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I initially wanted to create a culture that used tides to tell the seasons, I'm sure I still can, but a question on here (I can no longer find it) made me wonder if I could set up a world where one of the moons was much brighter on a few days of the year (once, twice, three times etc, but not 'monthly' or weekly).

I'd want a world roughly the size of earth, and between 1 and 3 moons if possible. I'd like the moon that gets unusually bright to be roughly (0.8 - 1.2 times) the size of our moon. I don't want any 'unnatural' circumstances (like one side of the moon is covered in mirrors). The brightness should be ~the same as our moon for the majority of the time, and then bright for a few days at most when it does become brighter (at least 2-5 times brighter).

I've read A brighter moon that's harder to see but I can't see how to adapt any of those answers.

Is this possible with just multiple moons and orbits?

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    $\begingroup$ Consider that a month “is approximately as long as a natural orbital period of the Moon”. If your brightening happens at regular intervals, your civilisation might be inclined to make that the duration of their months. $\endgroup$
    – user137369
    Aug 5 at 10:47
  • $\begingroup$ Can the moon be tectonically active? $\endgroup$
    – tuskiomi
    Aug 6 at 14:33
  • $\begingroup$ @tuskiomi yes but that's been answered here, and also I sort of wanted something regular. I don't know how to do that with tectonics $\endgroup$
    – Pureferret
    Aug 6 at 14:50
  • $\begingroup$ Can the moon support vegetation? The albedo of a Deciduous forest is 0.15 to 0.18, and the albedo of grass is 0.25, whereas the albedo of our moon is only 0.12. You could have some sort of vegetation bloom/grow during these periods. $\endgroup$
    – Austin
    Aug 6 at 19:45
  • $\begingroup$ @austin I don't know. You'd have to show that was possible for a moon with the specifications in my question. $\endgroup$
    – Pureferret
    Aug 7 at 0:46

17 Answers 17

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No tidal locking

The moon still hasn't developed a tidal lock with the planet. So, the hemisphere of this moon facing the planet is always different.

Because of its geological history, this moon's surface has areas with very different albedo, think something like Ceres bright spots, but more widespread.

This way, its brightness has some spikes only in certain days of the year, also taking into account the different lunar phases, since the bright areas must be facing the planet and be lit up by the sun at the same time.

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    $\begingroup$ I came here to say that. You've said it :-) $\endgroup$ Aug 4 at 12:10
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    $\begingroup$ Models show that our Moon became tidally locked very quickly. You would need some mechanism to stop that from happening, or to have disrupted the rotation in geologically recent times. $\endgroup$
    – JDługosz
    Aug 4 at 14:17
  • $\begingroup$ @JDługosz That is because our moon formed in an area with a strong gravitational gradient, near the Earth. A moon that would be a captured object that formed far from gravitational gradients yet was massive enough to differentiate, would be unlikely to have a lopsided hemisphere and thus would have comparatively little tidal torque applied to it. $\endgroup$
    – dotancohen
    Aug 4 at 15:22
  • $\begingroup$ @JDługosz I would also add that other possible explanations could be that this moon did settle in a resonant orbit (like Mercury and the Sun), or that it was captured only recently by the planet (recently in astronomical terms, of course) $\endgroup$
    – McTroopers
    Aug 4 at 15:31
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    $\begingroup$ Yes, there are situations where an odd-half-multiple (like Mercury) is preferred over tidal locking, and this is commonly overlooked as I have lamented many times. Make it lopsided and eccentric, and have another body farther out to drive the eccentricity (so it doesn't circularize instead). $\endgroup$
    – JDługosz
    Aug 5 at 14:06
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This could be achieved by placing the moon in a slightly inclined orbit outside an inner semi-transparent ring system.

(side view)
side view

For most of the year, people living on the planet will view the moon through the rings (which need to have some thickness, a "smoke ring"). They will see a dimmed version of the regular moon phases.

