Mercury水星・辰星
Always close to the Sun, so look low in the twilight. It goes backward when it passes between Earth and the Sun.
總是離太陽很近，要在黃昏或清晨的低空找。它從地球與太陽之間經過時會逆行。
Astronomy · Lesson 7 · 天文教育 第七課
Why Do Planets Wander Across the Sky?行星為什麼會在星空中遊走？
Watch Earth overtake Mars on real orbits, see why planets loop backward among the stars, and find tonight's planets over Changhua.
在真實軌道上看地球超車火星，看懂行星為什麼會在星空中倒退繞圈；再看今晚彰化的天上有哪些行星。
Big idea · 一句話看懂
Planets go around the Sun at different speeds. When Earth overtakes a slower outer planet, the planet seems to drift backward among the stars for a while.
行星繞太陽的速度各不相同。地球超車外側較慢的行星時，那顆行星看起來會在星空中倒退一陣子。
3D Model · 3D 模型
Earth Overtakes the Planets 地球超車行星
The orbits are real and to scale, and the planets move on real dates. The white sight line runs from Earth through the planet and on to the distant stars. Press Play and watch Earth, on the faster inside track, catch up with Mars and pass it. Around that moment, the planet's path among the stars, on the right, turns orange and runs backward. The numbers 1 to 9 mark the same nine dates in both pictures: Earth and the planet in the 3D model, and the planet's place among the stars on the chart, where the order goes forward, backward, and forward again.
軌道是真實比例，行星照真實日期運行。白色視線從地球穿過行星，一路延伸到遙遠的星空。按下播放，看跑在內圈、速度較快的地球追上火星、超過它；就在超車前後，右邊行星在星空中的路徑變成橘色、往回倒退。1 到 9 的號碼在兩張圖裡標的是同樣九個日期：3D 模型裡是地球和行星的位置，右邊路徑圖上是行星在星空中的位置——順序是前進、倒退、再前進。
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 path chart and the cards below still work.
這個瀏覽器不支援 WebGL，無法顯示 3D 模型；路徑圖與下方的卡片一樣能用。
- White line: the sight line from Earth through the planet to the stars
白線：從地球穿過行星、延伸到星空的視線 - Numbers 1 to 9: Earth and the planet on nine equally spaced dates, joined by sight lines; orange while the planet moves backward
號碼 1 到 9：九個等間隔日期的地球與行星位置，以視線相連；行星倒退時是橘色 - Path in the sky: teal while moving east as usual, orange while moving backward
天上的路徑：照常往東時是青色，倒退時是橘色 - Dashed gold circle: the ecliptic, the Sun's path. Planets always stay close to it
金色虛線圈：黃道（太陽的路線）；行星永遠在它附近
Orbits to scale (1 AU = 10 units); the Sun and planets are drawn hundreds of times too big. The stars are drawn as if infinitely far away, so the planet's place among them is correct. Positions come from real orbital elements, accurate to well under a degree.
軌道按真實比例（1 天文單位＝10 單位）；太陽和行星本身畫大了好幾百倍。星空當作無限遠，所以行星在星空中的位置是正確的。位置由真實的軌道根數算出，誤差遠小於一度。
Planet positions: approximate orbital elements from NASA/JPL Solar System Dynamics (E. M. Standish), valid 1800–2050; brightness from Meeus, Astronomical Algorithms. Stars: Yale Bright Star Catalogue, 5th ed. (public domain). Constellation lines drawn by twrses.org.
行星位置：NASA／JPL 太陽系動力學組的近似軌道根數（Standish），適用 1800–2050 年；亮度公式取自 Meeus《天文演算法》。星表：耶魯亮星星表第五版（公有領域）。星座連線為本站自繪。
Reading · 英文閱讀
7
Unit 7: Why Do Planets Wander Across the Sky?
The ancient Greeks noticed that five of the brightest "stars" do not stay put. Night after night, they drift slowly through the constellations of the zodiac, and every so often one of them stops, turns around, and moves backward for weeks before going forward again. The Greeks called them planetes, which means "wanderers," and that is where our word planet comes from. Chinese astronomers watched the same lights. They named Mars Yinghuo, "the dazzling puzzler," because its path was so hard to predict.
