶是由無數暗淡的星星組成的——它們太遠、又擠得太近，眼睛分不開。今天我們知道，銀河就是我們所在的星系：一個由幾千億顆恆星，加上氣體和塵埃，靠引力聚在一起的巨大扁平圓盤。我們的太陽只是其中一顆。
So why does it look like a band? We live inside the disk, about 26,000 light-years from its center. When we look along the disk, our view passes through enormous numbers of stars, and their light blends into a glowing band. When we look up or down, out of the disk, we see far fewer. Imagine standing in a crowd spread out across a wide, flat field: look straight ahead and you see people all around you, but look up and you see only sky.
那它為什麼看起來是一條帶子？因為我們住在盤子裡面，離中心大約 2.6 萬光年。順著盤面看出去，視線穿過數不清的星星，它們的光融成一條發光的帶子；往盤面上下看，星星就少多了。想像你站在一大片平坦的空地上，四周散滿了人：平視時四面八方都是人，往上看卻只有天空。
The disk is about 100,000 light-years across but only about 1,000 light-years thick, flatter than a pancake. At its center is a bulge of older stars and a black hole about four million times as heavy as the Sun. Spiral arms wind out from the center, and the whole galaxy turns. The Sun takes about 230 million years to go around once. The last time it was where it is now, the first dinosaurs were appearing.
這個盤子寬約 10 萬光年，厚度卻只有約 1,000 光年，比煎餅還扁。中心是一團由老恆星組成的核球，還有一個質量約太陽 400 萬倍的黑洞。旋臂從中心盤旋而出，整個星系都在轉。太陽繞一圈大約要 2.3 億年；上一次它在現在這個位置時，最早的恐龍才剛出現。
In Chinese stories, the Milky Way is the river that keeps the Cowherd and the Weaver Girl apart. Look for them in the evening sky: Altair and Vega shine on opposite banks of the band. The ancient Book of Songs describes the bright River of Clouds turning across the sky. The Greeks said it was milk spilled by the goddess Hera, and that is why our word galaxy comes from the Greek word for milk.
在中國的故事裡，銀河是隔開牛郎和織女的那條河。傍晚找找看：牛郎星和織女星正好在光帶的兩岸。古老的《詩經》寫過明亮的「雲漢」在天上流轉；希臘人則說它是天后赫拉灑出的乳汁，所以英文的 galaxy（星系）來自希臘文的「乳汁」。
Sadly, more than a third of the people on Earth can no longer see the Milky Way from home, because city lights hide it. In Taiwan, the best time to see its bright center is a summer evening, from a dark place such as the Hehuan Mountain Dark Sky Park. Use the chart below to find where the band lies over Changhua tonight.
可惜的是，地球上超過三分之一的人，在家已經看不到銀河了，因為城市燈光把它遮住了。在台灣，看明亮的銀河中心最好的時間是夏天傍晚，地點要夠暗，例如合歡山暗空公園。用下面的星圖找找看，今晚彰化天上的銀河在哪裡。
Read aloud · 課文朗讀
Questions · 閱讀理解
What is the Milky Way made of?
Hundreds of billions of stars, along with gas and dust. Most of the stars are too faint and too close together for our eyes to see one by one.
Why does the Milky Way look like a band across the sky?
We live inside a flat disk of stars. Looking along the disk, we see huge numbers of stars whose light blends into a band; looking out of the disk, we see far fewer.
About how long does the Sun take to go around the center of the galaxy once?
About 230 million years.
Words & Phrases · 生字及片語
band(n.)帶子；條紋She wore a red band in her hair.她頭髮上綁了一條紅色髮帶。disk(n.)圓盤The frisbee is a plastic disk that flies through the air.飛盤是一個能在空中飛的塑膠圓盤。billion(n.)十億About eight billion people live on Earth.地球上住著大約八十億人。countless(adj.)數不清的There were countless grains of sand on the beach.海灘上有數不清的沙粒。spiral(adj.)螺旋的We climbed a spiral staircase to the top of the lighthouse.我們爬螺旋梯到燈塔頂端。center(n.)中心There is a fountain in the center of the park.公園中央有一座噴泉。cloud(n.)雲A dark cloud covered the Sun before the storm.暴風雨前，一片烏雲遮住了太陽。pollution(n.)污染Light pollution makes it hard to see stars in the city.光害讓人在城市裡很難看到星星。
Quick Check · 小測驗
1. What did Galileo see when he looked at the Milky Way with a telescope?
2. Why does the Milky Way look like a band?
3. About how wide is our galaxy?
4. Which two stars shine on opposite banks of the Milky Way?
5. In Taiwan, when is the best time to see the bright center of the Milky Way?
Our Galaxy · 我們的星系
Four numbers to remember 記住四個數字
Press See it in 3D to fly to each view in the model.
