# What would make the moon flicker?

Maybe it was a trick of the light or a strange cloud, or just not enough sleep. A couple nights ago I could have sworn that thin crescent moon flickered.

Which led to an interesting question - what conditions could make the moon flicker?

Of course, there are all kinds of interesting consequences. I know we have a couple eclipses each year, and the moon has phases, but overall, we take the steady, ethereal glow of the moon for granted.

By flicker, I mean off and on within seconds to an hour. The phenomenon must be observed wherever the moon is visible. It can be instant or gradual, but must return to its original state within an hour.

I realize the physics of moonlight make this hard, so a habitable Terra-like planet in another system is ok. The moon just needs to be large enough to be extremely obvious and an important part of the night sky, so that its flickering would be impossible to miss. I'd especially like it if this were a recurring event, maybe even a regular feature of the night.

Edit: I'm not looking for an explanation of what I saw, that can probably be chalked up to sleep deprivation. I'm looking for a sci-fi or real explanation for a flickering moon. Math not required, just ideas.

• Stop looking at the moon through the fan. ;) – Frostfyre May 22 '15 at 14:50
• I was outside on my way home to a newborn baby after working 12 hours. So who knows what I saw. :D – Josiah May 22 '15 at 14:55
• Most ceiling fans go between 80 and 300 revs per minute, so I would say your cloud should pass in front of the moon at that speed (or the equivalent). I'll leave the math to the answerer. – JDSweetBeat May 22 '15 at 14:59
• The transition has to be instant, or can be gradual? I guess instant but you didn't specify. – o0'. May 22 '15 at 14:59
• there seems to be a few "dead pixel" in the dyson sphere come on please stop looking at the fan and do something about the dead pixels! – user6760 May 22 '15 at 15:32

First a brief explanation of what flickering is:
Flickering is caused by transient changes in light intensity. It is much more common to see flickering in small / point sources of light (e.g. stars or planets) because very small atmospheric changes affect all light arriving from that source.

Similar small atmospheric disturbances usually don't affect all the light coming from non-point sources like the moon.

Causes
The most simple and plausible causes of "moon flicker" (changing brightness) would fall into one of these categories:

1. Something interferes with the passage of light from Sun -> Moon
2. Something interferes with the passage of light from Moon -> Earth
3. Something changes the brightness of Moon
A. The Moon emits light
B. The Moon's reflectivity changes
C. Something else shines on the Moon
4. Something changes the brightness of the Sun

1. Interference of Sunlight reaching the Moon
I really can't think of much that could do this. It'd have to be huge to affect the brightness of the Moon such that we could see it.

The Earth's atmosphere isn't in the way so that wouldn't do it either.

Under the realm of SF, you might see something like this if you had something like a planet size solar reflector (e.g. to cool Venus or warm Mars) and it accidentally reflected light onto the Moon. That might be spectacular but I'm not going to do the math for you :P

2. Interference of light reflected from Moon -> Earth
This is by far most plausible since the Earth's atmosphere regularly causes flickering in the light coming from smaller astronomical bodies. To affect the overall brightness of the Moon, the turbulence would have to be much larger than normal (more energy in the atmosphere).

Alternatively something large would have to pass between the Earth and Moon to dim the Moon significantly. The closer the object is to the observer, the smaller it could be to have the desired effect. That object would probably possess low reflectivity (perhaps an undiscovered Near Earth Asteroid?) or the observer might notice the occultation.

3. Something changes the brightness of the Moon
This image of Inconstant Moon from Deviant Art

There are three phenomenon that could do this, two from a flash coming from the Moon and the last is something else shining on the Moon.

Moon emits light
The more common of these and one that has been observed is meteors striking the Moon. If you are not observing the Moon with the telescope, you'd just see a brightness change without knowing what caused it. The meteor actually emits enough light to compete with the reflected sunlight. It especially striking when the Moon is not full and the meteor strikes the unlit portion of the Moon.

Moon's reflectivity changes
Probably less common but possible in an SF story would be if human development left man-made structures on the Moon. When sunlight strikes flat surfaces (e.g. PV panels?), the terrestrial observer would see glinted sunlight. This might cause a "flickering" type of effect. The change in reflected could also be caused by any number of other SF occurrences.

Something else shines on the Moon
A variant of this would be if the Solar System were hit by a supernova/hypernova/gamma ray burst from relatively close range (10s - 100 light years). People on the sheltered side of the Earth might see spectacular effects on the Moon. In this scenario, the devastation would be total - it would kill everything on the planet.

4. Sun's brightness changes
A number of SF stories have been written about this. The most memorable to me was Larry Niven's Inconstant Moon (also made into an Outer Limits episode). In it the Sun emits and enormous flare that presumably kills most of the inhabitants of the days side of the planet. You wouldn't need such a huge flare to cause a noticeable difference in Moon brightness but I don't think the effects would be very benign either.