古希臘人注意到，天上最亮的「星」裡有五顆不肯待在原地。它們一夜接一夜，慢慢穿過黃道上的星座；每隔一陣子，其中一顆會停下來、掉頭，往回走好幾個星期，然後才再往前。希臘人叫它們 planetes，意思是「漫遊者」，英文的 planet（行星）就是這麼來的。中國的天文學家看著同樣的光點，把火星叫做「熒惑」——閃閃發光、又讓人困惑，因為它的路線實在太難預測了。
Today we know that the planets, Earth included, all travel around the Sun in the same direction and in nearly the same flat plane. That is why they always appear near the zodiac, along the Sun's yearly path. The closer a planet is to the Sun, the faster it goes. Mercury makes a full lap in 88 days, Earth in a year, Mars in about two years, Jupiter in about 12 years, and Saturn in almost 30. Seen from Earth, a planet usually creeps a little farther east among the stars each night.
今天我們知道，包括地球在內，所有行星都以同一個方向、在幾乎同一個平面上繞著太陽轉。所以行星總是出現在黃道附近，也就是太陽一年走過的路線上。離太陽越近的行星跑得越快：水星 88 天繞一圈，地球一年，火星大約兩年，木星大約 12 年，土星將近 30 年。從地球看，行星通常每天晚上在星空中往東多挪一點點。
So why does a planet sometimes go backward? Think of riding in a car on the highway. As you pass a slower truck, the truck seems to slide backward against the faraway mountains, even though it is still moving forward. Earth, on its faster inside track, passes Mars about every 26 months. For a couple of months around that time, Mars appears to drift west against the stars, then turns east again. Astronomers call this retrograde motion. It happens when the planet is on the opposite side of Earth from the Sun, so the planet is also at its closest, brightest, and up all night.
那行星為什麼有時候會倒退？想像你坐在高速公路上的車裡。超過一輛比較慢的卡車時，卡車好像在遠方山景前面往後滑，雖然它其實還在往前開。地球跑在比較快的內圈，大約每 26 個月超車火星一次。超車前後的兩個月左右，火星看起來在星空中往西倒退，然後才轉回往東。天文學家把這叫做「逆行」。逆行發生在行星位在地球另一邊、正對太陽的時候，所以這時它也離我們最近、最亮，而且整夜都看得到。
Explaining these loops took a very long time. About 1,900 years ago, the astronomer Ptolemy put Earth at the center and moved each planet on small circles riding on bigger circles. His system worked well enough to predict the planets, but it was complicated. In 1543, Copernicus put the Sun at the center instead, and the loops suddenly made simple sense: they are just what you see when a moving Earth overtakes another planet. Retrograde motion became one of the strongest clues that Earth itself is moving.
要解釋這些倒退繞圈，人類花了很久的時間。大約 1,900 年前，天文學家托勒密把地球放在中心，讓每顆行星在「大圓上的小圓」（本輪）上運行。他的系統足以預測行星的位置，但非常複雜。1543 年，哥白尼改把太陽放在中心，繞圈一下子就變得簡單：那只是會動的地球超過另一顆行星時看到的景象。逆行因此成了「地球本身在動」最有力的線索之一。
In Chinese, the five planets you can see with the naked eye are named after the five elements: Mercury is the water star, Venus the metal star, Mars the fire star, Jupiter the wood star, and Saturn the earth star. Their older names describe how they behave. Jupiter was the "year star," because it moves about one twelfth of the way around the sky each year, and people used it to count years. Venus was called Qiming, the morning star, when it rose before dawn, and Changgeng, the evening star, when it shone after sunset.
中文裡，肉眼看得到的五顆行星用五行命名：水星、金星、火星、木星、土星。它們更古老的名字描述了它們的行為：木星叫「歲星」，因為它每年大約在天上走十二分之一圈，古人拿它來紀年；金星在日出前升起時叫「啟明」，日落後閃耀時叫「長庚」。
You do not need a telescope to find a planet. Look near the zodiac for a bright point that shines with a steady light, because stars twinkle more than planets do. Venus is the brightest thing in the night sky after the Moon, and Jupiter comes next. A planet will not stay among the same stars for long, so it never appears on the star maps from Lesson 5. Check the list of tonight's planets below, then look up and watch a wanderer move from one week to the next.
