Moonlight bright enough to see by

Question

Is it possible for an Earth-like planet to exist where the light reflected by the moon is, not as powerful, but close in intensity to daylight?

And if so, what would be necessary for it? A moon much closer to the planet? Different chemical composition? Different moon cycle? Multiple moons??

What would be the effect of these on the projected moonlight? Would its spectrum shift towards blue or towards red?

Hesitant to tag this as hard-science as I'm not necessarily looking for exact equations for light propagation, etc...

Answer 1

I am not sure you understand the vast difference between the brightness of sunlight and moonlight when you ask for moonlight "almost" as bright as sunlight. And in fact the moonlight on Earth is quite adequate for many purposes, so it is possible that your story might work with moonlight no brighter than that of on Earth.

The magnitude scale for apparent brightness is a reverse logarithmic scale. The higher the magnitude number, the lower the apparent brightness of a light source. The lower the magnitude, the higher the brightness. A magnitude one magnitude lower corresponds to being 2.512 times brighter. A magnitude five magnitudes lower corresponds to being 100 times brighter.

The new moon, the Moon at its minimum brightness, has an apparent magnitude of -2.50, while the full moon, the Moon at its maximum brightness, has an apparent magnitude of -12.90, a difference of 10.4 magnitudes. A difference of only 10.00 magnitudes corresponds to a difference of 10,000 times in brightness.

The Sun, as seen in a clear sky on Earth, has an apparent magnitude of -26.74. That is a difference of 13.84 magnitudes. A difference of 13.00 magnitudes is a difference of 126,202 times the brightness, and a difference of 14 magnitudes is 317,021 times the brightness. So as seen from Earth the Sun has a few hundred thousand times the brightness of the full Moon.

Apparent magnitude

You might want to ask yourself exactly what you want the extra brightness of your planet's moon for in your story, and then do research to find out how much light is needed for that, and then figure out if it is possible to increase the brightness of moonlight on your planet that much.

On a clear night, you can see fairly well by starlight if you are far from man made light sources and the resulting light pollution.

I used to go out at night and walk up a hill to a grassy field and look at stars and astronomical bodies with binoculars. I didn't take a flashlight with me to light my way because I wanted my eyes to become dark adapted to see in the darkness better.

Human eyes adapt to see better in darkness after a few minutes in darkness. So amateur astronomers don't use flashlights or lanterns, or use only red artificial light, when setting up their equipment to observe the skies, because they don't want to interfere with their eyes adapting to see better in the dark.

Adaptation (eye)

On a cloudy night close to a big city, you can see fairly well by city lights reflected from the clouds due to man made light pollution.

On a clear moonlit night you can see fairly well without any artificial light sources.

Both history and fiction have many examples of single persons or entire armies sneaking around in the dark.

Of course if someone travels by night without artificial light sources they would probably have a higher than usual probability of tripping over something they don't notice or stepping into an unseen hole than if they traveled during the day. But if someone doesn't watch where they are going they could trip during broad daylight also.

The light of stars, planets, and even the full Moon, is not intense enough for most people to read by. Even the light of the full Moon is not intense enough to see colors, except that objects may look faintly blueish.

If you really want the moonlight on you planet to be "almost" as bright as daylight, then you do have a problem designing a different astronomical set up allowing the moonlight to be almost as bright as daylight, because on Earth daylight is hundreds of thousands of times as bright as moonlight.

To get moonlight a thousand times more intense than moonlight on Earth, you might have a moon that occupies a thousand times the area of the sky as seen from the planet as the Moon has seen from the Earth. The square root of 1,000 is 31.622776. The Moon has an angular diameter in Earth's sky of about 29 to 34 arc minutes, so if your fictional planet's moon has an angular diameter of about 916.4 to 1,074 arc minutes, or 15.273 to 17.9 arc degrees, it will have 1,000 times the angular area of the Moon.

If your fictional moon is at the same distance as Earth's Moon, it can have 31.622 times the angular diameter of the Moon if it has 31.622 times the actual physical diameter of the Moon. That would make the fictional moon several times the diameter of any Earth-like planet, so if the planet is supposed to be Earth-like and thus have an Earth-like size the "moon" in your story will actually be a large planet orbited by an Earth-like moon.

Or the moon in your story could be the same size as the Moon but orbit the planet 31.622 times as close as the Moon orbits the Earth. Other things being equal, that will make it appear to be 1,000 times as bright as the Moon seems in Earth's sky. It should actually be more than 1,000 times as bright since the moon will be closer to the planet and it's reflected light will be more concentrated when it hits the planet.

