Moon (2024)

月球 (2024)

In the vastness of empty space surrounding Earth, the Moon is our closest celestial neighbor. Its face, periodically filled with light and devoured by darkness, has an ever-changing, but dependable presence in our skies. In this article, we’ll learn about the Moon and its path around our planet, but to experience that journey first-hand, we have to enter the cosmos itself. 在环绕地球的广袤太空中，月球是我们最近的天体邻居。它的面貌周期性地被光芒填满，又被黑暗吞噬，在我们头顶的夜空中呈现出一种不断变化却又始终如一的存在。在本文中，我们将了解月球及其绕地球运行的轨道，但为了亲身体验这段旅程，我们必须进入宇宙本身。

Let’s take a look at the Moon as seen from space in all its sunlit glory. You can drag it around to change your point of view, and you can also use the slider to control the date and time: In this convenient view, we can freely pan the camera around to see the Moon and its marvelous craters and mountains from various angles. 让我们从太空中一睹月球在阳光照耀下的壮丽景象。你可以拖动它来改变视角，也可以使用滑块来控制日期和时间：在这个便捷的视图中，我们可以自由地平移镜头，从不同角度观察月球及其奇妙的陨石坑和山脉。

Unfortunately, we don’t have that freedom of motion in our daily experience – the Moon wanders on its own path across the daily and nightly skies. We can simulate these travels below, where you can see the current position of the Moon in the sky. You can drag that panorama around to adjust your viewing direction – this lets you see the breadth of the sky both above and below the horizon. 遗憾的是，在日常生活中我们并没有这种运动的自由——月球沿着它自己的路径在昼夜天空中漫步。我们可以在下方模拟这些运行轨迹，在那里你可以看到月球在天空中的当前位置。你可以拖动全景图来调整观察方向——这让你能够看到地平线上下广阔的天空。

By dragging the sliders you can witness how the position of the Moon changes in the sky across days and hours of your local time. As the Moon’s placement in the sky shifts, the little arrow will guide you to its position. You can also drag the little figurine on the globe in the bottom-right corner to see how the sky looks at that location on Earth. If your browser allows it, clicking/tapping the button will automatically put the figurine at your current location. 通过拖动滑块，你可以观察到月球在天空中的位置如何随着你当地时间的日期和小时而变化。随着月球在天空中的位置移动，小箭头会指引你找到它的方位。你还可以拖动右下角地球仪上的小人偶，查看地球上该位置的天空景象。如果你的浏览器允许，点击/轻触按钮将自动把人偶放置在你当前的位置。

This may all feel quite overwhelming at the moment, but we’ll eventually see how all these pieces fit together: Over the course of one day, the Moon travels on an arc in the sky almost completing a loop around the Earth. As the days pass, the Moon’s illumination also visibly changes. You’ll probably admit that it’s a little hard to focus on the tiny Moon as it shifts its position in the sky. To make things easier to see, I’ll zoom in the camera and lock its position on the Moon: 这一切目前可能让你感到有些眼花缭乱，但我们最终会明白这些碎片是如何拼凑在一起的：在一天的时间里，月球在天空中划出一道弧线，几乎完成了绕地球一周的轨迹。随着时间的推移，月球的亮度也会发生明显变化。你可能会承认，当月球在天空中移动时，要聚焦在这个小小的天体上确实有点困难。为了让观察更轻松，我将放大镜头并锁定月球的位置：

Notice that across a single day the Moon seems to rotate, and over many days it quite visibly wobbles. These wobbly variations let us occasionally see some hidden parts on the “edges” of the Moon, but our neighbor ultimately shows us only one of its sides. In our space-floating demo we could easily see the Moon from all sides, but on Earth we can never see most of the far side of the Moon. 请注意，在一天之内，月球似乎在旋转，而经过多日观察，它会有明显的晃动。这些晃动让我们偶尔能看到月球“边缘”的一些隐藏部分，但这位邻居最终只向我们展示了它的一面。在我们的太空漂浮演示中，我们可以轻松地从各个角度观察月球，但在地球上，我们永远无法看到月球背面的大部分区域。

Over the course of days, the lighting on the Moon also changes dramatically. The line between the lit and unlit parts of the Moon, known as the terminator, sweeps across the Moon, revealing the details of its surface. Although the Moon has a spherical shape, the fully lit Moon looks more like a flat disk. In this article I’ll explain all the effects we’ve just seen, and we’ll also learn about gravity, ocean tides, and eclipses. Let’s begin by exploring how celestial bodies move through space and how their mere presence influences the motion of their neighbors. 在几天的时间里，月球上的光照也会发生剧烈变化。月球明暗交界线（即晨昏线）扫过月球表面，揭示了其地表的细节。虽然月球是球形的，但完全被照亮时，它看起来更像是一个扁平的圆盘。在本文中，我将解释我们刚刚看到的所有现象，我们还将学习有关引力、海洋潮汐和日食的知识。让我们先从探索天体如何在太空中运动，以及它们的存在如何影响邻近天体的运动开始。