But two times every month, the moon will be far enough from the orbital plane of the smoke ring to be clearly visible over the edge. If this happens to align with a full moon, the resulting brightness could be several times brighter than normal.

How often this event occurs can be adjusted by changing the length of the time window where the moon can be viewed unobstructed. If the Earth had such a ring system, and the moon poked over the edge for a few days each time, you would expect roughly 1 extra bright full moon every year.

camera tracking

Visualisation with some example parameters. Camera on the surface tracking the moon, keeping the smoke ring in centre of the image.

Significant inclination differences between moons and rings can happen in real life. The Saturn moon Iapetus is for instance 15.5° out of the equatorial plane, although the size and thickness of the rings aren't right to make an arrangement similar to this.

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  • $\begingroup$ This is very unlikely due to orbital mechanics. The "smoke ring" would tend to align with the planet's equator, as would the orbits of moons (you get tilt because the planet's axis is tilted, and you get chaos if the moon is far enough out for the star's tug to compete with the planet's). Most likely the moon would be behind the smoke ring all the time... $\endgroup$
    – Zeiss Ikon
    Aug 4 at 12:24
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    $\begingroup$ @ZeissIkon Moons can be quite far away from the planet's equator. Our Moon, for instance is about 20 degrees out of plane. With a smoke ring added in the equatorial plane, that's a large enough difference. $\endgroup$ Aug 4 at 12:44
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    $\begingroup$ Our moon is so far out of plane because it's so far out -- no other moon in the System is anything like as far out relative to the mass of the primary. It's almost an independent satellite of the Sun. Look for any other major moon that's even 10 degrees from the planet's equatorial plane -- maybe one of Uranus's, I don't recall, but certainly none of the major, co-formed moons of Jupiter, Saturn, or Neptune. $\endgroup$
    – Zeiss Ikon
    Aug 4 at 13:21
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    $\begingroup$ I am more unsure about the smoke ring itself. Planetary rings are rather thin; if they where thicker they would experience a lot of friction. $\endgroup$
    – Taemyr
    Aug 6 at 8:51
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    $\begingroup$ Certainly such a constellation would not last for billions of years. But if it can last for 1 million that may be more than enough for the story. (Super nice visualization by the way!) $\endgroup$ Aug 6 at 17:34
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If your moon had an orbit significantly inclined to the plane of the ecliptic (i.e. the planet's orbital plane), it would only be full close to the ecliptic a couple times a year (depending on its orbital period relative to the planet's, perhaps you'd have occasional years with one or two more or fewer such full moons -- one pair would be split around the ecliptic crossing, and might both or neither be "close enough").

Due to a well known optical phenomenon, a full moon that's on or near the ecliptic will be brighter than one that's not -- a situation that could be enhanced if the moon's surface has a retroreflective component in the regolith (shock glass spheres just the right size?).

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  • $\begingroup$ polar orbit should do this $\endgroup$ Aug 4 at 6:35
  • $\begingroup$ @IvanBorsuk Wouldn't a polar orbit have every full moon on the ecliptic? $\endgroup$ Aug 4 at 9:36
  • $\begingroup$ yes, this answer says so: "it would only be full close to the ecliptic" $\endgroup$ Aug 4 at 9:50
  • $\begingroup$ @PaŭloEbermann Polar orbit would only ever be full on the ecliptic, so you wouldn't have brighter and less bright full phases. An orbit with, say, 10 degrees inclination from the ecliptic would be full near the ecliptic only two to four times a year, and would be significantly brighter at full at that time than when it's furthest from the ecliptic. $\endgroup$
    – Zeiss Ikon
    Aug 4 at 11:03
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    $\begingroup$ @IvanBorsuk Don't trim your quotes in ways that change the meaning. "it would only be full close to the ecliptic a couple times a year" is what I wrote. $\endgroup$
    – Zeiss Ikon
    Aug 4 at 11:07
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Everything is brighter.

elliptical orbit https://commons.wikimedia.org/wiki/File:Ellip-orbit.gif

Your planet and its moons are in an elliptical orbit around its star. When it is close the star is brighter. Because of that, reflected light from the moons is also brighter. The more elliptical the orbit is, the shorter the time near the star is.

