On earth there can be hours where the moon is up during the same time the sun is in the sky (daytime moon) due to the combined rotation of earth and orbit of the moon. I would like to have an earth-like planet which never has the moon and the sun visible in the sky at the same time from any point of the planets surface. Is this possible without too much interference with the earth-like state of the planet?

Ideally the reason for the lack of simultaneous visibility does not effect the stabilising function of the moon for the planet's hability as well as has no significant effects on the planets seasons and day and night cycle. In general consider the planet, the moon and the whole solar system to be the same than ours.

EDIT for clarification: Ideally the planet has a

  • similar axial tilt than earth (necessary for similar seasons)
  • similar rotation period than earth (no tidal lock, necessary for similar day length and day-night cycle)
  • similar orbit than earth (necessary for similar seasons)
  • similar gravity than earth

If deviations from the former are necessary than I prefer

  • stronger variations in seasons (stronger axial tilt) to less sesaons (axial tilt)
  • longer rotation period (longer days) to quicker rotation period (shorter days)
  • different orbit is ok (but please state the effect that might have on the planet)

EDIT 2: For the sake of this question "visible" means visible to the naked human eye and/or technology available in the late medieval period of our world (ca. 15th century).

A solution using atmospheric conditions are perfectly fine as long as this does not infringe on the habitability of the planet and these conditions are the same from any point of the planet.

EDIT 3: There are no restrictions on the perceived colour of the moon in the sky. It can be any colour, as long as it stays visible in the night and still give some light during the night.

You can postulate any changes to our system that have no or marginal effects like described above and achieve the desired result. If this effect is not achievable with the restrictions declared above, please state why.

This question asks for hard science. All answers to this question should be backed up by equations, empirical evidence, scientific papers, other citations, etc. Answers that do not satisfy this requirement might be removed. See the tag description for more information.

  • Some of your answers may require things like a tidally-locked planet (shows one face to the sun at all times), a planet with no axial tilt (Earth has a tilt), etc. So, when you say "Earth-like," do you only mean habitable with lush life, or do you mean same orbit, same tilt, same rotation, etc.? Please edit your question with the clarifications. – JBH Apr 10 at 0:11
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    Not scientifically. The moon has to be always on the other side of the planet to be not visible. On the other hand, moon has to be on some kind of stable orbit not to fall down or fly away. There is Lagrangian point L2 on the other side which provides some stability, but not enough to keep the moon there for astronomical time periods. – Alexander Apr 10 at 0:16
  • Would it be permissible if the moon is simply invisible during the day -- that is, due to some atmospheric condition, it simply cannot be seen by the naked eye while the sun is up? – Emory Bell Apr 10 at 0:30
  • @Emory Bell That would be perfectly fine as long as this condition allows the moon to be seen during the night and this condition exists from any point of the planet. Furthermore, the "not visible" means not visible to a human eye and/or technology comparable to our late medieval perios (15th century). I will edit this into the question. – DerGreif Apr 10 at 0:33
  • It might be possible if "the planet" is a moon of a gas giant and people think the gas giant is "the moon". The mass-radius ratios could be close enough for the opposition to last for a very long time. Although the solar eclipses would also last for a very long time and freeze the planet... Or is it a moon? – Ville Niemi Apr 10 at 10:54
up vote 13 down vote accepted

Decrease the brightness of your moon, and make it invisible during the day.

By making the surface of your moon darker, you reduce its brightness due to light reflection (albedo). Cover it with black lava fields or dark stones. If it's dark enough, your moon should be only visible during the night or twilight.

You can give your moon the same apparent magnitude of objects that appears after sunset. On Earth, the twilight sky allows the visibility of objects with magnitudes from -2.5 to +4.5.

-2.5 is also the apparent magnitude of the new moon, invisible during the day. Even if we can't see it, the new moon is not totally dark as it reflects the earthlight. But it's hidden by our atmosphere brightness. An object of similar magnitude would be visible during the night.