找行星不需要望遠鏡。在黃道附近找一個穩穩發亮的光點，因為恆星比行星更會閃爍。金星是夜空中除了月亮以外最亮的天體，其次是木星。行星不會在同一群星星之間待很久，所以第五課的星圖上從來沒有它們。看看下面今晚的行星清單，然後抬頭，一週一週地看一顆漫遊者怎麼移動。
Read aloud · 課文朗讀
Questions · 閱讀理解
Why does Mars sometimes seem to move backward among the stars?
Earth, on its faster inside orbit, overtakes Mars. Like a slower truck you pass on the highway, Mars seems to slide backward against the distant stars, even though it is still moving forward.
Why are planets always found near the zodiac constellations?
All the planets go around the Sun in nearly the same flat plane as Earth, so they always appear close to the Sun's path, the zodiac.
How did retrograde motion help show that Earth moves?
Copernicus showed that the loops are simply what you see when a moving Earth overtakes another planet, so the loops became a strong clue that Earth itself is moving.
Words & Phrases · 生字及片語
wander(v.)漫遊；閒逛We wandered around the night market for hours.我們在夜市裡逛了好幾個小時。drift(v.)漂移；慢慢移動The boat drifted slowly toward the shore.小船慢慢漂向岸邊。overtake(v.)超車；趕上The bus overtook us on the highway.公車在高速公路上超過了我們。loop(n.)環；圈The roller coaster goes through two loops.雲霄飛車會繞兩個大圈。steady(adj.)穩定的Keep the camera steady when you take the photo.拍照時要把相機拿穩。puzzle(v.)使困惑The missing cookies puzzled everyone.不見的餅乾讓大家一頭霧水。clue(n.)線索The muddy footprints were an important clue.泥巴腳印是重要的線索。naked eye(phr.)肉眼You can see the Pleiades with the naked eye.用肉眼就看得到昴宿星團。
Quick Check · 小測驗
1. What does the Greek word for planet mean?
2. When does Mars appear to move backward?
3. While an outer planet is in retrograde, it is...
4. Which planet can you NOT see without a telescope?
5. How can you tell a planet from a star?
The Five Wanderers · 五星
Five planets you can see without a telescope 肉眼看得到的五顆行星
The dates below are calculated in your browser, so they are always up to date. Uranus and Neptune are planets too, but they are too faint to see without a telescope.
下面的日期在你的瀏覽器裡現算，永遠是最新的。天王星和海王星也是行星，但太暗了，沒有望遠鏡看不到。
Venus金星・太白
The brightest planet. The morning star (Qiming) or the evening star (Changgeng), but never up at midnight.
最亮的行星。清晨叫啟明、黃昏叫長庚，但半夜永遠看不到。
Mars火星・熒惑
Orange-red, with the biggest backward loop. It is brightest when Earth overtakes it.
橘紅色，逆行的圈最大；地球超車它的時候最亮。
Jupiter木星・歲星
Brilliant and creamy white, second only to Venus. It moves about one constellation a year.
明亮的乳白色，僅次於金星；大約每年移動一個星座。
Saturn土星・鎮星
Soft and golden, the slowest wanderer. It spends more than four months of every year going backward.
柔和的金黃色，走得最慢的漫遊者；每年有四個多月在逆行。
Tonight · 今晚
Tonight's Planets over Changhua 今晚彰化的行星
Calculated in your browser for today. Evening planets are given for the end of dusk, morning planets for just before dawn, and the rest for 9 p.m.
依今天的日期在你的瀏覽器裡現算。黃昏的行星以天剛黑時為準，清晨的以天亮前為準，其餘以晚上九點為準。
Calculating… · 計算中…
East and West · 東西方的星空
Wanderers in Many Languages 各種語言裡的漫遊者
Every culture that watched the sky noticed the five lights that do not stay put, and gave them names that still live in our calendars.
每個觀察天空的文化，都注意到這五個不肯待在原地的光點，給了它們名字——有些到今天還留在我們的日曆裡。
熒惑守心 · Mars and Antares
When Mars Lingers at the Heart熒惑守心
When Mars slowed down, stopped, and lingered near Antares, the red heart of the Azure Dragon, Chinese court astronomers took it as a grave warning for the emperor. Today we know it is just a retrograde loop near the zodiac. Fittingly, the name Antares means "rival of Mars," because the two are so alike in color.
