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With all the talk these days about "leaping years,"
最近人们都在讨论闰年
we should probably be asking "what is a year, anyway?"
那么年是什么呢
Ignoring all cultural history,
抛开文化背景不谈
a year is basically the period of time that it takes for one physical body to orbit another.
一年是一个星体环绕另一个星体一周的时间
So an earth month could equally be called a "moon year."
所以地球上一个月就是一个"月球年"
But to under-simplify things, even if we stay on earth, there are several different ways to define a year
但即使我们在地球上 一年还是有几种不同的定义
— either the time it takes for the tilt of the earth's axis to come back to the same angle relative to the sun
地球的同一面两次面向太阳之间的时间
called a tropical year
称为"回归年"
— or the amount of time before we come back and see the same stars rising behind the sun
或是以恒星为参照 地球再次回到对恒星相同的位置所需的时间
called a sidereal year - pronounced like ethereal
称为恒星年
— or, since the earth follows an elliptical orbit which precesses,
因为地球是以椭圆轨道绕太阳运行
we could measure the amount of time between closest approaches to the sun
我们还可以测出地球两次经过近日点所需的时间
this is called the anomalistic year.
这称作近点年
Of course, all three "years" have slightly different lengths.
当然 这三种方法算出的年的长度略有不同
But our day-to-day civil calendar does its best to follow the tropical year,
但我们日常采用的是回归年
since its duration is defined by the tilt of the earth's axis which also determines the passage of the seasons.
因为地球面向太阳的方向不同 也就形成了四季
There are non-earth years, too:
另外 还有非地球年
Mercury's year is roughly 88 earth days, Neptune's is 165 earth years,
水星年相当于88个地球日 海王星年则有165个地球年
and the "Galactic" year
至于银河年
– which is how long the whole solar system takes to orbit once around the Milky Way –
也就是太阳系绕银河系一周的时间
is around a quarter of a billion earth years!
需要大约2.5亿年
But not all galactic years are long:
但不是所有恒星的银河年都很长
check out this timelapse of stars orbiting the supermassive black hole at the center of the Milky Way!
上面这张图是银河系中心的恒星围绕黑洞运行的图像
Even though you can't see the black hole, it's pretty obvious that it's there,
虽然你看不到黑洞 但很明显它就在中间
what with all those stars zipping around in orbits taking only ten earth years!
这些恒星只需要10个地球年就能公转一周
Astrophysicists have even used these orbits
天体物理学家甚至利用这些图像
to figure out that the black hole is as massive as 4 million suns.
算出中心黑洞的质量大约是太阳的400万倍
But enough about interstellar timekeeping…
关于恒星年就说到这
if you want to learn more about leap years here on Earth,
如果你想了解更多关于"闰年"的知识
check out Grey's new video over on his awesome channel, "cgpgrey"
去搜"cgpGRAY"频道
And a big thanks to the UCLA Galactic Center for doing sweet supermassive black hole research with the Keck telescope in Hawai'i,
最后感谢加州大学洛杉矶分校银河研究小组对黑洞所做的研究
and for sharing their images with the public.
以及他们提供的图像