According to Handbook of practical Astronomy (p. 402, 403), during a total lunar eclipse, the Moon's magnitude is -2 when the sky is clear, and totality is rather light and still visible. Your full moon should look similar in brightness, except that it won't have the red/brownish color of our Moon (due to the eclipse), it should be dark grey instead.

So, we can imagine having a black moon with a very low albedo, that reflects a very small percentage of sunlight, and that should be invisible from sunrise to sunset. Of course during the night it won't be as bright as our own moon, and will look more "greyish" than white... But if it's as big as our moon it should be easy to spot it anyway.

With such a moon, your nights should be quite dark, even during the full moon. You may not see your own shadow. The behaviour of nocturn animals/bugs should also be different, because the moonlight won't be as bright as what we know on Earth.

  • It would still be visible when both the Sun and the Moon are just over the horizon, but on different directions. – Renan Apr 10 at 12:18
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    @Renan When a moon is just over the horizon, its visibility decreases as the path of light through the atmosphere is longer and absorbs a part of the visible light. So if that moon is very dark and doesn't reflect much light, you shouldn't see it with naked eye until the sun sets. – Ghajini Apr 10 at 12:52
  • @Ghajini Your answer seems to be for now the only option I have. Could you elaborate on how this dark moon would affect the light situation in the night? – DerGreif Apr 10 at 14:00
  • @DerGreif I added a few info, I hope it will be helpfull. – Ghajini Apr 10 at 17:47
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    To add to @Pere's comment, the Moon's albedo is 0.136, which is about the same as dry asphalt. – Keith Morrison Oct 29 at 3:09

There's actually a way to make this happen, but it's done by following the letter of the request rather than the spirit: Have a large, low-density moon that sits in the planet's L1 point (the Lagrangian point between the planet and the sun.) If the moon is big enough and steady enough, the sun is always in eclipse and since the sun can be seen nowhere on the planet, it is also true that nowhere on the planet can both the sun and the moon be seen.

(That's a bit of rules-lawyering, and it wouldn't work for long anyway since the L1 point isn't stable (it's metastable). Also, I'm not sure what you'd make the moon of to be big enough and light enough.)

Taking what I think is the spirit of the question -- with the sun and the moon each visible for some part of the planet -- you come closest with the moon sitting in the planet's L2 point which will keep it opposite the sun in the sky. Unfortunately, that's also only metastable. Also, since the planet has an atmosphere, there will be a thin band around the world halfway between the subsolar point and the sublunar point where atmospheric bending probably makes and edge of the sun and an edge of the moon both visible.

This question asks for hard science. All answers to this question should be backed up by equations, empirical evidence, scientific papers, other citations, etc. Answers that do not satisfy this requirement might be removed. See the tag description for more information.

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    Yes, you are right with the spirit of the question. I have no problem with metastability, as long as this metastability exists during the period in question. But I presume that being metastable means I will have difficulty of this situation forming at all or at the right point in time for the planet to have life at all. – DerGreif Apr 10 at 0:41
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    From some articles touching in passing on metastability at the L1 and L2 points (but not actually discussing the problem in detail) it seems the typical stability time is a relatively small number of orbital periods: 5, 10, 20, but rarely as much as 100. (It's a chaotic system, so you can't do more than give a probabilistic answer.) So you've got maybe 20 years or maybe fifty years (but possibly only 5 years) from when the sun-planet-moon system went into this resonance to when the moon wanders out of the L2 point and probably goes into solar orbit with some funny resonance with the planet. – Mark Olson Apr 10 at 0:47
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    If you put something the size of a moon in a Lagrange point, it's no longer a Lagrange point. The Lagrange points are a solution to the restricted three-body-problem that requires the third body in question to be of negligible mass. A moon, by any definition I'm aware of, isn't a negligible mass. Size is also an issue. As L1 and L2 are on the edges of the planet's Hill sphere and its interaction with the star's Hill sphere, the gradients are not symmetrical, so things much larger than a point will not be stable for any significant time. – Samuel Apr 10 at 1:14
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    I don't think anyone believes that there is a possible way for the desired behavior to occur naturally. Given that, the problem is to see if there's a not-egregiously-unrealistic solution. The L1/L2 business can be fairly described as the minimally-unphysical solution to the problem. YMMV, of course. – Mark Olson Apr 10 at 1:22
  • @Samuel While the position of the L2 point will vary based on the size of the object there it should still exist. Being at the edge of the Hill sphere is a much bigger problem, though. – Loren Pechtel Apr 10 at 3:14

Is a large moon necessary to stabilize the axial tilt of an Earth-like planet and keep it habitable?