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The moon is still volcanically active. Volcanism has a periodic cycle, and every eruption explosively projects in the sky volcanic dust and rocks which significantly increase the albedo of the body itself, reflecting more light.

Since there is no atmosphere to slow down the deposition of the dust, the phenomenon quickly disappears.

The reason for the periodic volcanism can be in the tidal heating caused by the interaction between the moons.

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Your planet may have its three moons in a self-correcting orbital resonance configuration, with the same ratios as the system formed by Ganymede, Europa and Io over Jupiter.

If the period of the innermost moon is about a month, the one in the second highest orbit will have a period on about two months, and the outermost moon will have a period that is about four months. That means all three moons will align perfectly every 4 months or so, which will give a minimum brightness at those periods (since they will be in each other's shadows).

Similarly, they will be each 60° from the next at least three times per year. If during those times the one that appears "central" in the sky is full, it will reflect not only the light of the sun, but also any light it gets reflected from the other ones. If they all have very high albedos, this might cause an impressive shine on the one that is full.

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An ocean covered moon that occasionally freezes over due to the elliptical orbit of the planet around the sun. Sea ice has an reflectivity (albedo) of over 10x that of water. 1

Caveat: if the moon has liquid water, it probably also has some sort of atmosphere. And those together probably means there will be clouds, which also increase the brightness when there isn't any ice.

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Did you follow the exploration of Ceres?

Several bright spots were spotted (pun intended) as the craft approached.

You could have highly reflective bright minerals on your proposed moon, that are located in deep pits or are naturally retroreflective on a micro-scale for some reason. This would cause the bright reflection to only be visible during a full moon; perhaps it can be precise enough that it's only visible on the full moon sometimes, about as common as we get eclipses here on Earth as that's also caused when the alignment is precise.

Imagine our moon, but the maria are covered not with dark basalt but with a mineral crust that dried and cracked to form an egg-carton pattern, and is a huge expanse of bright retroreflector. The contrast would be especially great since being off-axis will appear darker than a simple specular surface, as all the reflection is concentrated in the beam direction instead of scattered in all directions. So, twice a year or so when it is near but not quite a lunar eclipse the dark seas turn golden bright!

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Since Niven's Inconstant Moon we know what it means when a moon becomes brighter: It must be because the sun illuminating it has become brighter.

Therefore, one reason for a moon to become brighter on occasion is an unstable central star. This would affect all moons around the planet though, and would impact the planet's day side.

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The star has a ring

enter image description here

A ring like this is very likely (always?) considered a protoplanetary disk, found around a very "young" star; yet even a wizened white dwarf can sport a showy ring from time to time. A white dwarf would be the better possibility here because it is such a small source of light that it can be blocked by a fairly narrow ring. The ring should be as 'dusty' as can be arranged, to block more light.

Planets are likely to orbit in the plane of the ring. If one of them does so almost precisely, but has a moon (perhaps captured) in an improbable polar orbit, the moon may occasionally stray out into the star's full fury, to the amazement of stargazers below.

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    $\begingroup$ That image... is the eye of Sauron. $\endgroup$
    – Pablo H
    Aug 5 at 14:10
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The earth's moon already does what you'd like. Because the moon's orbit is not quite circular, the full moon is sometimes closer to the earth than other times. This results in the moon being slightly larger and noticeably brighter. The moon's orbital eccentricity is only 5.5%, but if it were larger, the effect would be greater.

As other posters have pointed out, you could get additional brightness from the moon's rotation bringing a brighter side facing the planet, reflections of other moons, and the whole planet-moon system being closer to the sun. Additionally, the earth's moon is about 4% brighter when earth is in opposition, which is hypothesized to be because the sunlight is more direct, resulting is few shadows.