The Moon has an average distance of about 384,402 kilometers or 238,856 miles from Earth. Divided by 31.622 that makes about 12,156.157 kilometers or 7,553.4754 miles, which would be really close to Earth.

I believe that a moon that orbits an Earth-like planet that closely would actually be slowing spiraling in toward the planet and would break up into rubble or collide with the planet within a few million more years.

Or you could make the moon both larger than the Moon, and also much closer to the planet than the Moon, so that the two factors combined give the moon an angular diameter 31.622 times that of the Moon to make it 1,000 times brighter than the Moon, while still being far enough away to not be spiraling in to its doom.

You could also make the surface material of the moon in your story more reflective than the surface of the Moon. The Moon has a rather dark, dull surface and only reflects a small percentage of the light that hits it. So your fictional moon could be more reflective than the Moon. Maybe your Earth-like planet has several large and close moons orbiting it in different orbits. And maybe your planet could have a ring of large moons around it at a fairly close distance.

Recent calculations indicate that it is possible for many equally spaced objects of equal mass to share the same orbit, so a few dozen large moons sharing the same orbit around a planet would not be physically impossible, though such an arrangement would be extremely improbable.

The Ultimate Engineered Solar System

So you could make an astronomical arrangement where your planet has moonlight a few thousand times as bright as moonlight on Earth.

But sunlight on Earth might still be tens or hundreds of times as bright as the moonlight on your planet, even if you make the moonlight on your planet a few thousand times as bright as moonlight on Earth.

In my opinion, making your "planet" actually a giant, Earth-sized moon of a giant planet may be the way to get the other astronomical body as large as possible in the sky of your world, and thus reflect as much light as possible on to that world.

And if you decide that is the case you should look up other questions and answers in this site about stories set on the moons of giant planets.

But of course the astronomical set up necessary for your story depends on exactly what you want more moonlight for in your story, and thus how much brighter the moonlight needs to be.

Answer 2

The sun is about 400000 times brighter than the full moon. That's quite a lot.

The moon, despite looking quite white, is actually a surprisingly dingy grey with an average albedo of about 0.12 (equivalent to damp soil). If you painted the moon a brilliant glossy white and raised its albedo to 1, it would be a little over 8 times brighter, which still leaves it 1/48000 times as bright as the sun.

(Incidentally, an ideal material for the surface of your super white moon would be ice, which is a little implausible close in to the parent star but not entirely impossible)

A moon much closer to the planet? Different chemical composition? Different moon cycle? Multiple moons??

None of the above. The apparent brightness of our super-high-albedo moon is related to its size and its distance from the sun. Even if you had ten moons, and each one had four times the apparent size of the moon (so about twice the apparent angular diameter), you'd still be 1/1200th of the sun's brightness, and that's such an astonishingly unlikely and gravitationally unstable arrangement that it isn't really worth thinking about.

You'd either need to move much close to the parent star, or to substantially increase the brightness of the parent star. In either case, the apparent brightness of the sun during the daytime would be correspondingly higher, and that means that your planet is going to be roasted and won't be likely to support life (or even an atmosphere, to be honest).

If you want something almost as bright as the sun without incinerating the world, you should see about building giant orbital mirrors and have them oriented such that they reflect sunlight onto earth.

What would be the effect of these on the projected moonlight? Would its spectrum shift towards blue or towards red?

Moonlight actually has a slightly warmer colour temperature than sunlight (about 4100K vs about 50-5800K for sunlight). A perfectly colour balanced brilliant white moon should therefore have a slightly cooler colour temperature than the moon (e.g.: more blue), which isn't what I would have expected at all.

Answer 3

Salt deposits are more stable, and they are white when powdered.

The bright spots on Ceres are hydrated magnesium salts and brine deposits. I don't know if hydrated salts can retain their water content on our moon, but salts like sea water salt are white by nature and have an albedo much higher than that of Regolith.

https://en.m.wikipedia.org/wiki/Bright_spots_on_Ceres

Your setup may involve a shower of meteors made-up of frozen brine hitting the moon and changing its color.

Answer 4

Moonlight has the (lack of) intensity it does because the Moon's surface (bright as the full moon looks at night, against the black of space) is quite dark -- about like worn asphalt pavement, gravel with tar between the pebbles.

