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I've got an image in my head of a world where it's just normal that the star is black with a golden halo around it - in other words, where the planet is in a state of perpetual solar eclipse.

Are there any mechanisms that could lead to this occurring naturally? It seems like a tidally-locked moon around a tidally-locked planet could produce such an effect, but I might be misunderstanding the dynamics of such an arrangement.

And if that would do it, is there anything preventing such a system from developing naturally?

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    $\begingroup$ Not sure if that would be considered a moon...you are asking for a moon that doesnt revolve around a planet for this to work, just orbits the sun infront of the planet. A tidal locked planet also has one side permanently in darkness and one side always in the sun. Suspect you are more looking for a large planet orbittung a sun closely with a planet further out thats permanently in that larger planets shadow. Not sure on feasibilty as either the large planet casting the shadow is moving exceedingly slowly or the planet in the shadow is moving exceedingly quickly $\endgroup$
    – Twelfth
    Commented Nov 22, 2014 at 2:08
  • $\begingroup$ @Twelfth - That's a really good idea, but the planet in front would be moving with a much greater angular velocity because of Kepler's Third Law. $\endgroup$
    – HDE 226868 ♦
    Commented Nov 22, 2014 at 2:23
  • $\begingroup$ Yes, either the large body would move too slowly and collapse into the sun or the smaller body would be moving too fast and exit the solar system. I dont think permanemt solar eclispe is workable. $\endgroup$
    – Twelfth
    Commented Nov 22, 2014 at 2:26
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    $\begingroup$ What if instead of a moon you had a planet that was equivalent to our mercury or venus but that was in an orbit synchronized with the outer planet and always was "in-line" with the sun and outer planet? I'm guessing it would have to be a bit bigger then mercury or venus but maybe its possible? $\endgroup$
    – Culyx
    Commented Nov 24, 2014 at 17:12
  • $\begingroup$ @Twelfth While one of the planets would have to be either too slow or too fast to maintain conventional orbit, if their masses differed drastically and their eccentricity was ideal, the effect could in theory be maintained. But with varying eccentricity, the distance between the planets varies and the effect would become a sort of pulsing annular eclipse. Or can this still not be done with ideal factor tweaking? $\endgroup$
    – Nikola Radevic
    Commented Feb 4, 2016 at 13:16

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What you are describing is an annular eclipse, where the moon is not quite as big (visually) as the star it eclipses.

This is not possible.

First off. An eclipse is a localized phenomenon. The parallax of being in different places on the planet looking at the star and the moon will give them different relative positions. To eliminate this problem, the "moon" would have to be closer to the star than to the planet and almost as big as the star.

A global total solar eclipse is possible, if you are on a moon being eclipsed by the planet it orbits. The planet is bigger than the moon and casts a bigger shadow that the moon can fit entirely within. This won't give you the ring of light that an annular eclipse does though. If the planet has an atmosphere, then you might be able to see it lit up like a ring shaped sunset. That's why the moon is red when eclipsed by Earth.

Making that permanent though isn't going to work. Tidal locking is about the rotation of a body about its axis, not its movement though its orbit. The moon is locked to Earth which means we see the same face of.

You can't make the orbit of the moon around the planet the same duration as the orbit of the planet around its star (which is what you are probably trying to get at with your double tide lock idea) The moon would be so far away, it wouldn't be in orbit around the planet any more and certainly wouldn't be close enough to cause an annular eclipse.

To get something at a fixed position with respect to the star would require positioning at a Lagrange point. Either L1 or L2. These can be thought of as "orbits with periods equal to the orbit of the planet" but it's a bit more complicated and only two points work, not the whole orbit. Those points are directly in line with the star though so that might seem it would work.

L1 is between the planet and its star. If something were that big enough to block the star, you'd get a permanent eclipse, until it drifted away which would eventually happen as L1 is unstable.

L2 is on the far side of the planet so you might be able to get a planetoid (not exactly a moon or a planet) to sit there, although it's again unstable so the planetoid would drift away from the point eventually.

