Would the light reflected off of a blue moon and red moon combine into a purple hue on the surface of the local planet at night?

Question

Edited: I suppose the setting doesn't really matter.

Scenario: An earth-like planet in size and material and atmospheric composition experiences a phenomenon of purple light being the visible light at night when its two moons, one with a red surface and the other blue, both shine with equal luminosity, usually during a dual full moon. Both moons are the same size as Luna(ours) at a similar distance to ours, with the blue one being slightly closer to the planet than the red one but the difference in distance isn't enough to make the blue look too significantly larger than the red. The how and why of them each being different enough in material composition to so drastically reflect such different wavelengths doesn't matter, just whether or not that they do would actually lead to a combined hue on the local planet's surface.

Would the light of two moons, one with a red surface and the other with a blue surface, lead to a purple light shining on the surface of the local planet at night?

Answer 1

Yes. If you've ever spent a lot of time in a place with no electricity, you'll know that it gets very dark at night when there's no moon. And when the moon is full, there's only just enough light to make out colors (when our eyes adjust to light little darker than that, everything is grayscale).

Now, you can check whether reflecting light changes the color of a thing by shining a bright light on something colorful and letting it reflect onto white paper. The resulting light is the color of the thing it was reflecting off of. Your moons would definitely reflect light of their own color.

So, when the moons are full enough for there to be substantial light, so that you can make out color, it would be blue and red light, and the combination would give a slight purple tint to the landscape.

Answer 2

Yes, with both moons full in the sky, their combined light will appear purple to human eyes, and should give the landscape illuminated by them a purplish tint.

However, in practice the effect may not be as noticeable as you might expect, for two reasons:

1. Human color vision is most effective in bright light. In dim light, our eyes gradually switch to scotopic vision, which is more sensitive at low light levels but cannot distinguish color. The result of this is that in dim light, such as moonlight, we perceive colors only faintly, if at all.

2. Even in bright light, human eyes adapt to the color of ambient light and compensate for it to provide perceptual color constancy. This is why we're able to, for example, look at a piece of white cloth or paper under a bright blue sky, in dim reddish-yellow candlelight or in a forest under a canopy of green leaves, and in all cases agree that it is indeed white.

   When deliberately tricked, or simply lacking the correct cues, this adaptation can also produce some pretty weird color illusions, like the famous dress or this Rubik's cube illusion. It can even synthesize perceived colors out of nothing, merely based on the lack of other colors, as in this landscape drawn entirely with red and white pixels.

Anyway, here's a quick way to test what the night might look like in your world:

1. Turn up your screen's brightness as far up as it will go.

2. Turn off all the lights in the room and cover any windows etc. that might let stray light in. (This is easiest to do at night, or in a windowless room. If you're on a laptop, you can just take it with out into a closet or something.)

3. Click the image below to open it in full size and switch your browser to full screen mode. You should now see nothing on your screen except a red and a blue circle.

   

   (If you can't get the image to cover the full screen without some title bars etc. visible, try this Google presentation link and press Ctrl+Enter to switch to presentation mode.)

4. Look around you.

I tried this myself, and honestly, the purple tint itself is kind of underwhelming. Yes, I can tell it's there, but my eyes adapt to it quickly and it becomes barely noticeable.

The colored shadows from the two separate light sources, however, look really cool.

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Ps. Since the issue was raised in the comments, let me briefly discuss whether it's possible to actually have two moderately large moons visible in the sky at the same time.

The short answer, FWIW, is yes. The tricky part is actually having two large moons in stable orbits in the first place (and getting them to somehow have the colors you want) — if you can manage that, there's no way to keep them from at least sometimes appearing in the sky together.

Anyway, as far as we know, there are basically three ways two have a pair of similarly sized moons in stable orbits around a planet:

1. They can orbit at different distances from the planet, meaning that their orbital periods will be different. This is by far the most likely scenario, as it's what most moons in the solar system do.

   In some cases the moons might end up in a stable orbital resonance, like the Laplace resonance between Jupiter's moons Io, Europa and Ganymede, where Ganymede's orbital period is exactly twice that of Europa and four times that of Io. This can stabilize the system and allow the moons to stay in stable orbits closer to each other than would otherwise be possible.