To make it brighter, it would need to be covered with brighter material. One fine candidate is ice; a fresh ice surface, if it's finely divided, like snow, could reflect about 5 times as much light as the regolith we see. Unfortunately, ice doesn't stay white like snow over geological time when exposed to space; it darkens and turns red. The undisturbed ice surfaces of Kuiper Belt objects and first-time comets can be darker than the rock dust that covers most of the Moon's face, and redder than a building brick, so if it's to stay bright, it'll need some mechanism to replace the surface every few thousand years or so.

Most ices will reflect nearly white -- that is, they won't change the color of the light that strikes them much. Spectrography can tell what the ice is made of, to some extent, by what light it absorbs, but the color of the reflected light will read as white to the eye.

Answer 5

A Moon could supply a lot of light given the correct conditions. A much bigger moon in a much closer orbit would work, although such a situation might best be described as a double planet rather than a planet and a moon. Such a moon might well fill a large portion of the sky and even when only half lit by the sun would still be very bright especially if its surface where composed of highly reflective materials.

I should point out that there are a number of issues with this type of arrangement. Although the roche limit for similarly sized bodies would allow a close approach such an arrangement is unlikely to be very stable and would produce a lot of strange gravitational anomalies.

Multiple Moons might sound attractive but in close proximity to a planet multiple large moons would probably be highly unstable leading to a collision.

Answer 6

Bright enough to see by can mean many things. And the human eye can adapt to a very wide range of brightness values.

For example, the brightest noon sun can be 120,000 lux, but a very cloudy overcast day can be as low as 200 lux. Most people barely notice the difference because our pupils expand and contract to keep the perceived brightness roughly the same.

A full moon is about .25 lux, but when fully adapted it can be possible to read by moonlight. Interior light generally is between 100 and 250 lux (the latter for a bright office, for example). So if you can deal with the amount of light in a classroom or office, you only need to improve your moonlight by a factor of 400-1000.

If your people are aliens, you could also leave the moonlight about the same and simply give them slightly bigger eyes, or their eyes could have a tapetum lucidum structure at the back (that reflective surface that makes dogs and cats eyes glow when you shine a light in them), which greatly enhances their low light performance.

Answer 7

I did a bunch of math related to this concept on the Gearbox Borderlands (video game) forums. The original post is here (go to the bottom of the linked post and expand the "details" section by clicking the arrow).

Summary: An Earth-like setup (1) with a moon close enough to be $\frac{1}{6}$ the Sun's brightness would have civilization-destroying tidal forces unless it's tidally locked to the planet. We could handwave the density by calling it a giant comet (3) which makes the tides tolerable, or making it out of fictional, synthetic materials (4) so it has practically no tides.

Alternately, we could get a much more plausible setup (4) at 71 times our moon's brightness that still has massive tidal forces, but would only render the outer portions of continents unlivable. By reducing the water mass, you could get more livable area. This is nowhere near as bright as the Sun, but is still bright enough to see by. Using the handwavium from (3), you could reduce the tides to Earth-like levels.

Here are the setups I came up with:

First Setup

Moon is 14,430 miles above the planet, subtends 23.3°, and has a solid angle of 0.512 steradians (sr), which is 8000 times higher than our moon. Moon's surface is like Saturn’s moon Enceladus, with a 99% albedo, or 8.25 times brighter than Earth’s moon. This brings the total brightness to 66,000 times brighter than our moon. The Sun is about 400,000 times brighter than our moon, so this moon is about $\frac{1}{6}$ the Sun's brightness, or about 16000 lux.

Problems:

1. Tidal forces are 4538 times higher, meaning the tides would literally be miles high, which would destroy everything on the surface unless the planet is tidally locked to its moon. This means the orbital period of the moon is exactly equal to the rotational period of the planet, and also means the moon would never appear to move in the sky unless you moved to a different part of the planet. So some parts of the planet always see their moon, while others never see it.

2. You're quite close to the Roche limit of 6000 miles, which may cause other major effects I'm not seeing.

3. Your day length is about 9 hours, which may or may not be acceptable.

Second Setup

Moon is 41,178 miles above the planet, subtends 0.75°, has a solid angle of 0.00054 sr, which is 8.4 times that of our moon. Again, it has 99% albedo, for an 8.5 multiplier. Total brightness is 71 times our moon, or 1.8% of 1% of the Sun (0.00018).

Additionally, the moon is 41% larger, and twice as massive to keep its surface gravity the same. The planet is a quarter the mass of Earth and half the size.