The best information I could find was here.

You could also just leave it as a mystery. As far as I'm aware, Tolkien never stated what material was used in the construction of Orthanc or how/why it was invulnerable to any tools or Ent-roots.

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A species of bioluminescent creatures (like fireflies or glowworms) live everywhere on this moon. They mostly shine their lights at a particular time of the year, such as their breeding season.

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    $\begingroup$ Is that bright enough to be seen from the planet? $\endgroup$
    – Pablo H
    Aug 5 at 14:12
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You can let two moons move in opposite eliptical orbits. When they pass one another on the dark side of the Earth montly, this gives rise to a temporarily increase of both libght and grsvity pulling on Earth.

When they are on opposite sides (half moons) there will be no tides at all and when they are on the light side of the Earth (no moons) there will be a smaller tide as when they were on the dark side (full moons).

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    $\begingroup$ Are you sure you understand how lunar phases work? You seem to have given the wrong phases for different orbital positions in this answer... $\endgroup$
    – Zeiss Ikon
    Aug 3 at 16:15
  • $\begingroup$ @ZeissIkon I must have been out of phase... Thanks! $\endgroup$ Aug 3 at 16:20
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A cloudy atmosphere on the moon.

It could be a relatively dark surface, but occasionally conditions cause a release of water vapor and the resulting clouds are more reflective causing an apparent increase of brightness.

Alternatively, it could be that the moon is normally shrouded in clouds of compounds that tend to absorb light and occasionally conditions cause this layer of clouds to clear, exposing a surface that is more reflective.

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    $\begingroup$ Hi, as this question is science-based can you support your answer with some science based arguements? Like what size moon is big enough to have clouds ... And what makes then only occur so rarely? $\endgroup$
    – Pureferret
    Aug 3 at 21:40
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There are a lot of rocks in space (Some broken up comets/asteroids that had orbits that intersected with your planet/moon). Periodically the moon ploughs into these and the moon brightens from the impacts. This also also disturbs the underlying layers of your Moon which happen to be highly reflective so the Moon stays bright for a while before the surface gets re-weathered and turns back to a low reflective color.

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The moon is inhabited

The moon has a full atmosphere and there is an advanced civilization living there, with large, well-lit cities across a large area. It's possible they only settled on about a 3rd of the surface for reasons known only to them. Possibly it gets the best balance of sunlight and darkness so it doesn't get too hot or cold by being constantly lit or constantly in shadow.

  • When it's daytime on the planet, you won't see much at all through the atmosphere.
  • When it's nighttime on the planet:
    • When the settled side is facing away from the planet:
      • When it's their daytime (full moon), you see just the regular albedo of the lunar surface.
      • When it's their nighttime (new moon), you see not much at all because it's too dark.
    • When the settled side is facing the planet:
      • When it's their daytime, it's pretty much the same as when the settled side is facing away from you.
      • When it's their nighttime, you can see the glow from their cities. (It's obviously not brighter than when reflecting sunlight, but it is something that would be considered a very special sight to behold, especially since it occurs during a time period in the lunar cycle when the moon would not be visible at all because it's not lit by the sun.)

Note this does not require either the planetary and lunar civilizations to be in communication or even be aware of each others' existence. They could both have evolved independently and be pre-spaceflight in their development, and this effect would still be visible. All it requires is that the lunar civilization has developed electricity and is widely using it to light their cities at night.

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The moon has an active core.

It's simple really. This is a moon in which there is an active, molten core. The contents of the core (much like the contents of some asteroids) is made of platinum or silver or other lustrous elements. The exerted bits of metal are similar in surface quality to Float Glass, making them highly reflective, with a near-mirror surface finish. These clouds of stardust are much more reflective than the surface, and for a few (perhaps regular, perhaps not) weeks of the year, the moon shines with a dazzling glow.

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