火星減速、停留、在青龍的紅色心臟心宿二附近徘徊時，古代朝廷的天文官視為對帝王的嚴重警訊，這就是「熒惑守心」。今天我們知道那只是黃道附近的一次逆行繞圈。有趣的是，Antares 這個名字的意思正是「火星的對手」，因為兩者顏色太像了。
歲星 · The Year Star
Counting Years with Jupiter歲星紀年
Jupiter takes about 12 years to circle the sky, so it moves through about one twelfth each year. Ancient Chinese astronomers used it to count years, and invented Taisui, an imaginary partner that moves the opposite way, to keep the count tidy. That is the Taisui of the old year-counting customs.
木星大約 12 年在天上繞一圈，每年走大約十二分之一，古人就用它來紀年，稱為「歲星」。為了計算方便，又假想出一顆反方向運行的「太歲」——民間說的「太歲」就是從這裡來的。
啟明與長庚 · Morning and Evening
One Planet, Two Names東有啟明，西有長庚
東有啟明，西有長庚。——《詩經·小雅·大東》"In the east is Qiming; in the west is Changgeng." (The Book of Songs)
Long ago, people thought the bright morning star and the bright evening star were two different stars. Both are Venus, which swings from one side of the Sun to the other. The ancient Greeks also had two names for it, Phosphorus and Hesperus, before they realized it was one planet.
很久以前，人們以為清晨的亮星和黃昏的亮星是兩顆不同的星；其實都是金星，它在太陽的兩側來回擺盪。古希臘人也曾給它兩個名字：晨星 Phosphorus 和昏星 Hesperus，後來才發現是同一顆。
星期 · Days of the Week
Planets in the Calendar藏在星期裡的行星
The seven-day week was named for the Sun, the Moon, and the five wandering planets. You can still hear them: Saturday is Saturn's day, and in French, Tuesday through Friday are the days of Mars, Mercury, Jupiter, and Venus. In Japanese, the weekdays are fire, water, wood, metal, and earth, the same planets with their Chinese names.
一週七天原本是用太陽、月亮和五顆行星命名的，現在還聽得出來：英文的 Saturday 是土星日；法文的星期二到星期五是火星、水星、木星、金星的日子；日文的星期是「日月火水木金土」，正是行星的中文名字。
Myth vs. Fact · 常見迷思
Four things people get wrong 四個常見的誤會
✗ Myth · 迷思During retrograde, a planet really turns around and moves backward.逆行的時候，行星真的掉頭往回走。
✓ Fact · 事實It never does. Every planet keeps going around the Sun in the same direction. The backward drift is only how it looks from a moving Earth, the same way a slower truck seems to slide back as you pass it.從來沒有。每顆行星都一直朝同一個方向繞太陽。倒退只是從會動的地球看過去的樣子，就像你超車時，比較慢的卡車看起來往後滑。
✗ Myth · 迷思You need a telescope to see the planets.要有望遠鏡才看得到行星。
✓ Fact · 事實Mercury, Venus, Mars, Jupiter, and Saturn are all easy to see with the naked eye. People have watched them for thousands of years. A small telescope adds Jupiter's moons and Saturn's rings.水星、金星、火星、木星、土星用肉眼都看得到，人類已經看了幾千年。小望遠鏡則能多看到木星的衛星和土星的光環。
✗ Myth · 迷思Planets twinkle just like stars.行星和恆星一樣會閃爍。
✓ Fact · 事實Planets usually shine with a steadier light. A star is so far away that it is a single point, and our moving air makes it flicker. A planet is a tiny disk, so the flickering mostly averages out, except very low in the sky.行星的光通常比較穩。恆星遠到只是一個點，流動的空氣讓它忽明忽暗；行星是一個小圓盤，閃爍大多互相抵消了，只有很靠近地平線時才會閃。
✗ Myth · 迷思A planet can show up anywhere in the sky.行星可能出現在天空的任何地方。
✓ Fact · 事實Planets always stay close to the ecliptic, the Sun's path through the zodiac, because all the orbits lie in nearly the same plane. If a bright "star" is far from the zodiac, it is not a planet.行星永遠在黃道（太陽穿過黃道星座的路線）附近，因為所有軌道幾乎在同一個平面上。離黃道很遠的亮星，就不是行星。
Remember It · 記憶口訣
Spot a wanderer 一句話認出行星
Passing a truck超車的時候
When you pass a slower truck, it seems to slide backward against the mountains. When Earth passes a slower planet, it seems to slide backward against the stars.