Earth Sun L1 and L2 are about $1.5\times10^6\rm{\,km}$ from Earth. When it's causing an annular eclipse, the Moon is about $4.0\times10^5\,\rm{km}$ from Earth. Moving the moon out that far would reduce it's angular size by a factor of $3.75$ so we'd have to scale up its radius by the same amount to keep it visually the same. That would make it slightly larger than Earth! Even if it weren't for the instability of L1, it would need an incredibly low density to avoid disrupting the whole system.

Conversely you could do something like put Jupiter at $0.93\,\rm{AU}$ then put Earth at Jupiter-Sun L2 ($1\,\rm{AU}$ from the sun), Jupiter would be about 50 arc minutes in radius (if I did the math right). This would be a bit less than twice the angular size of the sun at that distance. You might see a bit of light around the edges of Jupiter diffracting through its atmosphere. This would be subject to parallax variation, but not as much as with an annular eclipse.

You have the basic problem of stability though. Earth might stay at L2 for a little while, but it would drift away without something holding it in place. It would end up as a moon of Jupiter, crashing into Jupiter or being flung out of the solar system.

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    $\begingroup$ Another problem with Langrangian points: They only work when the mass of the 3rd object is negligible small. $\endgroup$
    – Philipp
    Commented Nov 22, 2014 at 12:11
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    $\begingroup$ What about making it localized? This will eliminate the problems raised in the beginning, and the eclipse with the golden halo would only be visible from one specific location. This might lead to this location being considered sacred ground, the capital of a mighty empire, or something similar. If the moon would not be stable on its own, magic or technology (done by some ancient, log-forgotten precursor civilization) could do it. Makes for an interesting plot when the astronomers at last figure out that such a thing couldn't occur naturally. $\endgroup$
    – vsz
    Commented Nov 22, 2014 at 16:23
  • $\begingroup$ +1 - You were right; I was wrong. I'm going to delete the part of my answer regarding the tidal locking but I'll leave in the effects of a moon between the planet and the star, just for fun. But your answer definitely answers the question. $\endgroup$
    – HDE 226868 ♦
    Commented Nov 22, 2014 at 19:11
  • $\begingroup$ @Phillip Yes, although I wasn't sure of the threshold so I was a big vague about that. $\endgroup$
    – smithkm
    Commented Nov 22, 2014 at 22:01
  • $\begingroup$ @vsz Yes, if you could get your moon at L1 such that it's big enough to cause an annular eclipse at that distance (about the same size as Earth in the case of Earth) while still being light enough to take advantage of the Lagrange point without destablising it and then actively held it in place, you could have an annular eclipse that stayed in one place. Or you could move it around on the surface of the planet. Partial eclipses would be visible from a much larger area around the area where the annular eclipse was visible. $\endgroup$
    – smithkm
    Commented Nov 22, 2014 at 22:06
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As the impossibility of such a set-up is already stated by @smithkm, we have to look into other ways to make the effect you present possible.

A possible naturally occurring thing that would permanently block the star is a ring system or very thick asteroid belt closer to the star than the planet. A view may then look something like this:

cloud

Not exactly a halo, but rather two thin slices.

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  • $\begingroup$ Cool idea. Any idea how thick that would have to be? $\endgroup$
    – HDE 226868 ♦
    Commented Mar 6, 2016 at 23:26
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    $\begingroup$ Vastly thicker than is realistic, I think... Our real-life asteroid belt has one rock every million miles or so. It's not going to blot out a lot of sunlight. That image looks more like a "ring world" than an asteroid belt. I suppose that could work... $\endgroup$
    – Joanna Marietti
    Commented Apr 11, 2016 at 4:33
  • $\begingroup$ @JoannaMarietti Whatever works. It is not extremely unrealistic though, for example, the rings of Saturn do not look very transparent. $\endgroup$
    – SE - stop firing the good guys
    Commented Apr 11, 2016 at 4:41
  • $\begingroup$ So, what if our planet is an 'interior' moon? If the plane of the Giant's orbital debris is the same as the course around the star and our planet doesn't wobble or any such thing, the eclipse could be maintained quite easily, no? I wouldn't expect a black bar so much as a diffusion and warping of light here though. The OP's desired effect gets an awful lot easier to achieve if we imagine our planet-moon is tidally locked to our Giant...so all that's then required is another conveniently placed Giant-moon $\endgroup$
    – mensenisevirem
    Commented Apr 3, 2017 at 14:43
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Note: My original answer was incorrect; tidal locking would not produce such a scenario. For an excellent explanation of why this is the case, see smithkm's answer. I want to leave in some notes regarding what would happen if there was somehow an object between the planet and the star. Not, though, that such a scenario is essentially impossible.

