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Imagine a habitable, water covered, small earth-like orb in space. Let's call it P. From its surface, you see 2 large glowing masses, different sizes, in the sky, let's call them Bs. They are not stars, they are way way too big.

Could these Bs be giant planets, moon, or otherwise satellites that shine down in the night on their habitable neighbor P? Could P be its own planet orbiting the same star as two visible planets, or would an individual planet, by necessity, be too far away to see the Bs as anything but stars?

It would look something like huge moons in the sky, if what I'm asking is possible.

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  • $\begingroup$ Without some clarification to the question, what do you expect other than the answer that you've been given. Bad manners are not appreciated here, clear communication is. What are you asking? $\endgroup$ – We are Monica. Jul 6 at 23:03
  • $\begingroup$ I'm not using bad manners, I'm stating that I didn't ask the question that guy answered and then I DID clarify that it wasn't about stars or planets that look like stars. $\endgroup$ – Bett Struble Jul 7 at 0:36
  • $\begingroup$ You've perhaps looked at the sandbox as I suggested, what have you decided? $\endgroup$ – We are Monica. Jul 7 at 0:38
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    $\begingroup$ Could you define the parameters of the vision that this "planet" possesses. What is the spectrum of light that your "planet" can see, what is the visual acuity, the minimum angle of subtention that your "planet" can differentiate two incident photons on it's eye. Does your "Planet" have stereoscopic vision? Please clarify what it is that you are asking. $\endgroup$ – We are Monica. Jul 7 at 0:51
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    $\begingroup$ I think its obvious what he's asking, stop being pedantic and irritating. $\endgroup$ – Innovine Jul 7 at 8:14
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You are on a habitable moon, looking at your own planet.

earth from the moon

https://moon.nasa.gov/resources/187/apollo-11-mission-image-view-of-moon-limb-with-earth-on-the-horizon/

A fine image bought with American tax dollars!

There is no reason a moon cannot be planet sized and habitable. Titan is a moon of Saturn and is bigger than the planet Mercury. The other thing in the sky might be another moon of the same big planet. There are some spectacular images of Jupiter on this site that I was tempted to use: http://www.johnstonsarchive.net/spaceart/art-r.html which are art, not photos and all copyright the artist. But take a look - they show Jupiter and other sister moons. Jupiter is huge!

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  • $\begingroup$ Yes yes! So because I can't really wrap my noodle around phases... does a moon see it's planet in phases? Does titan see the other moons of Saturn? This is helpful so far, thank you $\endgroup$ – Bett Struble Jul 7 at 1:42
  • $\begingroup$ I love that second link!!!! Perfection $\endgroup$ – Bett Struble Jul 7 at 1:43
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    $\begingroup$ Any body in the solar system will have phases. Here is a fine shot of earth and the moon as seen from Mars - both earth and moon are in the same phase from the Martian perspective, which makes sense. baylor.edu/mediacommunications/…. From your habitable moon, the parent planet and sister moons should all be in the same phase. $\endgroup$ – Willk Jul 7 at 2:20
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    $\begingroup$ Good stuff here: en.wikipedia.org/wiki/Extraterrestrial_skies. Apparently Titan is too hazy to see much of anything in the sky. The Galilean moons of Jupiter could definitely see each other but probably not as moon sized objects. $\endgroup$ – Willk Jul 7 at 2:29
  • $\begingroup$ That's true, I forget how hazy titan is. Thanks! This helps! $\endgroup$ – Bett Struble Jul 7 at 3:12
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We see giant planets in the sky on Earth with our naked eyes, and they clearly appear different from the stars not because of their size, but because they follow a seemingly erratic path which is very different from the stars, which instead look like fixed in the sky.

Truth to say, the name "planet" comes from the Greek word meaning "wanderer".

If you instead want to know if it is possible for a planet to see another planet like an extended body in the sky, and not like a point like source of light, without orbiting it, then it's not possible.