Problems:

1. Tidal forces are still 391 times higher. We can cut this down to about 200 by leaving the moon's mass alone, but its still untenable for normal life in non-locked orbits, with waves over 1000 feet tall.

2. That said, you could probably have reasonable amounts of life nearer the center of continents (away from the reach of thousand-foot tidal waves), though your world would be very active with volcanoes and such. I think that once the waves start covering the continents, the increased surface area means the water is being spread into a shorter wave, so it would likely be "only" several hundred feet of elevation that gets submerged twice a day.

3. It's not really "near daylight" anymore, but it's certainly enough to see by.

4. The day length is 90 hours. This was specifically done for Pandora, because the first Borderlands game claims the day there is 90 hours. You could play with the numbers to get something closer to your world's parameters, but you're not going to get much better tides without losing a lot of light.

Third Setup

If we stop restricting ourselves to the Borderlands setup, we can try weird things.

Copy the first setup, but make the moon far less dense. Say, the density of a comet (0.6 $\frac{g}{cm^3}$). It's still about $\frac{1}{6}$ our moon's mass, and probably not physically realizable anyways (that much mass would condense into a much denser ball with gravity). But it brings the tides down to something like 100-200 feet, which would allow life to reasonably exist.

And if you handwave the size, you have the advantage that it's made of lots of ice, which is highly reflective, making our 99% albedo more believable. Of course, then you have to handwave the ice seeing as our Moon has surface temperatures well over boiling water. So I'm not sure this is a great solution.

Fourth Setup

Let's use some spacewavium. A synthetic moon made of an extremely lightweight shell might work. It would have negligible mass, and therefore no discernable tides, but would still have the broad surface area needed to reflect lots of sunlight down onto the planet. There are no known methods of constructing such a shell, but it's in the realm of "probably possible".

Notes on Lux

This Wikipedia article lists lux values for different conditions.

Our moonlight is 16,000 lux for the first, third, and fourth setups, and 17.5 lux for the second.

In comparison, sunlight is about 100,000 lux on a typical day, 20,000 lux in shade that's just illuminated by diffuse light from the sky, 1500 lux on an overcast day, and 40 lux when overcast at sunrise or sunset.

Nighttime house lighting is about 30 lux, 50-100 lux is acceptable for safely moving around a strange area without tripping over anything, and 200-500 lux is comfortable for reading.

I can navigate in full moonlight (0.1 to 0.25 lux) across unknown terrain without too much difficulty (fences and potholes can be troublesome though), so those "safe movement" thresholds are way above "required to see anything" limits.

The first setup would be very bright, much brighter than an overcast day and almost as bright as standing in the shade on a clear day.

The second setup would be nowhere near daylight, but still easily bright enough to navigate on foot and so forth. I don't think you could safely drive at highway speeds though.

Tidal Issues

The tide difference in Cape Disappointment, Washington is about 6 feet on average, and seems pretty typical for Earth.

The difference at the Bermuda Biological Station is only 2.5 feet, but the top of the island is only 249 feet above sea level. This height is pretty typical for areas around the Gulf of Mexico as well.

Places like Anchorage, Alaska get really high tides averaging 26 feet difference. Canada supposedly has places with 40 feet average difference.

_{Taken from Pinterest, but the map says it's Rand McNally, so I'm calling "fair use" given that maps at these resolutions are available straight off their online store.}

If we presume 5-foot average tides, then our first setup would have tides about 4.3 miles tall. That's most of Earth.

The second setup would have 1955 foot tides, which is everything in the green and green-yellow zones, or basically the eastern half of the United States.

Modifying the second setup to have a lower lunar mass brings us down to 977 feet, or about half of the green-yellow zone and all the green zone.

As I said above, I don't think you'd actually get these numbers though, since the water's height is proportional to how much area it covers. Also, if your planet has much less water covering the surface, the relative land area destroyed by tides goes way down.

Answer 8

This is a brief answer meant to complement others.
Some years back I designed portable solar powered lights.
I did substantial testing of what could be achieved with various light levels.
There was much available information of what was "needed" for eg colour vision, fine work such as embroidery, general hobbies, day to day activities, finding your way around, not being in quite absolute pitch blackness, ... .
In many cases the published "required" levels were well above what were adequate for the tasks.

Cutting to the chase, for now.
More later maybe.

Light levels are measured in lux = lumens per square meter.
Don't let it worry you.

