On the one hand, satellites in geosynchronous orbits look, from afar, like they're orbiting

一方面，在地球同步軌道上的衛星，從遠處看，就像它們在繞著軌道運行。

the earth just like any other satellite.

就像任何其他衛星一樣，在地球上運行。

But from down here, they appear to be floating, stationary, thirty-six thousand kilometers

但從這裡看，它們似乎是漂浮著的，靜止的，三萬六千公里

above our heads [a tenth of the way to the moon].

在我們頭頂上[到月球的十分之一]。

Because by definition, a geosynchronous orbit is in sync with the rotation of the earth

因為根據定義，地球同步軌道是與地球的旋轉同步的

- it takes the same amount of time for one orbit as the earth takes for one full rotation

- 一個軌道所需的時間與地球自轉一圈的時間相同

[aka a day] - so even though it's orbiting like normal around the earth's center of mass,

[又稱一天] - 所以即使它像正常一樣圍繞地球的質心運行。

it's stationary with respect to the earth's surface.

它相對於地球表面是靜止的。

[Fullscreen onscreen footnote: \* Ok technically only the geo*stationary* orbits are actually

[Fullscreen onscreen footnote: （* 好吧，在技術上只有地球*靜止*軌道是實際的。

stationary above you, because they orbit around the equator.

靜止在你的上方，因為它們繞著赤道運行。

Geo*synchronous* orbits take the same time as the earth to rotate, but are at other angles

地球*同步*軌道的旋轉時間與地球相同，但卻有其他角度

and so appear to drift up and down in latitude [\*and longitude] while while floating overhead.

是以，在漂浮在頭頂時，似乎在緯度[*和經度]上上下下漂移。

But regardless of whether they're geosynchronous or geostationary, they're possible because

但無論它們是地球同步軌道還是地球靜止軌道，它們都是可能的，因為

of two things:]

的兩件事：]

Geosynchronous orbits are possible because of two things: Kepler's laws, and the fact

地球同步軌道之所以可能，是因為有兩件事。開普勒定律，以及以下事實

that we live on a pile of rock.

我們生活在一堆岩石上。

Kepler's third law is the observation that the farther you are from a planet or dog or

開普勒第三定律是這樣觀察的：你離一個行星或狗或的越遠，你就越能看到它。

whatever, the longer it takes to complete an orbit.

不管是什麼，完成一個軌道所需的時間就越長。

The reasons are partly that you have to travel a longer distance to orbit around a bigger

部分原因是，你必須走更遠的距離才能繞著一個更大的物體運行。

circle [ellipse - or near-ellipse in GR], compounded by the fact that gravity is weaker

圓[橢圓--或GR中的近橢圓]，再加上引力較弱的事實

farther out so you can't go as fast along the circle [centripetal force & all that].

更遠，所以你不能沿著圓周走得那麼快[向心力和所有這些]。

But anyway, the point is that a super big orbit takes basically forever, and the closer

但無論如何，重點是，一個超級大的軌道基本上需要永遠，而越接近

you get the less and less time an orbit takes.

你得到一個軌道所需的時間就會越來越少。

So [by the intermediate value theorem,] somewhere in there is a time that's exactly in sync

是以，[根據中間值定理，]在那裡的某個地方有一個時間是完全同步的

with the amount of time it takes the planet itself to spin around once.

與行星本身旋轉一圈所需的時間有關。

Hence, "geosynchronous" [or dog-synchronous, or whatever.]

是以，"地球同步"[或狗同步，或其他什麼]。

And when you orbit in exactly the same time it takes the planet to spin, for someone on

而當你在軌道上運行的時間與地球自轉的時間完全相同時，對於一個在

the planet, you appear to just float overhead.

在這個星球上，你似乎只是漂浮在頭頂上。

[*onscreen note: again, technically that's only true for geostationary orbits... geosynchronous

[*螢幕說明：再次強調，技術上這隻適用於地球靜止軌道......地球同步軌道

ones have a bit of side-to-side drift]

的有一點側向漂移]。

[If you're in an orbit that's bigger than a geosynchronous orbit, you appear to travel

[如果你在一個比地球同步軌道大的軌道上，你似乎在旅行

"backwards" through the sky (to the west on earth), and if you're closer in, then you

"向後 "穿過天空（在地球上向西），如果你更靠近，那麼你

speed ahead to the east.

