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Our Definition For “Moon” Is Broken (Collab. w/ MinutePhysics)

嗨 我是凯特 欢迎收看『分钟地球』
Hi, this is Kate from MinuteEarth.
直到近代 人类一直相信我们能够看到的天上所有物体都是按照轨道
Until relatively recently, humans believed everything we could see in the sky was orbiting
around the earth.
当然 只有月亮是真正围绕地球轨道运行的 并且地球本身
Of course, the Moon was the only thing actually orbiting the earth, and the earth itself was
orbiting a star.
But there were other objects orbiting that star that also had things orbiting them, so
we decided to call all of those secondary satellites “moons”, too.
当前 当人们说“月亮”的时候 他们实际上指的是一个自然形成的卫星
Nowadays when people say “moon”, they essentially mean a natural satellite of a
这个卫星是围绕行星轨道运行的 这就会让我们在直觉上好奇 什么样的卫星才能算是
satellite of a star, which seems to capture our collective intuition for what distinguishes
a moon.
然而 这就遇到个问题:太空中的石头形状和尺寸各不相同
However, there’s a problem: space rocks come in all sorts of shapes and sizes and
运行轨迹千姿百态 他们当中的许多根本和我们传统上理解的“月亮”的
behave in many ways, many of which don’t at all fit into our traditional idea of a
我们当然可以就此打住 说“好吧 所有的这些东西都叫月亮” 但是
We could at this point give up and say “ok, all these things are moons” – but in this
本视频中我们将维护“月亮”这个概念的特殊性 我们不应该
video we’re going to defend the idea that “moon” is a special title and we shouldn’t
award it too broadly.
首先 随着技术的进步 我们会(发现并)把越来越小的
To start with, through improvements in technology, we’ve ended up calling smaller and smaller
things moons.
单单土星 我们就已经定义了60多个围绕它轨道运行的“月亮” 其中的
We’ve already identified more than 60 increasingly small moons orbiting Saturn alone, some of
一些实际上就是几公里直径的 藏在土星光环之中的岩石而已
which are essentially kilometer-wide rocks hiding in its rings.
我们不可避免的会把更小的 围绕行星轨道运行的物体定义为“月亮”
It’s inevitable that we’ll keep identifying ever smaller individual chunks orbiting planets.
所以 如果我们不为“月亮”的尺寸设定一个最低标准 我们就可以设想这个情景
So if we don’t set some lower limit on moon sizes, are we willing to think of each of
难道我们要把土星光环里面上亿的小石头和微粒尘埃 或者围绕地球轨道的每一粒微尘
the billions of tiny rocks and dust particles in Saturn’s rings – and every last speck
of dust orbiting the earth – as moons?
在另一个极端 有的“月亮”却因为太大而不能被称之为“月亮”:冥王星和
At the other extreme, some “moons” are too big to be considered moons: Pluto and
它的疑似“月亮”凯伦的尺寸就非常接近 实际上说它们相互
its maybe-moon Charon are close enough in size that it’s really fairer to say they
both orbit each other.
实际上 技术角度来看 任何时间的任何物体都在围绕其他物体的轨道运行 他们事实上都是
In fact, technically, any time anything orbits any other thing, they’re actually both orbiting
围绕着他们质量的公共中心运行的 被称之为质心
their common center of mass, called the barycenter.
Where that point is depends on the relative masses of the two objects – if one is way
远远大于另一个 比如地球 那么质心就会靠近甚至位于
bigger than the other, like the earth, the barycenter will be close to or even inside
该星球之内 因此说小的围绕大的轨道运行感觉很合适
it so it feels right to say the smaller one is orbiting around the bigger one.
但当他们的尺寸基本相等 例如90 Antiope这个双星系统的情况 质心
But when they’re almost the same size like with the binary asteroid 90 Antiope, the barycenter
将几乎处于两者中间的位置 这时再称呼其中一颗为“月亮”
will be almost halfway between, and it really doesn’t make sense to call one or the other
(or both) of them moons.
那么这其中的尺寸比率如何确定呢?这个问题就等于说“谁在围绕着谁转 怎么转?”
So where in the continuum of size ratios – aka “who’s really orbiting who, and by how
在什么界限之外 我们就说这个东西不是“月亮”了?
much” – Where should we stop calling something a moon?
Maybe the ratio of sizes doesn’t accurately capture your intuition about what a moon is
——也许以这些候选“月亮”的绝对尺寸 或者圆度来定义
– maybe absolute size would be better, or the roundness of the candidate moon, or whether
或者是否能够从其行星表面上观察到 或者是否具有规则的椭圆轨道
it can be seen from the surface of its planet, or whether it has a regular elliptical orbit.
Or maybe there’s simply no one definition for “moon” that captures both the complexity
包括物体围绕其他物体或者我们的星系轨道运行的不同方式 以及
of the different ways stuff can orbit other stuff and our nebulous, intuitive “I know
it when I see it” idea of what makes a moon.
与“月亮”不同 在宇宙中还有许多其它物体 比如恒星衰退死亡的过程中
Unlike moons, there are many other objects in space, like the different things a star
形成的各种物质 这些都是自然产生的 你可以
can become when it dies, that are actually delineated quite naturally, and you should
来我们的『分钟物理』节目 了解关于他们的更多知识
come over to MinutePhysics to find out more about them!
今天我们讲了足够多关于“如果一个“月亮”在另一个“月亮”的轨道上运行 它还是“月亮”吗?”的话题
Enough of this “if there’s a moon around a moon, is it still a moon?”
– come over to MinutePhysics, we’re going to talk about black holes and white dwarfs
and neutron stars!