Motion in Space 太空中的运动

Let me introduce a little cosmic playground in which we’ll do our experiments. Inside it, I put a little planet that floats freely in space. You can drag the planet around to change its position. The arrow symbolizes the initial velocity of this body – you can tweak this velocity by dragging the dashed outline at the end of the arrow. To get things going, you can press the button in the bottom-left corner: 让我介绍一个小型的宇宙游乐场，我们将在其中进行实验。在它内部，我放置了一个在太空中自由漂浮的小行星。你可以拖动行星来改变它的位置。箭头象征着该天体的初始速度——你可以通过拖动箭头末端的虚线轮廓来调整这个速度。要开始运行，你可以按下左下角的按钮：

Notice that I’m drawing a ghost trail behind the moving planet, making it easier to track its motion. As you can see, once you let the planet go, it travels through space in a straight line, only to eventually get out of visible bounds. Let’s complicate things a little by adding another body to this sandbox. You can tweak the positions and velocities of both bodies to see how their mutual presence impacts one another. I’m also marking the thin lines of trajectories that the bodies will take even before you let things go, making it easier to plan their motion: 请注意，我在移动的行星后面绘制了一条轨迹，使其更容易追踪其运动。正如你所见，一旦你松开行星，它就会在太空中沿直线飞行，最终飞出可见范围。让我们在这个沙盒中增加另一个天体，让情况变得复杂一点。你可以调整两个天体的位置和速度，看看它们的存在如何相互影响。在松开它们之前，我还标记了天体将要运行的细轨迹线，这使得规划它们的运动变得更加容易：

The motion we see now isn’t as straightforward as before. In some scenarios, the two bodies travel past each other after tweaking their initial trajectories. In other configurations, both objects roam through space together, permanently locked in a swinging dance. You may have also managed to make the two bodies run into each other. We’ll eventually see a more realistic visualization of that scenario, but in this simplified simulation when two objects collide, they just stick together and continue their coupled journey. 我们现在看到的运动不再像以前那样简单直接。在某些情况下，调整初始轨迹后，两个天体会擦肩而过。在其他配置中，两个天体会在太空中共同漫游，永久地锁定在一种摇摆的舞蹈中。你可能也成功地让两个天体撞在了一起。我们最终会看到该场景更真实的模拟，但在这种简化模拟中，当两个物体碰撞时，它们只是粘在一起并继续它们的共同旅程。

What’s responsible for all these effects is the force of gravity acting on the objects. Let’s explore that interaction up close. As before, you can drag the two bodies around, and you can also change their masses using the sliders below: 导致所有这些现象的原因是作用在物体上的引力。让我们近距离探索这种相互作用。和之前一样，你可以拖动这两个天体，也可以使用下方的滑块改变它们的质量：

The arrows represent the force of gravity acting on the two bodies – the longer the arrow, the larger the force. For completeness, I’m displaying the values and units of masses and distances, but the numbers aren’t particularly important here. What matters is that when we increase either the mass of the first body m1 or the mass of the second body m2, the force of gravity grows too. Moreover, the magnitude of gravity also depends on the distance r between the objects. As bodies move farther apart, the gravity weakens. 箭头代表作用在两个天体上的引力——箭头越长，力就越大。为了完整起见，我显示了质量和距离的数值及单位，但这些数字在这里并不是特别重要。重要的是，当我们增加第一个天体 m1 或第二个天体 m2 的质量时，引力也会随之增大。此外，引力的大小还取决于物体之间的距离 r。当物体距离越远，引力就越弱。

Notice how the forces acting on each body have the same magnitude, but they point towards the other body, which indicates an attractive force. If you paid close attention to the lengths of the arrows, you might have noticed that the force decreases quite rapidly with distance. We can visualize this with a plot, in which the white line shows the magnitude of gravity as a function of distance. More precisely, it shows that gravity is inversely proportional to the square of that distance: 请注意，作用在每个天体上的力大小相等，但方向都指向对方，这表明这是一种吸引力。如果你仔细观察箭头的长度，你可能会发现引力随距离的增加而迅速减小。我们可以通过图表将其可视化，其中的白线显示了引力随距离变化的函数关系。更准确地说，它表明引力与距离的平方成反比：

Let’s take a very brief mathematical interlude to describe what we’ve seen in more detail. All these dependencies are captured… 让我们进行一段简短的数学插曲，更详细地描述我们所看到的内容。所有这些依赖关系都被捕捉到了……