超過慢卡車時，卡車好像在山景前往後滑；地球超過慢的行星時，行星就好像在星空前往後滑。
Steady light, near the zodiac不眨眼，在黃道邊
A bright point that does not twinkle, close to the Sun's path, is almost always a planet.
在黃道附近、不太閃爍的亮點，幾乎一定是行星。
Backward, brightest, all night逆行時最亮、整夜可見
An outer planet goes backward when it is opposite the Sun: closest to Earth, at its brightest, and up from dusk to dawn. That is the best time to look.
外行星逆行時正對太陽：離地球最近、最亮、從黃昏掛到天亮——正是看它的最好時機。
Mars 26, Jupiter 13, Saturn 12½火星 26、木星 13、土星 12½
Months between backward loops: Mars about 26, Jupiter about 13, Saturn about 12½. The slower the planet, the sooner Earth laps it again.
兩次逆行相隔幾個月：火星約 26、木星約 13、土星約 12 個半。行星走得越慢，地球越快再超它一圈。
Classroom Activity 1 · 課堂活動一
Overtake Mars on the Playground 操場上超車火星
You need · 準備材料
- Chalk to draw two circles around a center point: a small one for Earth and a bigger one for Mars粉筆：繞著中心點畫兩個圓，小圓是地球的軌道，大圓是火星的軌道
- A long wall or fence beyond the circles, with paper signs numbered 1 to 15 spaced along it as distant stars圓圈外的一道長牆或圍欄，貼上編號 1 到 15 的紙卡，當作遙遠的星星
- Two students, Earth and Mars, plus a recorder with paper兩位學生扮地球和火星，一位記錄員拿紙筆
Steps · 步驟
- Earth and Mars stand on their circles on the same side of the center, Earth a little behind. Both walk counterclockwise; Earth takes two steps for every one step of Mars.地球和火星站在各自的圓上、在中心點的同一側，地球稍微落後。兩人都逆時針走，地球每走兩步，火星走一步。
- Every few steps, the teacher calls "Stop!" Earth points at Mars and keeps pointing past Mars to the wall, and the recorder writes down which numbered sign it hits.老師每隔幾步喊「停！」地球伸手指向火星，再一路指到牆上，記錄員寫下指到第幾號紙卡。
- Keep going until Earth has passed Mars and moved well ahead.一直走到地球超過火星、領先一大段為止。
- Read the list aloud. The numbers climb, then go back down for a while, then climb again: that is retrograde motion.把記錄念出來：數字先變大、接著有一陣子變小，然後又變大——這就是逆行。
Teaching tip: then turn on the numbered sight lines in the 3D model. On the path chart, the numbers 1 to 9 follow the same forward, backward, forward order as the class's list.
教學提示：接著打開 3D 模型的「編號視線」，右邊路徑圖上 1 到 9 的順序，就和全班記錄的一樣：前進、倒退、前進。
Classroom Activity 2 · 課堂活動二
Track a Planet for a Month 追蹤一顆行星一個月
You need · 準備材料
- The Tonight's Planets list on this page本頁的「今晚的行星」清單
- A printed star chart of that part of the zodiac, or the path chart in the 3D model那一段黃道星空的星圖（可以用 3D 模型旁的路徑圖）
- A pencil一支鉛筆
Steps · 步驟
- Pick a planet that is easy to see this month, such as Jupiter, Saturn, or Mars.挑一顆這個月好找的行星，例如木星、土星或火星。
- Once a week, at the same time of night, find it and mark a dot on your chart next to the stars around it. Write the date beside the dot.每週一次、在晚上同一個時刻找到它，在星圖上畫一個點，標出它和旁邊星星的相對位置，並寫上日期。
- After four or five weeks, connect the dots. Is it moving east (left) or west (right)?四、五週後把點連起來：它是往東（左）還是往西（右）走？
- Compare your drawing with the path chart in the 3D model for the same dates.和 3D 模型路徑圖上同一段日期比一比。
Teaching tip: a planet that is in retrograde this month, shown in the Tonight list, gives the most exciting result. Saturn and Jupiter move slowly, so compare them with a bright star right next to them.
教學提示：挑「今晚」清單上正在逆行的行星，結果最精采。土星和木星移動得慢，要拿緊鄰的一顆亮星當參考。