There would be some interesting effects due to this arrangement:

  1. Tides would be incredible - and not-existent. Tides are caused by the various alignments of the Sun, Moon and Earth. Tidal bulges are the result, and can vary depending upon their location relative to the Sun and Moon. This arrangement would mean that the moon is forever on one side of the planet, and so the tidal bulges are forever like the spring tides shown here:

enter image description here

  1. No change in the time of day. Well, this actually applies to any planet tidally locked to a star. One hemisphere would always be in sunlight, and one hemisphere would always be in shadow. It would be the same on the moon. One side would have a blazing view of the star, while the other would have a nice view of the planet.
  2. You could build a space elevator to the moon. I asked about building a space elevator between doubly tidally locked bodies on Space Exploration a while ago, and the answers seem to apply here. As HopDavid and aramis said, it's possible but not very feasible. Still, it would provide an interesting form of transportation, and a handy one. All you need is hundreds of thousands of kilometers of carbon nanotubes and a lot of luck.
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  • $\begingroup$ Not sure if that gets the permanent eclipse hes looking for, unless ive misunderstood his question...i think he is looking for the moon to always be between the planet and the star, which doesnt qualify as a moon anymore, does it? $\endgroup$
    – Twelfth
    Commented Nov 22, 2014 at 2:11
  • $\begingroup$ @Twelfth Sorry if I was unclear. I meant that I don't think there's anything that means that his scenario is impossible. $\endgroup$
    – HDE 226868 ♦
    Commented Nov 22, 2014 at 2:12
  • $\begingroup$ The scenario isnt tidal locked moon and tidal locked planet, its permanent eclipse all the time. I think...moon always between star and planet $\endgroup$
    – Twelfth
    Commented Nov 22, 2014 at 2:15
  • $\begingroup$ @Twelfth Yes it is. He mentioned that in his question. $\endgroup$
    – HDE 226868 ♦
    Commented Nov 22, 2014 at 2:16
  • $\begingroup$ Ah, my misunderstanding, i thought the question was with permanent eclispe causing the sun to always look black with a halo with tidal locked as a possibilty. You are right in the double tidal locked $\endgroup$
    – Twelfth
    Commented Nov 22, 2014 at 2:19
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There is one possibility for this to occur, but it can only happen around a certain star. As you can see from this answer,

Consider a neutron star. If the radius falls below 1.76 times the Schwarzschild radius for its mass, then due to the General Relativistic bending of light in curved space, then all of the surface is visible, when viewed from any direction

Any planet orbiting close to the neutron star will therefore be in the path of the light coming from the surface, obscuring it and generating an eclipse, and if the proportions and distance are in the proper range it might be well noticeable.

I guess it will hardly looks constantly like a perfect halo, but still it will be a shadow.

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  • $\begingroup$ I haven't worked out the numbers, but my intuition is that any planet orbiting this close to a neutron star would be inside the Hill Sphere and, therefore, not a planet, but it would be cool if that's not the case $\endgroup$
    – bendl
    Commented Jan 16, 2018 at 18:39
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I have one suggestion but that lies in more artificial/fantastical possibility:

Not quite tidally locked, but using a similar concept... given a star, a planet and a satellite (i.e. lunar body or otherwise). The planet and the satellite are both surrounded by some sort of gas/construct that holds them together and dampens any movement relative to each other, this whole system (planet+satellite+stabiliser) is tidally locked with the star, the planet or satellite being at the far side and recieving the permanent eclipse effect.

Essentially the planet and satellite act as one larger body that is tidally locked.

As for if this idea is naturally possible? I suspect not, otherwise very unlikely.

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