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  • $\begingroup$ That's definitely not what I'm asking and I'm not sure how to be any clearer about that. I specifically asked if they could see them "as anything but stars" and then added at then end that they would look like moons, because I don't want a lesson about the Greek wanderers. I know we see planets, but they look like stars. I know what the planan were. $\endgroup$ – Bett Struble Jul 6 at 22:34
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    $\begingroup$ @BettStruble Clarify your question then or perhaps your question would be a good candidate for development in the sandbox, up to you. $\endgroup$ – We are Monica. Jul 6 at 23:16
  • $\begingroup$ @L.Dutch ah, I'm not sure if perhaps I just didn't see that last bit or if it was added later. You say a planet cannot see a planet as an "extended body in the sky" but that's why I'm trying to be vague, using P and B's, because I'm not married to the idea that one, two or all 3 are or are not moons. $\endgroup$ – Bett Struble Jul 7 at 0:55
  • $\begingroup$ @Confoundedbybeigefish. changed $\endgroup$ – L.Dutch - Reinstate Monica Jul 7 at 3:04
  • $\begingroup$ @L Dutch It is possible to see a planet as an orb from the distance of another planet in their star system. The planets in our star system are separated by distances much too great to be seen as more than dots of light from other planets. But star systems are known where planets can be seen as orbs from neighboring planets. $\endgroup$ – M. A. Golding Jul 7 at 16:32
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Seven points to consider:

ONE:

Obviously the planet Earth has a companion object, the Moon, which looks like the Moon in the sky.

Obviously at any one moment the Earth will be as far from the Moon as the Moon is from the Earth.

Since the Earth has about four times the physical diameter of the Moon, obviously the Earth appears to have about four times the angular diameter as seen from The Moon as the Moon appears to have as seen from the Earth.

So in a story set on a moon base on the near side of the Moon the Earth will appear about four times as large in the sky as the Moon appears in the sky of Earth.

And someone could write a story where the Moon has been terraformed to be habitable and have a breathable atmosphere - kept from escaping by a forcefield or something - and people could walk around outside without spacesuits and enjoy the view of Earth.

TWO:

It may be noted that tidal interactions cause the Moon to very, very, very slowly increase its distance from Earth. So billions of years ago, the distance between the Earth and the Moon was just a small fraction of the present distance, and the Moon appeared many times larger in the sky of Earth than it does today, and the Earth appeared four times larger in the sky of the Moon than the Moon appeared in the sky of Earth.

Of course neither the Earth nor the moon was habitable in that era, so there were no native lifeforms to enjoy the views. But possibly advanced civilizations might discover young planet-moon systems where the two bodies are close together and terraform one or both of the worlds in such a system to make them habitable.

THREE:

And of course the small planet Mars looks gigantic in the skies of its two moons, Deimos and Phobos, that orbit close to Mars. Deimos and Phobos are very tiny bodies, probably captured asteroids, so from the surface of Mars they look like very small orbs or mere dots of light.

The dwarf planet Pluto and its largest moon Charon are sort of a double planet, being so close together and so similar in size, and each appears very large as seen from the other.

Of course Deimos, Phobos, Mars, Pluto, and Charon are not habitable for any multi celled Earth life forms, though there is speculation that Mars could be terraformed to be habitable.

FOUR:

The four giant planets in our solar system, Jupiter, Saturn, Uranus, and Neptune, all have several times the diameter of the Earth, and all have at least one moon orbiting them at the distance between the Earth and the Moon, or closer, so obviously they all appear to be many times the apparent angular diameter of the Moon as seen from their inner moons.

In old science fiction stories the four large Moons of Jupiter: Io, Europa, Ganymede, and Callisto, were often depicted as habitable worlds, thus making it possible to encounter a Jovian goon on a Jovian moon. Saturn's largest moon, Titan (and sometimes other Saturnian moons) was often depicted as habitable. Neptune's largest moon Triton, was sometimes depicted as habitable, and sometimes even the larger moons of Uranus, even though they are much smaller.

Of course the surfaces of those moons are now known to be deadly for Earthly lifeforms, as no doubt seemed scientifically very probable even when those stories were being written.

FIVE:

However, scientists have given serious thought to the possibility that giant planets that orbit in the habitable zones of stars could have giant, Earth-like, moons that could be habitable.

A habitable moon of a giant planet could orbit it close enough that the giant planet would appear enormous in the sky of the moon, just as the giant planets in our solar system appear enormous in the skies of their inner moons.

And if that giant planet has two or more giant, habitable moons orbiting it, the life forms on each of the habitable moons would not only see the giant planet occupying a large part of their sky, but also see the other large moon (or moons) as sometimes a mere dot of light, and sometimes when closer possibly appearing several times as large as Earth's Moon.