向東快速前進。

The fact that geosynchronous orbits don't move relative to the surface is why they're

事實上，地球同步軌道不會相對於表面移動，這就是為什麼它們是

useful:] And that's the reason geosynchronous orbits are useful - if you can put a satellite

有用：]。而這正是地球同步軌道有用的原因--如果你能把一個衛星

just floating above you at all times, it'll also be floating above most other people and

在任何時候都只是漂浮在你的上方，它也會漂浮在大多數其他人的上方，並且

places on your side of the earth, too, and you can use it to send messages or television

在地球的另一邊，你也可以用它來發送信息或電視。

signals to them.

向他們發出信號。

While mountains might block the view between you, the satellite will always be up there

雖然山脈可能會阻擋你們之間的視線，但衛星將永遠在上面。

with a clear line of sight.

有清晰的視線。

But there are two potential problems with geosynchronous orbits.

但地球同步軌道有兩個潛在問題。

The first is: they don't always exist.

首先是：它們並不總是存在。

The faster a planet is rotating, the closer you have to be to it in order to be in a geosynchronous

一顆行星的旋轉速度越快，你就必須離它越近，才能在地球同步軌道上。

orbit, so what if the planet is spinning so fast you'd have to be inside it to be in sync?

軌道上，如果這個星球旋轉得如此之快，你必須在它裡面才能同步，那又如何？

Well that's certainly a potential problem if you're orbiting a ball or a dog or something

如果你繞著一個球或一隻狗或其他東西轉，這當然是一個潛在的問題。

held together by internal tension forces [electromagnetic forces]: a solid steel ball with a 1meter

通過內部拉力[電磁力]保持在一起：一個1米長的實心鋼球

radius spinning once per hour has geosynchronous orbits that are inside it.

每小時旋轉一次的半徑有地球同步軌道，在它裡面。

But most things you can orbit are held together by gravity - the same force that causes you

但你能在軌道上運行的大多數東西都是由重力固定在一起的--同樣的力量導致你

to orbit - and a planet that's held together by gravity can only spin so fast before bits

到軌道上--而一個靠引力支撐的星球只能以如此快的速度旋轉，否則就會出現碎片。

of the planet itself start getting flung off.

的行星本身開始被甩掉。

When a planet is spinning at this maximum speed, the physics works out that a geosynchronous

當一個行星以這個最大速度旋轉時，物理學計算出，地球同步軌道的

orbit would exactly coincide with the surface of the planet.

軌道將與行星的表面完全重合。

For any slower spin rate, a geosynchronous orbit is farther from the planet's surface.

對於任何較慢的旋轉速度，地球同步軌道離地球表面更遠。

So if you're orbiting something like the earth, which is basically a pile of rock held together

是以，如果你圍繞著像地球這樣的東西運行，它基本上是一堆岩石固定在一起的

by gravity, there will always technically be geosynchronous orbits.

在重力作用下，技術上將永遠存在地球同步軌道。

And this brings us to the second potential problem with geosynchronous orbits: even if

這讓我們看到了地球同步軌道的第二個潛在問題：即使

they exist, they're not guaranteed to be useful.

它們的存在，並不能保證它們是有用的。

If a planet is spinning quickly, geosynchronous orbits around it might be too close to see

如果一顆行星正在快速旋轉，圍繞它的地球同步軌道可能太近，無法看到

much of the planet's surface.

地球表面的大部分地區。

For example, if earth took 90 minutes to spin instead of 24 hours, geosynchronous orbits

例如，如果地球自轉需要90分鐘而不是24小時，地球同步軌道

would be at an altitude of around 280 kilometers – beneath the orbit of the international

將在大約280公里的高度上--在國際社會的軌道之下。

space station – and satellites there could only see 2% of the earth's surface at one

空間站--而那裡的衛星在一次只能看到地球表面的2%。

time – not very useful for communications!

