如果我是神,我决定把欧洲和月球交换位置,我们多久能注意到变化?会有什么变化?If I were God and decided to swap the Moon and Europa, for fun and because I could, would we notice any changes on Earth and how long before we started to notice changes?

【翻译自Quora,作者Jonathan Devor, PhD Astronomy, Harvard University (2008)】

We would notice immediately, since the Moon and Europa look very different (see below). Even if we couldn’t make out the surface features, it would be very obvious that Europa is much brighter since it has an albedo of 0.67 compared to the Moon’s 0.136. This is because Europa’s surface is mostly water ice, while the Moon’s surface is covered with a dark rocky powder. With this increased brightness of a “full Europa” it might well be possible to read a book at night. Nocturnal predators would likely be very pleased.


Europa is slightly smaller than the Moon, at 1560km radius versus the Moon’s 1737km, so we would no longer enjoy complete solar eclipses. Also Europa is less massive than the Moon, at 0.008 Earth masses compared to the Moon’s mass of 0.012 Earth masses, so the ocean tides would be about 30% smaller.

欧洲地区的直径略小于月球,半径为1560公里,而月球的半径为1737公里,因此我们看不到日全食了。 此外,欧洲地区的质量小于月球,质量为0.008个地球质量,而月球的质量为0.012个地球质量,因此海洋潮汐将减小约30%。

In short, there wouldn’t be much difference for us. Europa, on the other hand, now receiving 25 times more solar radiation, would undergo major changes. Its surface ice would rapidly sublimate, transitioning directly from solid to vapor, and it’s quite likely that it would create a significant atmosphere, possibly with clouds and weather. While this atmosphere would eventually dissipate into space, over billions of years, because of Europa’s weak surface gravity, it could quite possibly become habitable for Earth-life. At least for anaerobic organisms, since there would be very little oxygen.

简而言之,对地球上的我们来说并没有太大的区别。 但对被换到月球那儿的欧洲地区来说,接受的太阳辐射要比原来多25倍,将会发生重大变化。 它的表层冰会迅速升华,直接从固体转变为蒸气,很可能会营造出浓厚的大气层,可能有云层和天气。 尽管这种空气最终会消散到太空中,但由于欧洲地区的地心引力弱,这个过程可能要数十亿年之久,所以它很可能成为地球生命的宜居之地。 至少对于厌氧生物来说,因为氧气很少。

Image: The Moon

Image: Europa



There seems to be interest among the commenters about how the orbit of Europa would be different from the Moon’s. The answer is that despite the fact that Europa has only about 2/3 of the mass of the Moon, there will be very little difference in its orbit.

评论者似乎对欧州地区的轨道与月球的轨道有何不同感兴趣。 答案是,尽管事实上欧洲地区只有月球质量的2/3,但其轨道上的差异却很小。

This is because, in the two-body problem, when one mass is much greater than the other, the exact mass of the smaller body has very little influence on the orbital dynamics. If you do the math, you’ll find that Europa’s orbital period will be only about 0.2% longer than the Moon’s, which means that the “lunar” month will be lengthened by less than 1.3 hours. Also the center of mass, around which both Earth and Europa would orbit, will shift from about 4670km to 3050km from the center of the Earth (both well within the volume of the Earth).

这是因为,在两体问题中,当一个质量比另一个质量大得多时,较小物体的精确质量对轨道动力学的影响很小。 如果进行数学计算,您会发现欧洲地区的轨道周期仅比月球周期长约0.2%,这意味着每个“月”将延长不到1.3小时。 对地月系统的质心也有影响,原来地月系统的质心在离地球中心的4679km处,但现在只有3050千米处了。但这两个距离都在地球表层内。


@ Alex Davis

The quintupling of the brightness would not be quite as pronounced as you say. Human perception of vision works on a logarithmic scale so an increase in brightness of order unity would not appear to be that much- not enough to read a book by anyway. The Sun is actually 100,000 times brighter than the full moon on Earth, and an overcast winter’s day is something like 2000 times dimmer than a clear equatorial summer’s day at high noon.



You are of course correct about the logarithmic nature of visual perception. But a 5-fold increase would still be noticeable. Reading in the light of a full moon in is on the edge of what humans can do. For example, in the Jewish ritual of kiddush levana, participants are required to read with moonlight. So large-print payer books are used. A 5-fold increase would certainly make such reading much easier.

对于视觉感知的对数性质,您当然是正确的。但是5倍的增长仍然很明显。满月读书是人类可以做的事情。例如,在犹太仪式中,基达杜瓦列瓦纳(kiddush levana)要求参与者在月光下阅读。因此,那些人读的书字体都是很大的。5倍的亮度增加将让阅读更加容易。

@ Alex Davis

Oh yes it would certainly be noticable. And since moonlight is almost enough to read by anyway you might be right. But from my experience in an optics lab if you increase the brightness of a source by a factor of 5, people don’t believe it. They think it’s maybe a 30% increase.


@ T S Acklin

I was in the Shawnee National Forest and read the instruction manual for my GPS device, under the light of a full moon. The text was small but it wasn’t a problem. I know, who asked for my two cents. Just saying.


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