Lux

0.01 - 0.05 - Stumble around in the not quite dark
0.1 - 02.5 - . Bright moonlight
5 ................ Read normal with difficulty
10 -20 .........Read "OK" but poor colour rendition
25-50 ......... Colours reasonably discernible but brighter would be nicer 50 -
100+ .......... Colours Good
300 ............ LCD screen surface full white
100,000 ..... Noonday sunlight

So, at 5 lux = 50 x good moonlight with longterm familiarity, and/or biological adaptation, you may be able to do OK.

Chart from here - not otherwsie worth looking at.

Answer 9

Don't overlook the composition of the planet's atmosphere. One with lots of moisture droplets floating around would diffuse the moonlight and increase the apparent brightness at the surface.

Answer 10

In short, no. And for the TL;DR version, the long answer is also no.

M. A. Golding has given a lot of very good details about why in his answer. What it boils down to is that the moon receives about the same amount of light per square foot as the Earth does (at the upper atmosphere at least) and only a small proportion of that light is reflected from the surface and only a fraction of what is reflected ends up on Earth. It doesn't matter how much you raise the albedo the resultant midnight illumination from a full moon is still going to be drastically reduced from any direct light from the sun. See M. A. Golding's answer for some math around this.

Since reflection won't cut the mustard the only option is to have the 'moon' be a light source, and a fairly damned bright one at that.

As far as I can tell there's nothing in current physics, astronomy or cosmology that fits the bill. It can't be a small star, it would have to be far more massive than any conventional planet to even get started. Fission is either too slow or way too fast. If the moon were white-hot it would work for a while but the surface would cool rapidly due to the escaping radiant energy and you'd still have to figure out what caused the temperature to be that high in the first place.

The only other options I can see are artificial, and those are still big problems. A flat perfect mirror the same diameter as your planet would do it. A smaller convex perfect mirror would look like a smaller sun and deliver a smaller amount. You could have an array of mirrors directing extra light towards the moon, both heating it and raising the surface temperature. Don't ask me how you'd keep them aligned. The torque would probably shred your mirror. And that's without trying to figure an orbit that keeps it out of the planet's shadow.

The total energy we receive from the Sun is around 175 petawatts at the 'surface' of the atmosphere. If you cut that down to just visible light - about 43% - you'll need 75 petawatts of visible light to match daylight. You could probably get away with maybe a quarter of that to maintain reasonable light levels on a cloudless night, so let's settle on about 20 petawatts of visible light to satisfy your needs.

That's on the order of 1000 times the total global energy production right now.

So natural sources are, I'm sorry to say, almost completely out of the question and artificial sources are going to be hugely problematic. You could throw some junk science at the problem or just go full fantasy. White holes? Crazy minerals that tap the zero point energy field or convert dark energy? Spontaneous antimatter conversion? Dyson Swarm transmitting power to light the dark side for the ignorant savages on the surface?

Answer 11

It needs mentioning that 'moonlight' isn't the same as 'nightlight'. Earth's moon reflects light during the day too but isn't generally significant compared to sunlight. A bright moon would add to 'daylight' as well as 'nightlight'.

Generally 'daylight' will be 'sunlight' + 'moonlight' + 'starlight' etc, and 'nightlight' will be 'daylight' - 'sunlight'.

So if you want a negligible sunlight so that night and day are prerry close, you want a dim sun and a LOT of stars.

Answer 12

Consider a world that is face locked to its "sun" - as Mercury was once thought to be and as the Moon is to earth.

Make the 'sun' side inhospitably bright and hot and unlivable.
Provide some form of thermal circulation system that allows the outward side not to freeze.
eg Hand wavium
Hot rivers
Thermal cycling of water or magma or ...
Maybe a hot rain's gonna fall"? :-)

Then - piece de resistance - A series of 'moons' that orbit sequentially )or maybe chaotically relatively, or ..., and which provide light from the other-side star.

Extra points, Moons are, for whatever reason, in elliptical orbits with fast transition of bright side and high elliptical loops over dark side. Add (implausible) titanium dioxide surfaces if you want really high albedo.

Maybe the long passed on fathers set it up ? :-) .

Answer 13

An alternative way to think about this perhaps is the terrain of the Earth. If you're in a valley surrounded by forest or bush (dark green, no reflections) then a full moon seems very dim. If you're on the top of a rolling hill surrounded by a snow-covered landscape then a full moon is almost dazzling. Apparent brightness is about more than just the source of the light.