This star trail over the Rocky Mountains shows the Polar North Star at the center of Earth's rotation.
落基山脉上空的这条恒星轨迹显示了地球自转中心的北极星。
Every 24 hours, the Earth makes a full rotation, spinning west to east, which is why the sun rises in the east and sets in the west and the stars at night appear to move across the sky.
每24小时，地球自转一周，从西向东旋转，这就是为什么太阳从东方升起，在西方落下，晚上的星星似乎在天空中移动。
Earth spins along an imaginary line between the North Pole and the South Pole called the rotation axis. The rotation axis points toward a bright star, Polaris, which is visible on clear nights in the Northern Hemisphere.
地球沿着北极和南极之间的一条假想线旋转，这条线被称为旋转轴。旋转轴指向一颗明亮的恒星北极星，在北半球晴朗的夜晚可以看到。
But why does Earth spin in the first place? And what would happen if it suddenly stopped?
但是地球为什么会旋转呢?如果它突然停止会发生什么?
To understand why, let's see what we can learn from other bodies in space.
为了理解其中的原因，让我们看看我们能从太空中的其他物体中学到什么。
Contents 目录
- Everything in the Solar System Spins 太阳系中的一切都在旋转
- Stars Are Born and Slowly Rotate 恒星的诞生和缓慢旋转
- What Would Happen if Earth Didn't Spin? 如果地球不旋转会发生什么?
The sun completes a rotation on its axis once in about 27 days. Earth finishes revolving the sun once in every 365 days, six hours and nine minutes.
太阳大约27天绕它的轴转一周。地球每365天6小时9分钟绕太阳公转一周。
The sun spins. In fact, it spins in the same direction Earth does, counterclockwise.
太阳旋转。事实上，它的自转方向与地球相同，都是逆时针。
Not only that, Earth orbits the sun in the same direction, as do most other planets and more than a million asteroids and dwarf planets.
不仅如此，地球绕太阳公转的方向是相同的，大多数其他行星以及100多万颗小行星和矮行星也是如此。
Jupiter and Saturn spin quite a bit faster than Earth, taking only about 10 hours to rotate. Saturn's spin is a little bit tilted, so we get to see changing views of its rings over time.
木星和土星的自转速度比地球快得多，只需要大约10个小时。土星的旋转有点倾斜，所以我们可以看到它的光环随着时间的推移而变化。
There are two exceptions: Uranus and Venus. Uranus is tilted so far over, it's virtually spinning on its side. Nobody knows exactly how or why it got this way. Maybe it collided with another planet. Venus is also odd — it spins clockwise.
有两个例外:天王星和金星。天王星倾斜得太厉害了，它几乎在一边旋转。没有人确切知道它是如何或为什么会这样。也许它和另一颗行星相撞了。金星也很奇怪——它是顺时针旋转的。
We don't know for sure whether it formed that way or got knocked over. Most scientists now think its spin has been reversed over time by tidal forces involving the sun and Venus' thick atmosphere. Both Venus and Uranus rotate in what's known as retrograde manner, meaning opposite the rotation of the sun.
我们不确定它是这样形成的还是被撞翻的。大多数科学家现在认为，由于太阳和金星厚厚的大气层所产生的潮汐力，金星的自转随着时间的推移发生了逆转。金星和天王星都以逆行的方式旋转，这意味着与太阳的旋转相反。
All that leads astronomers to wonder: Is there something about how the solar system formed that kind of "baked in" that direction of spin?
所有这一切都让天文学家想知道:太阳系是如何形成的，是在那种旋转方向上“烘烤”的吗?
Stars Are Born and Slowly Rotate 恒星的诞生和缓慢旋转
This composite image of the young star Beta Pictoris, which has a temperature of around 2192 degrees Fahrenheit (1200 degrees Celsius), is how astronomers believe our own solar system might have looked when it was being birthed.
这是一颗年轻恒星贝塔的合成图像，它的温度约为2192华氏度(1200摄氏度)，这是天文学家认为我们太阳系诞生时的样子。
For more clues, we can look at a young star, one that is just forming its system of planets.