按「在模型中看」，模型就會飛到那個視角。
Where We Live我們住在哪裡
26,000light-years from the center · 光年，離中心
In a small arm called the Orion Spur, between two large spiral arms, about halfway out from the center.
在一條叫「獵戶臂」的小旋臂上，夾在兩條大旋臂之間，大約在中心到邊緣的一半。
One Galactic Year一個銀河年
230 millionyears for one trip around · 年繞一圈
The Sun races along at about 230 kilometers a second. One galactic year ago, the first dinosaurs were appearing.
太陽以每秒約 230 公里的速度前進。一個銀河年以前，最早的恐龍才剛出現。
Looking Along the Disk順著盤面看
360°the band circles the whole sky · 光帶繞天空一整圈
The band goes all the way around the sky, but half of it is always below the horizon. Its brightest part, the center, lies in Sagittarius.
光帶繞天空一整圈，但永遠有一半在地平線下。最亮的銀河中心在人馬座。
Tonight · 今晚
The Milky Way over Changhua Tonight 今晚彰化的銀河
Where the band lies at 9 p.m. tonight, whether its bright center is up, and how the Moon will affect your view, plus the best months for the center. Everything is calculated in your browser for today's date.
今晚九點銀河光帶在哪裡、明亮的銀心升起了沒有、月光會不會干擾，以及一年中看銀心最好的月份。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 River in the Sky 天上的河
East and West, people saw the Milky Way as a river, a road, or a stream of milk, long before anyone knew it was made of stars.
在知道它是由星星組成之前很久，東西方的人就把銀河看成一條河、一條路，或一道乳汁。
雲漢 · Book of Songs
The Shining River of Clouds倬彼雲漢
倬彼雲漢，昭回于天。——《詩經．大雅．雲漢》"Bright is that River of Clouds, shining and turning across the sky." (Book of Songs)
In this ancient poem, a king looks up at the Milky Way during a terrible drought and prays for rain. Chinese has many names for it: the River of Clouds, the River of Heaven, and the Silver River.
這首古詩裡，一位君王在大旱時仰望銀河、祈求降雨。中文替銀河取了很多名字：雲漢、天河、河漢、銀漢、銀河。
銀漢 · Qin Guan
Two Stars on Two Banks銀漢迢迢暗度
纖雲弄巧，飛星傳恨，銀漢迢迢暗度。——秦觀〈鵲橋仙〉"Thin clouds weave their art, shooting stars carry sorrow, and far across the Silver River she quietly crosses." (Qin Guan)
On the night of Qixi, the Weaver Girl crosses the Milky Way to meet the Cowherd. In the sky, Vega and Altair really do sit on opposite sides of the band, about 15 light-years apart (Lesson 10).
七夕夜，織女渡過銀河與牛郎相會。在天上，織女星和牛郎星真的分在銀河兩岸，相距約 15 光年（第十課）。
Galaxias
A River of Milk一道乳汁
The ancient Greeks called it the milky circle and told of milk spilled by the goddess Hera. The Latin name became the Milky Way, and the Greek word for milk, gala, gave us the word galaxy.
古希臘人叫它「乳白色的圓環」，說是天后赫拉灑出的乳汁。拉丁文變成英文的 Milky Way，而希臘文的「乳汁」（gala）就是英文 galaxy（星系）的字源。
1610 · 1918
Finding Our Place找到我們的位置
Galileo first saw that the Milky Way is made of stars. Later astronomers thought the Sun sat near the middle, until Harlow Shapley showed in 1918, using star clusters, that the center lies far away in the direction of Sagittarius.