   Note that it's also not strictly necessary for the moons to orbit the planet in the same direction, and in fact simulations show that having one of the moons in a retrograde orbit can sometimes help stabilize the system. However, it's unlikely for a moon to form in a retrograde orbit, so it's thought that all such irregular moons in the solar system (which, except for Neptune's moon Triton, are all small and far from their planets) have been captured into their current orbits from elsewhere.

2. It's also possible for two moons of similar size to share approximately the same orbit. There's exactly one known pair of such co-orbital moons in the solar system: Saturn's moons Janus and Epimetheus.

   However, such moons will not stay a fixed distance apart — that would be an unstable arrangement. Rather, whichever moon happens to be slightly closer to the planet will orbit faster and gradually catch up to the other one. But once the moons get close enough to each other, gravitational interaction between them will cause them to swap orbits, with the outer, slower one getting pulled into a closer, faster orbit and vice versa. This makes the moons start moving slowly apart again, until eventually the now faster moon will complete a (nearly) full extra orbit and catch up to the other one from behind, at which point another orbit swap will happen and the cycle repeats.

3. Finally, it's of course also possible for the two moons to orbit one another as a "double moon". Obviously, this would make the moons always stay close to each other in the sky, although they can also regularly pass in front of each other.

   There are no known examples of such double moons in the solar system, although several double planets (including, by some definitions, the Earth–Moon system) and double asteroids are known. One reason for this is believed to be that, for moons close to their parent planet, the same tidal interactions that drive tidal locking will also destabilize double moons, as they cannot simultaneously lock to each other and to the planet. That said, you could probably handwave that issue away somehow.

If one of the moons was significantly smaller than (i.e. less than ~4% of the mass of) the other, it could also orbit stably at or near the other moon's 4th or 5th Lagrange point, i.e. 60° ahead or behind it on the same orbit. No such "trojan moons" are known in the solar system, but several planets have small asteroids orbiting at their L4 and/or L5 points, including the several thousands of Jupiter trojans. However, if you want your moons to look roughly equally big and bright in the sky, this is unlikely to be what you want.

In general, however, note that your moons do not have to be anywhere near the same size in order to look roughly equally large in the sky, as long as the smaller one orbits closer in. For example, Mars's inner moon Phobos, despite having only 0.00001% of the mass of the Earth's moon, looks fairly comparable in size as seen from the surface of Mars due to its much lower orbit.

Answer 3

This is advice to Rubrikon and a reply to Goodies's comment that:

+1 wonderful scenario but I'm glad there isn't any "reality check" involved. I think the two large moons would eventually stabilize their orbits in eternal opposition. I'm afraid your two large moons won't have a common full moon. That would mean they are on the same side of the planet. Considering the large relative weight of our moon, a second one, forming a binary moon system with an Earth-like planet, will probably stabilize with the moons in orbit on either side of the planet, with no common full moon.

That would not happen in real life.

There are many examples of multiple moon systems in our solar system. There are no examples of 2 moons orbiting at the same distance from the planet and always opposite to each other. And moons in different obits will always change their relative positions as they orbit.

Part One: Moons in different orbits.

If there are two moons, They will (almost always) orbit the planet at different distances. And so the moon with the inner orbit will orbit faster and pull ahead of the outer moon until it is 180 degrees ahead, on the opposite side of the planet, and then catch up with the outer moon until it eventually passes it, over and over again. Sometimes the two moons will appear close together, sometimes only one will be seen at a time.

Sometimes one moon will be seen in the day, and the other moon will be seen in the night. Sometimes both moons will be seen in the day. Sometimes both moons will be seen in the night, making the night purple as requested.

With different red and blue moons, there will be constant changes in the color of the night.

Part Two: Moons in Exchange Orbits.

It is commonly said that the moons Epimetheus and Janus of Saturn share an orbit, but that is not exactly true. They have very similar orbits, but one orbits slightly closer to Saturn than the other. Because of the very similar orbits and orbital speeds, it takes many orbits round the planet for the inner one to pull ahead of and then catch up with the outer one, and they orbit so close that every time the inner one catches up their gravitational interactions make them switch obits.

https://en.wikipedia.org/wiki/Janus_(moon)