There have been many other questions about possible Earth sized habitable moons of gas giant planets.

I have answered a number of those questions.

Here is a link to a previous question and my answer to that question includes a link to a question and answer which has links to previous questions and answers.

What are the day and night fluctuations for a moon orbiting a planet the size of Jupiter?1

The article "Exomoon Habitability Constrained by Illumination and Tidal heating" by Rene Heller and Roy Barnes, Astrobiology, January 2013, discusses factors affecting the habitability of exomoons.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3549631/2

SIX:

The distances between planets in our Solar System are so vast that no planet can be seen as more than a dot of light from another planet. And it might be assumed that all other planetary systems would be similar.

But astronomers have now discovered a little more than four thousand planets orbiting other stars, some of them in systems with more than one known planet. And there are some systems where the distances between planetary orbits are many times greater than in our system, and some other systems where the distances between planetary orbits are many times smaller than in our system.

In the Kepler-70 system, the orbits of Kepler-70a and Kepler-70b are so close that:

During closest approach, Kepler-70c would appear 5 times the size of the Moon in Kepler-70b's sky.

https://en.wikipedia.org/wiki/List_of_exoplanet_extremes3

Among potentially habitable exoplanets, exoplanets that orbit in the habitable zones of their stars, the potentially habitable planets in the TRAPPIST-1 system orbit closest to each other.

https://en.wikipedia.org/wiki/List_of_potentially_habitable_exoplanets4

The orbits of the TRAPPIST-1 planetary system are very flat and compact. All seven of TRAPPIST-1's planets orbit much closer than Mercury orbits the Sun. Except for b, they orbit farther than the Galilean satellites do around Jupiter,[41] but closer than most of the other moons of Jupiter. The distance between the orbits of b and c is only 1.6 times the distance between the Earth and the Moon. The planets should appear prominently in each other's skies, in some cases appearing several times larger than the Moon appears from Earth.[40] A year on the closest planet passes in only 1.5 Earth days, while the seventh planet's year passes in only 18.8 days.[37][34]

https://en.wikipedia.org/wiki/TRAPPIST-15

SEVEN:

Added August 12, 2019:

I have just found a discussion of another type of planetary configuration where a large planet and a small planet could sometimes be close enough that each would appear as a disc or orb in the skies of the other planet. At Sean Raymond's PlanetPlanet site, is this post:

https://planetplanet.net/2018/07/02/horseshoe-planetary-system/6

About so-called "horseshoe orbits". An Earth like planet in a horseshoe orbit relative to a second planet could periodically approach the second planet close enough that during the period of closest approach the other planet would appear as large as the Moon does from Earth.

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Can you see one large planet? Sure, if your viewpoint is from a moon.

Can you see two large planets? Well, probably not. To grossly oversimplify, moons and planets that aren't either very small, or very widely separated, tend to have sufficiently strong gravitational interactions with each other that their orbits are changed, usually with one or more parties being flung out of orbit. Have a read up on Hill spheres for more information on this subject.

You can get a great view of jupiter or saturn from their moons, but most or all of the other moons in those systems look like little points of light. Earth's fellow planets, even the largest ones, are just points of light without the aid of decent telescopes. WillK's link above shows a second moon being visible as a small but non-point-like object; you can probably manage this effect for a single moon, but you won't get anything that looks as big as the parent planet.

It probably isn't even possible to get a Tatooine-style double sunrise... one or more of the suns have to be a long way away (though again, it might be a small but non-point-like blob rather than merely a really bright star).

If the situation you're describing is artificial, you might be able to get away with it. Otherwise, you're almost certainly out of luck.

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  • $\begingroup$ You might be able to see two planets in the sky if they're a dual planet system rotating around a common centre of gravity, and the habitable moon is orbiting at one of their Lagrange points. Of course, the two planets would need to be significantly bigger than the lone habitable planet- probably both gas giants. $\endgroup$ – nick012000 Jul 7 at 11:46
  • $\begingroup$ @nick012000 a dual gas-giant system sounds unlikely to have arisen naturally, but I Am Not An Astrologer. There might be enough room to handwave something in there. $\endgroup$ – Starfish Prime Jul 7 at 12:45
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    $\begingroup$ @Starfish Prime What does an Astrologer know about Astronomy or Planetary Sciences? $\endgroup$ – M. A. Golding Jul 7 at 16:35

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