can be brighter around the edges and darker in the middle. Which is weird, because a flame

可以在邊緣更亮，中間更暗。這很奇怪，因為火焰

is normally *brightest* in the middle, since that's where you're looking at the most

通常是中間最亮的，因為那是你看的最多的地方。

hot burning gas.

燃燒的熱氣。

I mean,

我的意思是

Look at the sun – actually, don't do that without a solar filter – but, it's a

看太陽--其實，沒有太陽濾光片就不要這樣做--但是，這是一

big ball of hot glowing gas, and it's definitely brightest in the middle. In fact, not only

大球的熱發光氣體，中間絕對是最亮的。事實上，不僅是

does it get darker towards the edges, it gets redder. These dark, red edges of the sun are

它的邊緣會變得更黑，會變得更紅。太陽的這些暗紅色的邊緣是...

called “limbs,” since “limbus” is latin for “edge”. And the limbs of the

被稱為 "肢體"，因為 "limbus "是拉丁語 "邊緣 "的意思。而四肢的

sun get darker and redder near the edge for three simple reasons:

太陽靠近邊緣會變黑變紅，原因很簡單，有三個。

First, the sun is cooler closer to its “surface” and hotter deeper in.

首先，太陽越靠近 "表面 "越涼，越往深處越熱。

Second, hotter stuff glows brighter and yellower, which you've probably seen with a hot poker

第二，熱的東西會發亮發黃，這一點你可能見過，用熱的撲克牌

or candle flame.

或燭火。

And third, if you look across the edge of a sphere, you have to look through more of

第三，如果你從球體的邊緣看過去，你必須看穿更多的東西

the sphere for your line of sight to reach a given depth than when you look directly

比起直接看球體，你的視線要達到一個給定的深度。

into the middle. If the sphere is only partially transparent (like the sun), when looking at

到中間。如果球體只是部分透明的（如太陽），當看到

the edge, you won't see as deep into the depths before your vision is blocked.

邊緣，在視線受阻之前，你不會看到那麼深的地方。

So, when you look directly – through a solar filter – into the middle of the sun, you

所以，當你通過太陽濾光片直接看向太陽中間時，你會發現

see deeper in to hot gas which glows bright and yellow. And when you look at the edge

深入到熱氣中，它發出明亮的黃色光芒。而當你看到邊緣

of the sun, you see shallower, cooler, gas which glows darker and redder.

你會看到更淺、更冷、更深、更紅的氣體。

The same effect in reverse explains why some flames have bright edges and darker centers

同樣的效果反過來解釋了為什麼有些火焰的邊緣很亮，而中心較暗。

– these flames are clearly hotter and brighter near their surfaces, likely because that's

- 這些火焰在其表面附近顯然更熱更亮，很可能是因為那是...

where the best mixture of oxygen and fuel is, so when we look at their edges, our line

氧氣和燃料的最佳混合體在哪裡，所以當我們看它們的邊緣時，我們的線...

of sight passes through more hot light-emitting gases than when we look straight-on. Take

視線穿過的熱發光氣體比我們直視時更多。以

a candelabra and turn it side-on, and you'll see exactly what's going on.

一個燭臺，然後把它側面打開，你就會看到到底發生了什麼。

The optical effect I describe in the video is called limb darkening

我在視頻中描述的光學效果叫做肢體變黑

or limb brightening.

或肢體亮化。

It also explains the theory the appearance observe nebulae

這也解釋了星雲的出現觀測理論。

which are actually hollow expanding shell of glowing gases

發光氣體的空心膨脹外殼