With news about the Faroe island solar eclipse, I wondered if a constant solar eclipse would be possible.

Is it theoretically possible to move the moon or some large object into the L1 earth-sun Lagrange point or would it be too large?

How else could a constant solar eclipse be achieved?

What would be the maximum size of such an object or could some other shape like a disk more effectively achieve this?

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    $\begingroup$ L1, L2, and L3 are unstable, so whatever object you put there will have to be actively controlled to keep it there. Note that satellites near the Earth-Sun L1 aren't at the Lagrange point itself, but are actually in a halo orbit around it; to an observer on Earth, such an orbit seems to trace out an oval shape around the Sun, never coming within a few degrees of it. (This is partly due to ease of stationkeeping, and partly to avoid solar interference when communicating with the satellite.) $\endgroup$ – 2012rcampion Mar 26 '15 at 15:03
  • $\begingroup$ I edited the title of you question. Feel free to roll back if I somehow screwed something up :) $\endgroup$ – JDSweetBeat Mar 26 '15 at 16:08
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    $\begingroup$ Whats the point of the eclipse? Are you talking about a normal solar eclipse that's very localized? Meaning every day about noon one path along the equator has the sun hidden for a few minutes? Or is this an eternal night type of scenario? $\endgroup$ – AndyD273 Mar 26 '15 at 17:54

Math Time!

What do you need in order to block out the sun at the L1 point?

Well, the Earth/Sun L1 point is located roughly 1,500,000 km away from Earth. in order for a 'solar eclipse' to occur, we need an Umbra (total shadow) to reach Earth from the object. This means that the angular diameter of the object (apparent size) must be greater than the sun. Wikipedia says that at perihelion, the sun has an angular diameter of 0.5450 degrees So, we need something that is apparently that big or bigger.

The equation to find Angular Diameter is:

$$\delta = 2\arctan(\frac{d}{2D}) $$

where d is the actual diameter of the object and D is the distance to the object. Rearranging, we get this.

$$ d = 2D \cdot \tan(\frac{\delta}{2}) $$

plug in values

$$ d = 2\cdot 1{,}500{,}000\cdot\tan(\frac{.5450}{2} ) $$

and solve...

$$ d = 14{,}268.174\,\rm km $$

For reference, the Earth's diameter is 12,742 kilometers, so we are going to need something bigger than our planet.

As others have mentioned, the L1 Lagrange point is unstable...if you drift off of it, you fall towards one body or the other. So, you are going to need a monstrously powerful system to stabilize it. If you are willing to put in serious effort in keeping it from sliding around, you could potentially make it out of something light (like several layers of graphene) but you have to contend with the Solar Wind at that point.

On the other hand, if you make it too massive, something that big is going to have a gravity well, in which case we have just jumped down the rabbit hole of chaos theory.

And, to be clear, this is not to eclipse the entire planet...this is just to have a moon-sized solar eclipse at all times.

Something this big also has to contend with getting hit by space debris (and it doesn't have an atmosphere to protect it from meteorites). Graphene is really tough, and might be able to survive the impact, but that is yet another thing trying to knock you off station.

Assuming you use graphene, with is about the strongest thing we currently have...a single sheet of graphene would block out roughly 2.3% of available light. To block out all light, you need 44 layers. Graphene has an approximate mass of 0.8mg per square meter. So a square meter of light blocking graphene would weigh 35.2mg.

Our eclipse creating disc has an area of

$$1.6\cdot 10^{14}\,\rm m $$

Multiply that all out....

$$ 56{,}320{,}000{,}000\,\rm kg $$

That is a lot of weight, but not as much as any of the larger asteroids in our system. But bear in mind that this is only for the sheet of graphene...which at that size would be tremendously flexible and need to be braced, reinforced, and then still add the stabilization system. All of which would increase the mass dramatically. And the bracing would need to be extremely robust...stabilizing something that massive would be...insane.

In short, I don't think this is feasible without some serious future tech, and maybe a bit of applied phlebotinum.

For other alternatives, try using a ring around the planet, or something else that doesn't have to be nearly so enormous.

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    $\begingroup$ Reflecting the light might be better than absorbing it. If you absorb it, eventually your object heats up until it is glowing about as bright as the sun would be. No eclipse. $\endgroup$ – Oldcat Mar 26 '15 at 17:07
  • $\begingroup$ True...a reflective layer on the graphene would be a better idea...yay, more mass! Though you could probably thin out the graphene by a few layers...still have to keep it thick enough to be resistant to impacts though. $\endgroup$ – guildsbounty Mar 26 '15 at 17:08
  • $\begingroup$ Not really - you might as well just let impacts just punch a hole and repair it later. $\endgroup$ – Oldcat Mar 26 '15 at 17:14
  • $\begingroup$ Repairing graphene is not easy. It's a mono-atomic structure. Much better to not let it get damaged in the first place. $\endgroup$ – guildsbounty Mar 26 '15 at 17:17
  • $\begingroup$ Not really possible with micrometeors. Swap it out with mylar then. $\endgroup$ – Oldcat Mar 26 '15 at 17:25

For stability, the best way to permanently eclipse the sun would be with a ring.

If the Earth had a ring like Saturn, it could filter the sun from one hemisphere for the whole winter. Maximum size, a dyson ring inside earth's orbit could be set up so that some of it would always be blocking some(or all) of the sun.


Since creating an object 8 times the size of the moon is impressively hard, there might be another way that is still impressively hard, but more practical.

Why isn't there a solar eclipse every new moon? It is because the moon's orbit is inclined relative to the Earth's orbital plane. On average, the moon is a bit smaller than the sun, so the moon needs to be nearer perigee to have a total eclipse.