Epimetheus's orbit is co-orbital with that of Janus. Janus's mean orbital radius from Saturn is, as of 2006 (as shown by green color in the adjacent picture), only 50 km less than that of Epimetheus, a distance smaller than either moon's mean radius. In accordance with Kepler's laws of planetary motion, the closer orbit is completed more quickly. Because of the small difference it is completed in only about 30 seconds less. Each day, the inner moon is an additional 0.25° farther around Saturn than the outer moon. As the inner moon catches up to the outer moon, their mutual gravitational attraction increases the inner moon's momentum and decreases that of the outer moon. This added momentum means that the inner moon's distance from Saturn and orbital period are increased, and the outer moon's are decreased. The timing and magnitude of the momentum exchange is such that the moons effectively swap orbits, never approaching closer than about 10,000 km. At each encounter Janus's orbital radius changes by ~20 km and Epimetheus's by ~80 km: Janus's orbit is less affected because it is four times more massive than Epimetheus. The exchange takes place close to every four years; the last close approaches occurred in January 2006,[14] 2010, 2014 and 2018. This is the only such orbital configuration of moons known in the Solar System[15] (although, 3753 Cruithne is an asteroid which is co-orbital with Earth).

https://en.wikipedia.org/wiki/Epimetheus_(moon)#Orbit

If your planet had two moons in very similar orbits like Epimetheus and Janus, they would be close together in the sky for a long time, and be farther away in the sky for a long time, and would be far enough apart that only one was visible at a time for a long time. Years could pass when only one moon was visible at a time, when the red and blue moon alternated in visibility and when only one lights the night sky at a time. And then after that there could be years when the two moons were close enough that they usually were seen together and light the night sky together.

If you want such a sequence of lighting effects you should go with a Janus and Epimetheus type orbit. And naturally some characters would worry that maybe something will go wrong the next time the moons exchange orbits, and fear that one of the moons might crash onto the planet.

Part Three: Moons in a Trojan Orbit.

With 2 moons in a trojan type orbit, they will both be the same distance from the planet, and one will alwys be about 60 degrees from the other. Being separated that far will mean that sometimes they will be seen together and sometimes one will be below the horizon while the other is visible (and sometimes both will be below the horizon). More importantly, one of the moons would have to be tiny compared to the other for long term orbital stability, and so it will reflect only a tiny amount of the light that the other one does.

Part Four: A Ring of Moons.

You could have a ring of 7 to 42 moons of equal mass equally spaced in the same orbit and alternating red and blue in color of surface material.

https://planetplanet.net/2017/05/03/the-ultimate-engineered-solar-system/

https://ui.adsabs.harvard.edu/abs/2010CeMDA.107..487S/abstract

But if the moons were massive enough to reflect enough light, orbital stability would probably require that the planet be too massive to be habitable for humans.

Maybe the habitable world would be a giant moon orbiting around a giant planet, and the ring of alternately red and blue moons would orbit the planet outside the orbit of the habitable moon.

Part Five: Double Moon - Two Moons Orbiting Each Other as They Orbit the Planet.

In a comment Ilmari Karonen suggested a double moon, with the two moons orbiting each other as they orbited the planet.

That might be the best way to have moons visble together for the mos time. They would be two equal size moons orbiting around their center of gravity or barycenter as they orbit the planet. The orbital distance between the two moons would be a rather small fraction of the total circumference of their orbit around the planet. Both the moons would be visible almost half the time. Both the moons would be out of sight below the horizon almost half the time. The red moon would be seen alone low above the horizon a small percentage of the time, and The blue moon would be seen alone low above the horizon a small percentage of the time.

They don't have to be exactly the same size. If one had half the diameter of the other one it would have a quarter of the surface area to reflect light, and one eighth of the mass of the larger one. It might appear to be only one quarter as bright as the other moon which may be a problem mixing colors to make purple light.

But if the two moons have different surface materials which have different colors, those surface materials could have different albedos, reflecting different percentages of the light that hits them. The larger moon's surface could refect a relatively small percentage of the light that hit it and the smaller moon could reflect a higher percentage of the light that hit it.

The variation in the albedo of various objects in the solar system is great enough that a moon with only one quarter the surface area of another one could reflect much more light than the larger moon.

And of course the star of the system might not emit equal amounts of red and blue wavelengths of light for the moons to reflect back at the planet. In fact it would probably be almost impossible for a star with a habitable planet to do so.

Of course designing a stable planet-moon system with a double moon orbiting the planet might be tricky, but it should be possible.