Thursday, August 9, 2012

Replacing our Moon with a Planet

Do you know how big the Moon is compared to the other planets?

io9 recently had an article showing how the different planets would look like if they were placed at the distance the Moon is from the Earth. This is not the first time I've seen it done (I embed a video I found a few years ago), but I wanted to comment on it as well for those who may not have considered this.
Click through to find out more.

Placing the planets at the Moon's distance is a useful exercise as you may recall that every so often there are news or articles about how big Mars will look like in the sky. Unfortunately, in many cases people think that Mars will look as big as the Moon. At its closest, Mars is about 55 million kilometers away from the Earth, whereas the Moon is on average 385,000 kilometers away. While Mars is larger than the Moon, given that it is at least 140 times farther away (and on average, ~5 times more distant), it always appears very small: just a bright point of light to our eyes.

But what what Mars look like if it were as close as the Moon? io9 to the rescue:
You should compare that to the image above, or check out the video at the end of this post.
Mars is about 3400 km in radius, whereas the Moon is just 1700 km. Hence, at the same distance, Mars would appear to be roughly twice the size as the Moon (when considering the radius or diameter; if we talk about area we must remember that it scales like the radius squared). For comparison: Earth's equatorial radius is about 6400 km, or about 4 times the size of the Moon.

The io9 article I've linked to has other interesting diagrams replacing the Moon with the rest of the planets in our solar system. I encourage you to go check it out and see them all, but I include below only my two favorite ones: that of the planets Saturn and Jupiter. Recall that Jupiter is about ten times the radius of the Earth and the Earth is about four times the radius as the Moon. Therefore, Jupiter would appear 40 times larger in radius (or diameter) when compared to the Moon if located at the same distance. This makes it very large.

Wouldn't these look awesome in real life or in a movie?

Author N.K. Jemisin had a recent book where one of the 'moons' was actually a gas giant planet similar to Jupiter. It turns out these pictures are not too far from what she had in mind: The shot of Jupiter is almost how I visualize Dreaming Moon, in Dreamblood. What if we had a planet instead of a moon? [...] - @nkjemisin

Io, a moon of Jupiter. Credit: NASA
The article does point out that if Jupiter were that close to the us there would be significant tidal influences that could change the nature of the Earth and affect the prospects for life on this planet. Consider, for example, the situation of Io, depicted at left. The gravitational tug-of-war between Jupiter and its other large moons have produced so much volcanism on Io that it is unlikely life could ever develop on such a world. However, if the Earth were the sole moon of such a giant planet, perhaps the interaction may not be enough to produce such an extreme environment and may only cause us to be tidally locked. This is the case for our own Moon and the result is that it always faces the same side to us.
For the sake of this post, though, we can gloss over these details and just enjoy the pretty pictures. It does, however, present some interesting concepts to keep in mind for science fiction stories set in the moons of giant planets.

Finally, here is a video of the same thing with some added music. It really helps drive home the scales involved:

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