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Long ago, I made a post with a scenario on how planet Earth would be affected if our moon were the size of Mars. But I've decided that it be more reasonable if our moon were closer in size to Titan, the largest moon of the planet Saturn.

Let's see how our moon stands as it is now:

  • Diameter: $3,475 \text{km} (2,159 \text{mi})$
  • Mass: $7.3459 \times 10^{22} \text{kg}$
  • Gravity: $0.165 \text{g}$ (17% that of Earth's)

The size of Titan is as follows:

  • Diameter: $5,150 \text{km} (3,200 \text{mi})$
  • Mass: $1.3452 \times 10^{23} \text{kg}$
  • Gravity: $0.138 \text{g}$ (13.8% of Earth gravity) on the surface

If our moon is the size of Titan, questions follow as such:

  • Would the Roche Limit be any different?
  • What would the nightscape look like?
  • How would tides and eclipses be affected?
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  • $\begingroup$ What would this moon be made of? Our moon is relatively homogeneous rocky material made up of a few common elements, and has no atmosphere to speak of. Titan, on the other hand, has a significant nitrogen-rich atmosphere, and a surface composed of water ice, probably some liquid water down deeper, and denser elements like silicates and heavy metals as you get closer to the core. So we really need to know what your moon will be made of, as the composition will have an enormous impact on your question. $\endgroup$ – type_outcast Jan 3 '16 at 5:57
  • $\begingroup$ The moon will still be airless rock. $\endgroup$ – JohnWDailey Jan 3 '16 at 6:00
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Terminology, to shorten my answer

  • Luna = Our real moon
  • Titan = Saturn's real moon
  • Lutan = Your fictional moon

Would the Roche Limit be any different?

Luna orbits at $\approx385,000 \text{km}$. The Roche Limit is at $\approx 10,000 \text{km}$. There would still be no danger. (Keep in mind the Earth's diameter is $12,742 \text{km})$

What would the nightscape look like?

Given your comment that Lutan "will still be airless rock," then the night sky would look almost identical to how it does now in terms of specularity (shininess) and color.

The main difference is that it will appear somewhat bigger in the sky.

The angular diameter, $\delta$, is essentially the actual diameter of an object divided by how far away it is, expressed as a simple fraction:

$$\delta = \frac{d}{D}$$

Luna appears $\approx 30 \text{ arcminutes}$ (it varies). Lutan would be $\approx 45.8 \text{ arcminutes}$.

Due to its increased size, it will also provide more than double the light during a full moon (about 133% more, by my calculation.)

How would tides and eclipses be affected?

Tides

Since Lutan is nearly twice as massive as Luna, tides would no doubt be affected. They would be more pronounced, so coastal regions would experience larger swells, and related weather patterns would be more significant.

The tidal locking would have happened earlier in the planet's history, which might have resulted in subtle changes. Also, the drag effect from Lutan on the Earth would be more significant, so our days would probably be somewhat longer. It's hard to estimate how much, but I don't think it would be out of the question to have a 25 hour day or so, depending on a few factors.

Eclipses

Total solar eclipses will be more frequent, because Lutan's disc is larger and would block more of the sun. Conversely, you would have fewer annular eclipses (these happen when Luna is farther from the Earth and doesn't completely block out the sun, as shown below):

enter image description here

Lunar eclipses would look a bit different, and might be more interesting to observe, due to the significantly larger area of the moon to view.

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  • $\begingroup$ What kinds of "subtle changes"? $\endgroup$ – JohnWDailey Jan 3 '16 at 6:38
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    $\begingroup$ @JohnWDailey Dang, I was hoping you'd accept my glossing-over of that. :-) When two objects are not yet tidally locked, the shape and magnitude of the tidal bulging is different. (Requires some discussion of prolate spheroids, ellipsoids, and other equally engrossing subjects.) The take-away from this is that the differences in the tides would have caused some changes with ocean tides, erosion, and weather, which might be evident in, say, fossil studies. Some coastlines may look different than they do today, but it would take some powerful computer modeling to create a picture. $\endgroup$ – type_outcast Jan 3 '16 at 6:48

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