My story has two planets separated by ~35,000 miles of empty space- tidally locked through a magic "planetary anchor" light rope that connects the two planets and keeps them at that exact distance.

The two planets are earth-like and I want them to be earth-ish habitability, with a bit more wildness due to the rotating nature of the binary planets. The planets orbit around the tether every 36 hours. I'm thinking they're on the solar plane or with <10 degrees of deviation.

Year length, if it's relevant, is earth year. Sun= earthlike/to be decided.

Would there be a normal 18hr day/ 18 hr night cycle (with slight seasonal variation), or would there be two nights- one at midday and one at the normal time?

  • $\begingroup$ Related question: How long would a day be on two tidally locked earth sized habitable planets?. $\endgroup$
    – JBH
    Mar 23, 2021 at 17:15
  • $\begingroup$ You have binary planets and binary stars both? Or is binary star a typo? $\endgroup$
    – Willk
    Mar 23, 2021 at 17:53
  • $\begingroup$ @Willk thank you for catching that. It was a typo. $\endgroup$ Mar 23, 2021 at 18:24
  • $\begingroup$ Good news, I don't think that your planets need any magical tethers to stay tidally locked, it's the natural consequence of two objects of similar mass closely orbiting each other. Tether can still be there if it's needed for plot or worldbuilding purposes, it will just connect the bodies instead of holding them. $\endgroup$ Mar 24, 2021 at 12:56

5 Answers 5


In a give spot on the planet surface you would have two dark times in a day:

  • a night when the sun is below the horizon
  • an solar eclipse when the other planet covers the sun in the sky

The duration of the solar eclipse will vary depending on the place on the planet surface: it will be as a minimum 0 on the opposite face with respect to other planet, and as a maximum it will last the entire day in the place closest to the other planet, except maybe a faint sunrise/sunset.


The other answers have covered the basic aspects of the main thrust of your question however I should point out an important issue. The orbital period, orbital radius and mass of orbiting bodies cannot be specified on an arbitrary basis as they are related by the gravitational laws such as Kepler's 3rd law.

There are a number of alternatives that could be used: The natural orbital period should be about nine and a half hours if the separation is 36,000 miles.

The separation should be around 86,000 miles if the natural orbital period is 36,000 miles.

Making the planetary mass greater will only decrease the natural orbital period.

The only way that the specified arrangement could happen would be in a state far removed from equilibrium with astronomical levels of force constantly applied. Similar to a centrifuge on a planetary scale. But this would have serious consequences for the stability of the planets surface.


  • 1
    $\begingroup$ Given that the original question posits an explicit "magic planetary anchor", and neither the hard-science or reality-check tags are in play, it's probably fine to disregard Kepler's third with regard to the double-planets. $\endgroup$
    – notovny
    Mar 23, 2021 at 16:01
  • $\begingroup$ It's a matter of how realistic you wish to be. You can specify any arbitrary rotations, but then again you can specify any arbitrary periods of light and darkness in fiction depending on which laws of physics are to be ignored or modified. I was merely pointing out the consequences of the situation described. The magic planetary anchor and planetary surface would be under huge stress continously. But sure that could simply be ignored. $\endgroup$
    – Slarty
    Mar 23, 2021 at 16:21
  • $\begingroup$ So @Slarty, you're saying that to maintain the 36-hour timeframe it should be 86k miles away? $\endgroup$ Mar 23, 2021 at 18:27
  • $\begingroup$ Yes radius of 43k $\endgroup$
    – Slarty
    Mar 23, 2021 at 19:34

Don't forget the nightlight!

You would have day; yes yes. And night; well and good. Also eclipses during the day.

But consider the night. Your brother planet is 10 times closer than the moon and presumably a lot bigger. It will fill a lot of the sky!

The brother planet would have phases, just like our moon. Being closer and larger, the brother moon would reflect a lot more light than our moon. A full - brother planet? - would be very bright.

This would make the night brighter when the brother planet was in the sky. I think it would also make the daytime sky some brighter.


Except at the poles, for regions where the other world is in the sky, there would be a solid 18 hour night, but the day would include a rather long eclipse.

Eclipses in the Earth-Luna system are short because our Moon is just the same angular size as the Sun (give or take a minute or arc or so), so the cast umbral shadow is only a couple hundred kilometers across and moves rapidly due to the motions of the two bodies.

Make the eclipsing body the same size as Earth and only 35,000 miles away, you'd have it eclipsing through nearly half of the day, and what time of day the eclipse begins and ends will depend on where the observer is relative to the "central pole" nearest the other body.


For your binary planet system, you don't need anything that sci-fi. You just need the planets to be orbiting a center of mass at a reasonable distance and velocity, and when one planet passes in front of the other, its shadow will make it night for the entirety of its twin. Eccentricity is allowed, as long as the two worlds won't drift apart or collide into each other. Also, we need not to just count the time it takes each planet to complete its orbit of the center of mass, but also each planet's individual rotation. However, I think that it's likely that they will be tidally locked to each other, which means that the near sides will only get indirect sunlight, and will thus be consistently cold.

The far sides will get the largest deviation in distance from the sun, but will get lots of sunlight. That will make them consistently warm, and when combined with their orbits around each other, will create weather patterns that are quite unlike Earth's.


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