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太阳的肢体

The Limb of the Sun

从这张照片上很容易看出,火焰的各个部分并不相同。
From this photo it’s easy to see that not all parts of a flame are made the same – they
边缘可能比中心更亮。这有点奇怪,
can be brighter around the edges and darker in the middle. Which is weird, because a flame
一般来说应该中心更亮,
is normally *brightest* in the middle, since that’s where you’re looking at the most
因为那里有高温气体燃烧。
hot burning gas.
我的意思是说,
I mean,
看看太阳吧-当然,要戴个护目镜-它是个
Look at the sun – actually, don’t do that without a solar filter – but, it’s a
由灼热发亮气体组成的大球,而且绝对是中间更亮。实际上,
big ball of hot glowing gas, and it’s definitely brightest in the middle. In fact, not only
边缘不仅更暗,还更红。这些暗红的边缘被称为肢端,
does it get darker towards the edges, it gets redder. These dark, red edges of the sun are
因为“limbus”在拉丁语里是边缘的意思。
called “limbs,” since “limbus” is latin for “edge”. And the limbs of the
太阳的肢端暗红的原因有三点:
sun get darker and redder near the edge for three simple reasons:
第一,太阳的表面温度低,而深层温度高。
First, the sun is cooler closer to its “surface” and hotter deeper in.
第二,温度越高的物体发出的光越黄,你大概也见过烧红的铁棍
Second, hotter stuff glows brighter and yellower, which you’ve probably seen with a hot poker
或者蜡烛是啥样吧。
or candle flame.
第三,如果你看向球体的边缘,你的视线需要穿过的球体部分
And third, if you look across the edge of a sphere, you have to look through more of
要比你看中心时候多出一部分。
the sphere for your line of sight to reach a given depth than when you look directly
如果球体是半透明的(太阳就是),
into the middle. If the sphere is only partially transparent (like the sun), when looking at
当你看太阳的边缘时,看不了很深你的视线就会被遮挡住。
the edge, you won’t see as deep into the depths before your vision is blocked.
所以,当你直视太阳中心时(再次提醒要戴护目镜),
So, when you look directly – through a solar filter – into the middle of the sun, you
你的视线能看到中心高温气体发出的亮黄光。而当你看向
see deeper in to hot gas which glows bright and yellow. And when you look at the edge
太阳的肢端时,你只能看到表层低温气体发出的暗红光。
of the sun, you see shallower, cooler, gas which glows darker and redder.
同样的效应反过来又解释了为啥火焰边缘更亮而中间更暗。
The same effect in reverse explains why some flames have bright edges and darker centers
这些火焰明显表面更热更亮,很可能因为
– these flames are clearly hotter and brighter near their surfaces, likely because that’s
在表面氧气和燃料气体混合得更好。因此当我们看火焰的边缘时,
where the best mixture of oxygen and fuel is, so when we look at their edges, our line
我们的视线比看中心时穿过更多的发亮光的高温气体。
of sight passes through more hot light-emitting gases than when we look straight-on. Take
拿个排状烛台侧过来看看,就能看得非常清楚。
a candelabra and turn it side-on, and you’ll see exactly what’s going on.
我视频里讲的这一光学效应(音频原文为几何效应)被称为肢端暗化
The optical effect I describe in the video is called limb darkening
或者肢端亮化。
or limb brightening.
这一效应也解释了星云的外貌(边缘更亮),
It also explains the theory the appearance observe nebulae
因为星云实际上是一层不断膨胀的发亮气体。
which are actually hollow expanding shell of glowing gases

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