I am curious if a planet that is tidally locked to a sun could have another planetary object in orbit that could eclipse the sun each day, effectively creating a day and night cycle similar to earth?

1) How large would the object or objects have to be to block the sun?

2) Is there a way it could block the sun for many hours a day (8-10) or would it be moving too fast?

3) What effects would this have on the tides the tidal locked planet?

4) Would this allow the light side to become habitable?

  • $\begingroup$ If a planet has a very large satellite, capable of causing long eclipses, it would be tidally locked with this satellite and not the sun. $\endgroup$ – Alexander Oct 26 '17 at 21:32
  • $\begingroup$ Maybe possible with an artificial structure, but definitely not a natural one. It could be a shell stretching half way around the planet - which is at least possible, but I doubt the orbit would be stable. $\endgroup$ – sdfgeoff Oct 26 '17 at 21:33
  • $\begingroup$ Habitability is mostly temperature, and you can control that by putting the planet closer or farther from the sun. You may want to look at the moons of Jupiter, Io orbits in 2 days, and spends 2hrs in the shadow, if you were even closer the orbit would be faster, and the shadow longer. Locking to the sun rather than Jupiter is still pretty unlikely. $\endgroup$ – user25818 Oct 26 '17 at 22:28

No it could not. A planet cannot be tidally locked both to the sun and to a large satellite if it is in a stable orbit.

If you want to calculate how large an object would have to be to just block out the sun, this can easily be achieved by using similar triangles. If any three of the following parameters are specified the fourth is automatically defined: diameter of the moon, diameter of the sun, distance of planet to the sun, distance of the moon to the sun.

Yes the sun could be blocked by a moon for many hours, depending on a lot of factors. I suspect in most cases it would require a double planet / very large tidally locked satellite orbiting relatively close.

The moon is tidally locked to earth but earth is not tidally locked to the moon. That is why we have tides on earth. In the case of a very large very close moon both moon and planet would probably be tidally locked to each other and the tides would be “locked” ie there would be no net tidal forces between these two bodies.

The issue of a “light side” and a dark side are only relevant in the case of a planet that is tidally locked to its parent star not in the case of a moon being tidally locked to its parent planet. In the case of our moon the so called “dark side of the moon” actually receives 14 days of light and 14 days of darkness (just like the other side). Much better terms are the far side and the near side of the moon.

  • $\begingroup$ In Earth was tidally locked to the Moon, there would still be Sun-caused tides. But that's beyond the main point. $\endgroup$ – Alexander Oct 27 '17 at 0:03
  • $\begingroup$ I appreciate the response, but I wasn't asking about the planet being tidally locked to both, only the sun. Like in the case of an eyeball planet. So, for this planet, unlike the moon, there would be a side that received constant sunlight and one that received no light at all. The part about the "other planetary object" was me asking if it was possible for this constant sunlight to be interrupted for a long enough period of time each day to create a semblance of Earth's day/night cycle. $\endgroup$ – BobDylan Oct 27 '17 at 0:12
  • $\begingroup$ A tidally locked planet would not normally have a large satellite. One way it might happen is if a tidally locked planet acquired a large moon that had been ejected from some other orbit. In this case I suggest there would have to be a period where the tidally locked planet also had a large moon orbiting it that could also be close in. I suspect this arrangement would be highly unstable, but I’m not sure how unstable. It might take years, centuries, millennia or millions or years before the moon-planet tidal locking over whelmed the planet-sun tidal locking. Thoughts anyone? $\endgroup$ – Slarty Oct 27 '17 at 13:14

Just an idea, it's not a full blockage, but a periodic dimming effect:

Make it a binary star system with a big cool star and a small hot star. Have the small star orbit the much heavier big star in a strong elliptic orbit whose major axis is aligned with the planet. You would have a significant dimming effect whenever the small star is behind the big star and thanks to the strong elliptic orbit that could be a significant amount of time.

Big problem is if the planetary orbit could be stable enough with such a binary system at the center.

  • $\begingroup$ why not making your "dark star" a black hole ;-) $\endgroup$ – Henning M. Oct 27 '17 at 12:24
  • $\begingroup$ I think OP said something about the planet to be potentially habitable. :) $\endgroup$ – Khris Oct 27 '17 at 12:48
  • $\begingroup$ make it a small black hole with not much more mass than your sun. if the dimming effect is then to little, add an accretion disk. A hot super jovian or a hot brown dwarf might be easier though. $\endgroup$ – Henning M. Oct 29 '17 at 23:39

Not if you want to use a single object.

There are two good places where you can put an object that will periodically block sunlight to a planet: in orbit around the planet, or in orbit around the planet-sun L1 point. In orbit around the planet is no good for your purposes: any orbit will move too fast to produce the desired day-night cycles.

This leaves the L1 point. If you place an object somewhat above or below the plane of the planet's orbit, it will cycle across the orbit roughly once a year, blocking the sun. You can adjust the blocking time by changing how far above and below the orbit the object goes, which in turn changes how fast it's going as it crosses the orbital plane.

Since the L1 point is unstable, this situation can't arise naturally: it would require constant station-keeping to last more than a few years. Further, in order to cast a shadow, such an object would need to be huge: something at Sun-Earth L1 would need to have a diameter slightly greater than Earth's in order to cast even a lunar-eclipse-sized shadow -- but at the same time, it would need to have a mass much less than that of Earth, in order to avoid destabilizing the L1 point.

Basically, what you're looking at is a collection of huge solar sails or similar, wobbling across the planet's orbital plane on a regular basis. Sounds like the sort of thing a race of sufficiently advanced alien pranksters might do.


A double planet could be locked to each other in close orbit around a dim star. If just outside the Roche limit, you would end up with two somewhat egg shaped planets with each taking about 30 degrees of sky. At the equator this would generate something like 1/6 of a period of darkness. (So the sub partner point would in effect have 2/3 night 1/3 day, but the day would be split in 2 chunks. This might be the coldest place on the planet.)

If the orbit of the plane of the double planet is coplaner with it's orbit around the sun, then you don't have seasons. Tilt it by 20-30 degrees and the zone of max darkness moves north or south of hte equator depending on the time of year. Put it in a moderately eccentric orbit and you will have a yearly variation due to the varying distance from the sun.

You would then have a Fimbulwinter and Fimbulsummer depending on the how the yearly orbit interacted with the double planet revolution. (One of them will precess. How fast?)


Have you considered using a binary sun instead of a moon? If your planet was in the same elliptical plane as a binary system with one bright star and one much dimmer one, you could have several transits of one star in front of the other, which could produce a "nighttime" that consists of it getting dim, but not dark. The day/night cycle you need would be determined by the distances and mass ratios of the suns.

Necessarily, tides would be small as you'd only have sun tides, but there would be some light activity.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.