伽利略第一個看出銀河是由星星組成。後來的天文學家以為太陽在銀河正中央，直到 1918 年，沙普利用星團證明銀河中心其實遠在人馬座方向。
Myth vs. Fact · 常見迷思
Four things people get wrong 四個常見的誤會
✗ Myth · 迷思The Milky Way is a cloud in our sky.銀河是天上的一片雲。
✓ Fact · 事實It is the light of countless stars thousands of light-years away, too faint and crowded for our eyes to see one by one.它是幾千光年外數不清的星星發出的光，太暗又太擠，眼睛分不出一顆一顆。
✗ Myth · 迷思Pictures of a spiral galaxy show our Milky Way from outside.那些漩渦星系的圖片，是從外面拍的銀河。
✓ Fact · 事實No spacecraft has ever left our galaxy. Pictures of the Milky Way from outside are drawings based on measurements, or photos of other galaxies.沒有任何太空船離開過銀河。從外面看的銀河圖，不是依測量資料畫的示意圖，就是其他星系的照片。
✗ Myth · 迷思The Sun is at the center of the galaxy.太陽在銀河的中心。
✓ Fact · 事實The Sun is about 26,000 light-years from the center, roughly halfway out. The center lies in the direction of Sagittarius.太陽離中心約 2.6 萬光年，大約在一半的地方。銀河中心在人馬座的方向。
✗ Myth · 迷思Only the stars in the band belong to the Milky Way.只有光帶裡的星星屬於銀河。
✓ Fact · 事實Every star you can see without a telescope belongs to our galaxy. They are just our nearer neighbors, scattered all around us. The Andromeda Galaxy, 2.5 million light-years away, is one of the very few other galaxies visible to the eye.肉眼看得到的每一顆星都屬於銀河，只是比較近、散在我們周圍。250 萬光年外的仙女座星系，是極少數肉眼看得到的其他星系之一。
Remember It · 記憶口訣
Find your place in the galaxy 一句話找到我們在銀河的位置
Inside the pancake住在煎餅裡
We live inside a flat disk of stars, so we see it edge-on, as a band.
我們住在扁平的星盤裡，從側面看出去，就是一條帶子。
100,000 by 1,000十萬寬、一千厚
The disk is about 100,000 light-years wide and 1,000 light-years thick.
盤面寬約 10 萬光年，厚約 1,000 光年。
Two stars, two banks牛郎織女隔河望
Altair and Vega shine on opposite sides of the Milky Way.
牛郎星和織女星分在銀河兩岸。
Summer center, winter faint夏看銀心，冬看淡河
The bright center is up on summer evenings; in winter we look toward the faint outer edge.
夏天傍晚看得到明亮的銀心；冬天看的是比較淡的外緣。
Classroom Activity 1 · 課堂活動一
Be the Galaxy 全班當銀河
You need · 準備材料
- The whole class and a playground全班同學和一片操場
- Chalk to mark a large circle粉筆，畫一個大圓圈
- A hat or sign for the student who plays the Sun一頂帽子或牌子，給扮演太陽的同學
Steps · 步驟
- Draw a large circle. Everyone stands inside it, spread out but more crowded near the middle: this is the galaxy's disk.畫一個大圓。全班站在圈裡，分散開來，但越靠中間越擠：這就是銀河的盤面。
- One student, the Sun, stands about halfway between the middle and the edge.一位同學扮太陽，站在中心到邊緣大約一半的地方。
- The Sun turns slowly and looks straight ahead: classmates fill the view all the way around, most toward the middle.太陽慢慢轉一圈、平視前方：四周都是同學，往中心的方向最多。
- Now the Sun looks up: only sky. Discuss: how is this like the Milky Way band?再往上看：只有天空。討論：這和天上的銀河光帶有什麼像的地方？
Teaching tip: then open the 3D model and switch from Outside to From the Sun.
教學提示：接著打開 3D 模型，從「從外面看」切換到「從太陽看」。
Classroom Activity 2 · 課堂活動二
How Dark Is Your Sky? 你家的天空有多暗？
You need · 準備材料
- The sky chart on this page, to find the Summer Triangle (Vega, Altair, and Deneb)本頁的星圖，用來找夏季大三角（織女星、牛郎星、天津四）
- A red flashlight and a worksheet一支紅光手電筒和一張學習單
- An adult to come along一位大人陪同
Steps · 步驟
- On a clear, moonless evening, find the Summer Triangle from your home or school.在晴朗、沒有月亮的傍晚，從家裡或學校找到夏季大三角。
- Let your eyes adjust for 15 minutes, then count how many stars you can see inside the triangle.讓眼睛適應 15 分鐘，數數三角形裡看得到幾顆星。
- If you can, count again from a darker place, such as the countryside or a mountain.有機會的話，到更暗的地方（鄉下或山上）再數一次。
- Compare the class's results. Where could you see the Milky Way running through the triangle?比較全班的結果：在哪些地方看得到銀河穿過三角形？
Tip: this is how the citizen-science project Globe at Night measures light pollution around the world.
提示：國際的公民科學計畫「Globe at Night」就是用這種方法，測量世界各地的光害。