This article suggests that the lack of a large moon would not make a planet uninhabitable:

https://www.npr.org/2011/11/18/142512088/is-a-moon-necessary-for-a-planet-to-support-life1

This article suggests that the super high tides when the Moon was much closer to Earth may have caused life to evolve in vast tidal flats. Thus a large close moon might have been necessary for life to appear.

https://www.scientificamerican.com/article/moon-life-tides/2

This article also suggests that the early Moon started plate tectonics and plate tectonics may be necessary for life on Earth.

https://phys.org/news/2015-11-moon-life.html3

So a large close moon early in a planet's life may or may not have been necessary for the planet to be habitable and for life to form. And a large moon much farther out billions of years later may or may not be necessary for the planet to have stable enough climate for life.

So turn your attention to the planet's atmosphere for a moment.

Earth's atmosphere scatters sunlight making it appear to come from every direction and making the sky appear blue. An object has to be brighter than the sky's background brightness to be visible in broad daylight. And during twilight it has to be brighter than the twilight sky's background to be visible.

In the apparent magnitude scale lower numbers signify greater apparent magnitude. It is said that the objects visible when the sun is less than 10 degrees above the horizon have to have an apparent magnitude of -2.5, while the faintest objects visible while the Sun is high in the sky have to have an apparent magnitude of -4.0. The apparent magnitude of the Moon varies from -12.90 when full down to -2.50 when new.

https://en.wikipedia.org/wiki/Apparent_magnitude4

Since the new moon is seen when very close to the sun in the sky, it should never been visible unless the Sun is about to set or has already set, and so should hardly ever be visible when the Sun is. The much brighter full moon should always be opposite in direction to the Sun and thus only been seen at night.

So if the sky scatters enough light to become a few magnitudes brighter, its scattered light should be enough to drown out the light of the moon and make it invisible. Right?

I remember late one afternoon I was near Convention Hall, on the boardwalk of Cape May, New Jersey, and saw what looked like a full moon, as far as I could tell with the naked eye, rising low over the Atlantic Ocean.

And then I thought that a full looking Moon should be nearly 180 degrees from the Sun. But I was seeing a full looking Moon, close enough to full that I couldn't see the difference, not in the night sky, not even in twilight, but in the afternoon in broad daylight with blue sky.

So I turned around and looked back, and there was a red setting Sun low in the sky, opposite to the Moon. Because the Earth is a sphere, and not flat, the ground falls away in the distance, and one can sometimes see objects more than 90 degrees lower than the zenith of the sky. And atmosphere refraction makes objects near the horizon appear to be several degrees higher. Thus I could see the Sun and an apparently full Moon in the sky together.

Bu there's more!

One time I was in a low place, surrounded by hills, buildings, and trees, with much less than a full hemisphere of the sky visible, about 9 or 10 in the morning. And I saw the moon, and it looked round and full as far as I could tell. And of course the Sun was also visible. And it looked like the Sun and the Moon were much less than 180 degrees apart, despite the Moon looking full as far as I could tell. And if the Moon was close enough to full to look full to me, its apparent magnitude was probably close to -12.50.

So the problem is to increase the light scattering of the atmosphere of your planet enough increase the brightness of the sky so that the apparent magnitude of the full moon of your planet is reduced by about 8 magnitudes when the Sun is high and reduced by about 10 magnitudes when the Sun is low in the sky.

So you may need an atmospheric expert to calculate how much water vapor, dust, various gases, etc. you may need to add to your planet's atmosphere to make your planet's full moon invisible in the day sky, and whether that atmosphere will still be breathable.