時間--對通信不是很有用!

On the other hand, if a planet is spinning too slowly, geosynchronous orbits will be

另一方面，如果一個行星旋轉得太慢，地球同步軌道將被

super far away.

超遠的距離。

Sure, a satellite might see nearly half of the planet's surface from there, but it would

當然，一顆衛星可能從那裡看到近一半的地球表面，但它會

be much harder to put satellites into that orbit, you'd need super powerful antennas

將衛星送入該軌道要難得多，你需要超級強大的天線

to send signals back and forth, and there would be a long delay while you wait for the

來回發送信號，而且在你等待的過程中會有很長的延遲。

signal to get there and back.

有信號才能到達和返回。

For example, satellites in a geosynchronous orbit around Venus - I mean, a venusynchronous

例如，圍繞金星的地球同步軌道上的衛星--我是說，金星同步軌道上的衛星

orbit - would be 4 times farther from Venus than the distance from the earth to the moon,

軌道 - 將比從地球到月球的距離遠4倍的金星。

so all communications signals would have a 10-second round-trip delay: Satellite TV wouldn't

是以，所有通信信號將有10秒的往返延遲。衛星電視將不會

work on Venus.

在金星上工作。

And a geosynchronous orbit around the sun - I mean, a helio-synchronous orbit - would

而圍繞太陽的地球同步軌道--我的意思是，地球同步軌道--將

be half way to the planet Mercury with a nearly 3 minute round-trip signal delay!

是半路上的水星，往返信號延遲近3分鐘!

[note: a day on the sun is not well defined since different parts of it rotate at different

[注：太陽上的一天並不是很明確，因為它的不同部分以不同的速度旋轉。

speeds].

速度]。

So as weird as geosynchronous orbits are, it's perhaps even weirder that we happen to

是以，儘管地球同步軌道很奇怪，但也許更奇怪的是，我們碰巧在

live on a planet that's not just in the goldilocks zone for life, but also in the goldilocks

生活在一個不僅處於生命的黃金地帶，而且還處於黃金地帶的星球上。

zone for satellite TV.

衛星電視的區域。

This video was sponsored in part by GiveWell, the non-profit that researches & vets other

本視頻部分由GiveWell贊助，GiveWell是一家研究和審查其他國家的非營利組織。

charities for you.

為您提供的慈善機構。

In particular, GiveWell searches out the non-profits that immediately and directly benefit or save

特別是，GiveWell搜索出了那些能夠立即和直接受益於或拯救的非營利組織

the most peoples' lives for the least money, like health & economic programs in developing

用最少的錢為最多的人服務，如開發中國家的衛生和經濟項目。

countries.

國家。

You can look at GiveWell's full charity recommendation list on their website, and you can also give

你可以在GiveWell的網站上查看其完整的慈善推薦名單，你也可以給

directly to any of those charities (or to a fund directed to a few current top choices)

直接捐給這些慈善機構中的任何一個（或捐給目前幾個首選的基金）。

through GiveWell; 100% of your donation goes to the charities.

通過GiveWell；你的捐款100%歸慈善機構所有。

On top of that, Givewell is matching donations from first time donors dollar for dollar up

除此之外，Givewell還對首次捐款者的捐款進行一美元對一美元的匹配，直至完成。

to 100 dollars.

到100美元。

Go to Givewell.org to donate, and to make sure your donation gets matched, at checkout

前往Givewell.org捐款，並確保你的捐款得到匹配，在結賬時

on the "how did you hear about GiveWell" question, select "YouTube" and then "Minutephysics".

在 "你是如何聽說GiveWell的 "問題上，選擇 "YouTube"，然後選擇 "Minutephysics"。

Again, that's "Youtube" with fundraiser code minutephysics and your donation to GiveWell

同樣，這就是 "Youtube"，用募捐者代碼minuteephysics和你對GiveWell的捐款。

recommended charities will get matched up to 100 dollars.

推薦的慈善機構將獲得最高100美元的匹配。