为了获得更多的线索，我们可以观察一颗年轻的恒星，一颗刚刚形成行星系统的恒星。
A famous one is called Beta Pictoris. It is surrounded by a thin disk of dust, gas and little bits called planetesimals that range in size from grains of sand and large rocks to objects the size of mountains. Astronomers are pretty sure the disk formed from material left over when the star was born 20 million years ago.
最著名的一个叫做贝塔图托里斯。它被一个由尘埃、气体和被称为星子的小块组成的薄圆盘所包围，这些小块的大小从沙粒、大岩石到山脉大小的物体不等。天文学家非常确定，这个圆盘是由2000万年前恒星诞生时留下的物质形成的。
Every star is born from a cloud of gas and dust that moves through space surrounded by other similar clouds. The force of gravity causes these clouds to tug on one another as they pass, which makes them slowly rotate.
每一颗恒星都诞生于一团气体云和尘埃云，这些气体云在空间中移动，周围环绕着其他类似的云团。重力使这些云在经过时相互拉扯，从而使它们缓慢旋转。
Even when one of these clouds collapses to form a star, it continues to rotate. The star forms, spinning at the center of a flat pancake of rotating gas and dust called a protoplanetary disk. All of it — the star, the gas, the dust — spins in the same direction.
即使其中一个云坍缩形成恒星，它也会继续旋转。恒星形成，在一个由旋转气体和尘埃组成的扁平圆盘的中心旋转，称为原行星盘。所有这些——恒星、气体、尘埃——都朝着同一个方向旋转。
Astronomers think that our solar system looked a lot like Beta Pictoris in its early years.
天文学家认为，我们的太阳系在早期看起来很像绘架座β。
We think that inside the disk, the gas and dust can stick together in a process called accretion. As a baby planet starts to grow, it gets heavier, and its gravity attracts more and more little pieces.
我们认为在圆盘内部，气体和尘埃可以在一个叫做吸积的过程中粘在一起。当一颗小行星开始成长时，它会变得越来越重，它的引力会吸引越来越多的小碎片。
When the baby planet gets massive enough, the force of gravity begins crushing it, making it denser. Because of the force of gravity, the planet spins faster, like an ice skater drawing in her arms to spin.
当这颗新生行星的质量足够大时，重力就会开始挤压它，使其密度更大。由于地心引力的作用，地球自转得更快，就像滑冰运动员缩臂旋转一样。
Rising pressure in the core then causes the core to melt. Denser materials sink toward the core, and lighter materials float to the planet's surface. We end up with a planet with an iron core surrounded by rock, and maybe water and ice on the outer edges.
核心不断上升的压力会导致核心融化。密度较大的物质下沉到核心，而较轻的物质则漂浮到行星表面。我们最终得到的是一颗由岩石包围的铁核行星，也许外层边缘有水和冰。
That's what we see in our solar system.
这就是我们在太阳系中看到的。
What Would Happen if Earth Didn't Spin? 如果地球不旋转会发生什么?Earth's spin is important for life, mainly because it causes day and night. On Mercury, which spins very slowly, temperatures vary drastically. During the day, temperatures can reach as high as 800 degrees Fahrenheit (430 degrees Celsius) and then drop as low as minus 279 degrees Fahrenheit (minus 173 Celsius) at night.
地球自转对生命很重要，主要是因为它造成了白天和黑夜。水星自转非常慢，温度变化很大。白天，气温最高可达800华氏度(430摄氏度)，晚上则降至零下279华氏度(零下173摄氏度)。
It's also important for ocean tides. Without the daily ebb and flow of water, it's possible life on Earth would never have emerged from the sea onto land.
这对海洋潮汐也很重要。如果没有每天的潮起潮落，地球上的生命可能永远不会从海洋出现在陆地上。
So, astronomers believe Earth spins because the entire solar system was already rotating when Earth formed.
所以，天文学家认为地球自转是因为地球形成时整个太阳系已经在旋转了。