And you can make your planet's moon a bit dimmer, which would help a bit. Since we don't know for certain how important a large moon is for planetary habitability you might decide to make the planet's moon much smaller than Earth's Moon, or might be afraid to make it too much smaller than Earth's Moon.

But you should be able to reduce the mass of the planet's moon to be considerably less than that of Earth's moon. Then increase the average density of the planet's moon so that the same mass can be within a smaller volume. Those two effects should make the planet's moon significantly smaller than Earth's Moon and thus it would look much smaller and be dimmer at the same distance as Earth's Moon.

And you can lower the planet's moon's albedo, the amount of light it reflects, to make it dimmer. Unfortunately, Earth's Moon already has a low average albedo of 0.137. Dark solar system objects like comets and asteroids have albedos down to 0.05 and 0.05. This can significantly reduce the apparent brightness of your planet's moon.

When the Moon is new and near the Sun in the sky, the Moon's visible side is almost totally in shadow. But sunlight reflected from the Earth, and thus becoming Earth light, does light up the Moon a little and make it a little brighter when it is new. The albedo of the Earth is about 0.30 to 0.35.

Decreasing the albedo of your planet would decrease the amount of light the planet reflects back onto it's moon when the moon is new, and thus will make the moon somewhat darker when new. It would also mean the planet could be slightly farther from its sun and have the same temperature as the Earth. Thus there would be less sunlight for the planet's moon to reflect back to the planet, making the moon dimmer.

I am not sure how well decreasing the planet's albedo would fit in with making the atmosphere scatter more like and obscure the light of the moon during daylight. So changes in the atmosphere and in the planet's moon might make the planet's moon visible in the night sky and invisible in the day sky and even at sunset.

So making the planet's moon less bright and making the planet's atmosphere obscure the moon more might be enough to make the planet's moon visible during the night and invisible during the day.

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    This sounds exactly like the answer I was looking for! I will give this question still a little bit time until finally decidig which answer to accept. – DerGreif Apr 11 at 7:44

Previous answers touch on many of the reasons why it is difficult to accomplish this particular orbital arrangement. I think I can come close but not quite to achieving the arrangement without anything beyond orbital mechanics, so I'll leave this here perhaps as inspiration for further work.

First Try - Tidal Locking Orbit to the Planet

My first guess was to tidal lock the Earth and Moon to each other, so that the Moon's orbital period around the Earth was equal to the Earth's rotational orbit. This achieves your desired arrangement, but only for half of the planet. Half of the planet never sees the Moon (so they can never see the Sun and Moon together, but unfortunately the other half always sees the Moon.

Second Try - Slow Down the Moon's Orbital Period

Okay, so that didn't work, but there might be another way. What if we slowed down the Moon's orbit more and more (of course at the same time pushing it further from the Earth)? It takes about 1 month for the Moon to go around the Earth, but what if we made this longer and longer until it took approximately a year for the Moon to go around the Earth?

First let's look at what happens to the background stars as the Earth orbits the Sun. On June 20th (summer solstice) at midnight on the Equator we see a specific set of stars corresponding half of the sky. The stars we can see at midnight change ever so slowly due to the Earth's orbit around the Sun until, on December 21st (winter solstice) at midnight, we can no longer see any of those stars. Instead we see the half of the night sky we couldn't see in June. These visible night sky continues to change until, on June 20th of the next year, when we're back to the same night sky we saw the previous June 20th. This is how we define a complete orbit around the Sun.

So now let's consider the a frame of reference with Earth at the center; we are ignoring the sun for now and focusing on just the Earth. The Earth is rotating, but we can also ignore this for now, since it is not important. The winter solstice night sky is to the left and the summer solstice night sky is to the right. From here on out we'll call the summer solstice night sky stars, Distant Stars. Place the moon in the center of the winter solstice sky, to the left of the Earth and let it orbit. We have the configuration, Moon, Earth, Distant Stars. In 6 months the moon has moved the center of the summer solstice sky, i.e. the configuration is now Earth, Moon, Distant Stars. In another 6 months the moon has moved back to the center of the winter solstice sky, back to Moon, Earth, Distant Stars. But the Earth is going around the Sun! What does the entire Earth, Sun, Moon system look like?

