Earth and Sky:Galactic Rotation 银河系自转

Galactic Rotation¹ 银河系自转

Astronomers² calculate there is ten times more matter in our Milky³ Way galaxy⁴ than we can actually see. Discover how the orbital motions of stars in the galaxy reveal dark matter -- on today's Earth and Sky.

JB: I'm Joel Block.

DB: And I'm Deborah Byrd for Earth and Sky. A listener writes, "I was wondering if the sun (and its solar system) revolve⁵ around the center of the Milky Way? In other words, does the Milky Way rotate?"

JB: Our sun and solar system orbit the center of the Milky Way galaxy at about 800 thousand kilometers an hour -- that's about 500 thousand miles an hour. So in the 90 seconds of this radio broadcast, we all move some 20,000 kilometers -- or 12,500 miles -- in orbit around the center of the galaxy. But the galaxy is a big place. Even at this blazing speed, it takes the sun about 200 million years to complete one journey around the galaxy's core.

DB: The orbital speed of a star around the center of our galaxy depends on its distance from the center. A star's orbital speed can be predicted from the galaxy's mass, and the star's location. But something mysterious is taking place at distances in the galaxy beyond the orbit of our sun. The speed of these outermost⁶ objects doesn't decrease as much as expected.

JB: So astronomers think there may be a large amount of undetected mass around our galaxy -- part of the unseen, dark matter of the universe. That's our show for today. Ask your Earth or sky question at our website, earthsky.org. With thanks to the National Science Foundation, I'm Joel Block, with Deborah Byrd, for Earth and Sky.

1 rotation   

n.旋转；循环，轮流

参考例句：

• Crop rotation helps prevent soil erosion.农作物轮作有助于防止水土流失。
• The workers in this workshop do day and night shifts in weekly rotation.这个车间的工人上白班和上夜班每周轮换一次。

2 astronomers   

n.天文学者，天文学家( astronomer的名词复数 )

参考例句：

• Astronomers can accurately foretell the date,time,and length of future eclipses. 天文学家能精确地预告未来日食月食的日期、时刻和时长。 来自《简明英汉词典》
• Astronomers used to ask why only Saturn has rings. 天文学家们过去一直感到奇怪，为什么只有土星有光环。 来自《简明英汉词典》

3 milky   

adj.牛奶的，多奶的；乳白色的

参考例句：

• Alexander always has milky coffee at lunchtime.亚历山大总是在午餐时喝掺奶的咖啡。
• I like a hot milky drink at bedtime.我喜欢睡前喝杯热奶饮料。

4 galaxy   

n.星系；银河系；一群(杰出或著名的人物)

参考例句：

• The earth is one of the planets in the Galaxy.地球是银河系中的星球之一。
• The company has a galaxy of talent.该公司拥有一批优秀的人才。

5 revolve   

vi.(使)旋转；循环出现

参考例句：

• The planets revolve around the sun.行星绕着太阳运转。
• The wheels began to revolve slowly.车轮开始慢慢转动。

6 outermost   

adj.最外面的，远离中心的

参考例句：

• He fired and hit the outermost ring of the target.他开枪射中了靶子的最外一环。
• The outermost electron is shielded from the nucleus.原子核对最外层电子的作用受到屏蔽。