So the obvious solution is to adjust the inclination and distance of the moon so that you have a total eclipse every month!

Since this only happens over a small region (and if you make the orbit inclinaton exactly 0 only at the equator) I can't see there being any real effect on a monthly eclipse except make it boring and mundane.


It is possible.

  1. Put boosters with power enough to make the Earth and moon stay in same place in Solar System (1 pointing the direction opposite to where our Earth is going now and 1 pointing to the sun so that our Earth won't go straight to the sun) and put them on the moon too, later (when the speed of Earth and moon is zero), stop the booster that is not pointing to the sun

Edit:yeah, it will be unstable and by the fact that Earth is always spinning around, we need to make it a booster train that can go across the Earth. and let's just put in on the moon so it will be a lot easier.

2.Make a Satellite that is big enough (with size that have the same size of sun on the distance it is from Earth looking from the ground) to cover the sky that is always in the Lagrange point to use it as artificial moon.

(these require much resource, fuel and energy more than we know now)

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    $\begingroup$ You will most likely need a bunch of stabilizers, too, since your setup is instable. Why not "just" put boosters on the moon? Given it's much smaller mass, that should be a lot easier. $\endgroup$ – Burki Mar 26 '15 at 8:50

The only way would be to have it artificially induced. Moons orbit a planet, so putting a moon in one spot between the planet and the sun (or any other large enough object) it would need stabilizers to keep it in that position. If you are thinking about some other object orbiting the sun that stays in between, well, the speed of the orbiting body is dependent on the distance from the sun, so a body closer to the sun would travel faster. Granted if is was large enough and close enough to our orbit, every couple thousand years we could have a month or 2 long eclipse as the planet passes us by.

One thing to think of in that case is Janus and Epimetheus, they are two body's that are so close in orbit that they switch orbits when they get close, they never pass each other, they are more like relay racers, one 'hands' off to the other then slows down to the outside lane and the first speeds up into the inside lane. Planets would likely cause tectonic activity when they approach each other that close too.


A soletta/solar mirror could work.

In Green Mars a giant solar mirror is put in Mars L1 point, and magnifies the sunlight to make Mars brighter and warmer. It's held in place by stabilizers and also through a solar sail type effect. Another is put in Venus L1 point to block all sunlight and freeze the atmosphere, so it could be replaced with something less lethal.

“Well, and now with this soletta pouring sunlight onto the surface!” Jessica exclaimed. She shook her head, as if disapproving. “Natural insolation averaged forty-five percent of Earth’s, and with the soletta it’s supposed to be up to fifty-four.”

“Tell me more about this soletta,” Sax said carefully. They told him in a kind of round. A group of transnationals, led by Subarashii, had built a circular slatted array of solar sail mirrors, placed between the sun and Mars and aligned to focus inward sunlight that would have just missed the planet. An annular support mirror, rotating in a polar orbit, reflected light back to the soletta to counterbalance the pressure of the sunlight, and that light was bounced back onto Mars as well. Both these mirror systems were truly huge compared to the early freighter sails Sax had enlisted to reflect light onto the surface, and the reflected light they were adding to the system was really significant.

“It must have cost a fortune to build them,” Sax murmured.

If a soletta was put into the L1 point, but then sabotaged or just glitched, it could direct the light away from earth and cause a permanate eclipse of the sun.

Now for a reason why they might put a soletta in Earth's L1: Global warming. Earth getting a little to hot, so direct 5% of the light away, and cool things down a bit, like a giant thermostat... but something goes wrong.

Addressing comments
You wouldn't need it all the way out in the L1 point. It could even be inside the moons orbit, using the pressure of the solar wind to keep it in place, along with thrusters and stabilizers. This would reduce the size needed.

  • $\begingroup$ You realize that this object would have to be about 8 times the size of the moon? $\endgroup$ – Oldcat Mar 26 '15 at 17:05
  • $\begingroup$ @Oldcat Ok, that's fine. Or it could be smaller, a lot closer than the L1 point, and just use the solar sail properties to help keep it in place. Either way, it would be super thin and light in order to work, so even if it was that big it would mass very little, and would have to be built in orbit. This is global warming we're talking about! Is any amount of money to much? And it would be several orders of magnitude smaller and more stable than Fungo's Dyson Ring. $\endgroup$ – AndyD273 Mar 26 '15 at 17:36
  • $\begingroup$ It could be the size of an umbrella and held over one's head, that's true. And the shade would cool you off considerably. $\endgroup$ – Oldcat Mar 26 '15 at 17:47
  • $\begingroup$ The numbers are to give a tiny spot of darkness on the planet. This would have no effect of global warming at all. To change the temperature of the planet, you have to do much more. $\endgroup$ – Oldcat Mar 26 '15 at 17:49
  • $\begingroup$ @Oldcat So it would take two fortunes... You know, I just realized that we're thinking of two different things. Your thinking of a normal eclipse that is the small area where the moons umbra hits the earth, and lasts for a few minutes until the umbra moves on. I'm more thinking of an eternal night situation, mostly because of the wording "Is it possible for the earth to have an eternal solar eclipse?" So yeah, a giant soletta would be super expensive, but cheaper and easier than changing the orbit of the moon, or building a ring. $\endgroup$ – AndyD273 Mar 26 '15 at 18:06

Another theory could be that some mass just large enough could smash into the solar system. If conditions were just right, and it got caught by the sun's gravity, it could match the Earth's orbit and block the sun. It's just my own idea lol, not saying its totally possible.


A personal drone hovers above each person, blocking the sun for glare reduction purposes. They either recharge at night, or take turns seamlessly from a fleet.

Might be cheaper than any astronomical solutions. It's surely more sustainable for the environment.

If costs are still too high, it may be scaled down, e.g. for the rich only.


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