We are now going to put our two scenarios together. To aid in visualization I've included this following image.

Sidereal vs Solar Time

In this image, the winter solstice sky is again to the left and the summer solstice sky (Distant Stars) is to the right. We know how the Moon, Earth, and Distant Stars change over a year, so we can use this image to help add the Sun in. On Day 1 (in the picture) the order is Moon, Earth, Sun, summer solstice midnight stars. 6 months later, we know Moon needs to be between Earth and the Distant Stars, but the Earth is also now on the other side of the Sun, and we have the order Sun, Earth, Moon, Distant Stars. In both cases the Moon is opposite the Sun! And if we think about it a bit longer we realize that the Moon is always opposite the Sun!

To see how the Moon moved in a single day, we can return to the image. If you continue the line from the Sun point to the Earth, beyond the Earth you can place the Moon there. You can see the Moon is still rotating around the Earth, with respect to the Distant Stars, but is doing so at such a slow rate that the amount of rotates around the Earth exactly matches the amount the Earth has rotated around the Sun!

Alas, Houston, We Have A Problem

In theory you can obtain your desired arrangement by slowing the Moon's orbit down to be 1 year, as I have shown. However there are a few problems. Due to Kepler's Third Law, the Moon is now approximately 5.6 times further away than before. It is probable the Earth cannot hold onto the Moon that far away, given the gravitational effects of the other planets, or the Sun. Even if the Earth can keep a grip on the Moon, at those distances the effect of the Earth's gravity compared to the Sun's gravity may be close enough in magnitude, that you might need to treat this as a full 3-body problem! This means we cannot use Kepler's Laws or simple assumptions like treating the Moon and Earth as a single 2-body problem, and then the Sun and Moon+Earth as a separate 2-body problem.

Apologies if this is a little rushed, if there are questions I can return to elaborate and maybe work on some more of the math.

If the planet is massive enough and its orbital period around its star short enough then it may be possible. If the moon is orbiting around the planet in the same prograde as the planet around the sun, and the moon's orbital period lasted exactly a year, then it would be plausible for the bodies to be permanently eclipsed (theoretically), however, practically, it is not actually stable because there is nothing keeping the moon in check. But you never know; our moon has almost the exact same apparent size as the Sun, allowing for a very cool solar eclipse, and that's completely by chance. While perhaps improbable, it is certainly possible that the Moon could stay in that state for quite some time under these circumstances.

EDIT: just saw your edits mentioning the planet being similar in gravity and orbital period to Earth, so this won't work for your purposes.

This question asks for hard science. All answers to this question should be backed up by equations, empirical evidence, scientific papers, other citations, etc. Answers that do not satisfy this requirement might be removed. See the tag description for more information.

  • Unfortunately astronomers have calculated that that a moon should orbits its planet at least 9 times during a year of the planet in order for the moon's orbit to be stable. Thus the moon should be on the same side of the planet as the sun at least 9 times per year. – M. A. Golding Apr 11 at 3:34

I think the answer is straight-up no. For the moon to be constantly on the opposite side of the Earth from the Sun, it could not be in orbit around the Earth. I would have to be

  1. in orbit around the Sun,
  2. but further out with a greater orbital radius (to be on the other side of the Earth), and
  3. it would have to have the same angular velocity going around the Sun to stay on the opposite side of the Earth.

It all falls apart on the third item. You can use Kepler's Third Law to calculate the orbital periods, and it is impossible to have the same period and different radii.

You can put the Moon in a Molniya-like orbit. A Molniya orbit is a very eccentric Earth orbit which turns with the sun. An small Moon can have an eccentric orbit that keeps it in the day side of the planet at apogee (therefore invisible due to distance and presence of Sun) to fly by the night side at perigee.

However there are a couple of caveats:

  • To synchronise that orbit with the Sun, the real Moon is also needed. Therefore, that satellite won't be "the Moon" but just a lesser satellite.
  • It may still be possible to see Sun and that satellite at sunrise or at sunset.

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