• Great ideas! #2 - maybe we could use something akin to earthshine on a new moon. Also like #3, in which my prehistoric denizens would be correct that the change in the moon's brightness portends doom. +1 again for the video. :) – Josiah May 22 '15 at 16:23
• For an SF story, you might state that a (rare) Moonquake causes a landslide which uncovers an alien artifact with a reflective surface. Occasionally sunlight glints off it and begins to make astronomers wonder about it... – Jim2B May 22 '15 at 16:29
• For #1, a very large, very fast-moving object or series of objects could pass through the Solar system, temporarily blocking the line of sight between the Moon and the Sun. They would have to be at least planet-sized and quite close to the Earth, so one would expect them to disturb its orbit as they passed. – Nathaniel May 23 '15 at 3:26
• Unless they were hollow. Imagine a bunch of 1000 mile wide mylar ballons (99 red balloons, where is Nena when you need her?). That could do it :) – Jim2B May 23 '15 at 3:56
• #1 A mega structure. The beginings of a dyson swarm. – PCSgtL Nov 10 '17 at 16:29

The moon is visible because it reflects light from the sun and some of that light deflects in our general direction. To make the moon flicker, you either need to block the light that is hitting the moon or block the light after it has reflected off. A third alternative is to momentarilly turn off the source of the light, but since that is our life giving sun, my continuing sanity requires that I ignore that option.

Blocking the light that hits the moon is possible. If a dense enough cloud of particles fell into our sun's gravity well it would block some of the sun's emitted light as it passed. If it happened to obscure the particular rays which were destine to reflect off our moon and into our eyes, and if we were looking skyward at exactly the right second, we would see a flicker.

Blocking the light after it reflects off the moon requires that something get in between us and the moon. Clouds within our own atmosphere could serve that function but they usually don't move fast enough to appear as a flicker. Also, you can usually see previously obscuring clouds as they continue on their journey, providing you with an instant explanation for what you have just seen. We have all lived on this planet for our entire lives, so we are pretty used to cloud shadows. Outside of our atmosphere, objects might thread the needle between our gravity well and that of our moon, and might be big enough to block the moonlight, but we would have heard about such a near-miss on the news either before or after it occurred.

The final option which I can think of, involves not an object (either in or out of our atmosphere) blocking the light, but rather our atmosphere itself. I know of no energy which could do the job, but I can imagine that there exists one that upon stricking the upper layers of our atmosphere, momentarilly rendered that layer opaque. Perhaps there are turbulence effects in high-energy as there are in fluids, and we just haven't encountered them until now. Perhaps in that fateful millisecond on that fateful night, when you and your child were starring at the universe, the first such energy collision in human history occurred, and you alone noticed it.

Consider yourself blessed and go back to enjoying your newborn!

• The cloud of particles is a great idea. Maybe not enough to make it flicker out, but could be noticeable. – Josiah May 22 '15 at 16:19

Bats!

I used to go camping in a valley in the Rocky Mountains where I'd see the moon flicker regularly, every night without fail. After inquiring, I found out that the reason for this were bats.

They are tiny, so you can't really see them or be sure they are responsible for the effect

They take flight from their cave at a consistent time causing the moon to 'flicker' at around the same time every night

There is a lot of them, so odds are at least one will fly between your eye and the moon several times in a short period of time

The amount of time they actually block the light from the moon from getting to you is tiny, so you get a full flickering 'completely bright' to 'not there' and back

Imagine a planet with 2 moons, like Mars. Unlike Mars+Phobos+Deimos, one of the moons of this fictitious planet is far away, and the other is very close to the planet. At some points in the cycle, the far-away moon is lit and the nearby, tiny moon is completely in the planet's shadow. This black disk will pass (quickly) over the bright, faraway moon, making it disappear and then reappear.

If, as you say in the comments, a gradual on to off over a few minutes is alright, then a lunar eclipse fits the bill. This would provide the effect to many people on the surface of the Earth.

For more drastic flickering, then the effect will have to be more localized, like a cloud or hot air balloon between the affected person and the moon.

Of course, you could also just block the sun from the Earth-moon system by flying a gas giant nearby. This would give the people on the day side something to look at too,

• Well, I'm looking for the scale of up to an hour, while a lunar eclipse, though dramatic, is on the scale of a couple hours. Not quite fast enough for what I'm looking for. The gas giant flyby would be on the scale of days wouldn't it? – Josiah May 22 '15 at 16:12
• @Josiah Ah, well the total 'off time' for an eclipse is on the order of minutes. The gas giant's effect depends on its velocity, this would be a rogue planet, not in a stable orbit. – Samuel May 22 '15 at 16:17
• @Samuel would a gas giant between us and the sun destabilize our planet too much to continue in our orbit after the lunar event? – Mikey May 22 '15 at 17:07
• @Mikey Not necessarily. But I'm not being too serious with this answer, so I'm not planning on backing that up :) – Samuel May 22 '15 at 17:13
• I'm guessing a sufficiently high velocity rogue gas giant would have very little effect on our orbit, but would still likely cause devastating tidal events, affecting the ocean only a little in such a short time frame, but affecting the crust enough to cause worldwide earthquakes. The ocean rising half a millimetre would go unnoticed, but the same movement in the crust applied suddenly would release all the earthquakes that have been waiting to happen at once. – trichoplax May 22 '15 at 22:26

Iapetus, a moon of Saturn is a moon with two colors: one side is snow-white, the other is pitch-black. It could be that a planet possesses such a moon and which is not (or is not yet) tidally locked. At one time it shows the white side, and then the black one at another. Now, if the surface is white and pockmarked with 50 shades of grey, then the brightness appears to change. The "new moon" phase is when the moon crescent is the thinnest. It is the phase which responds the most to changes in albedo. The moon will appear to flicker, albeit slowly.