I think that my rough calculations show that it might be possible to design a system where there are two co-orbital planets with very similar orbits, and where every few decades the outer planet suffers an eclipse from the inner planet lasting for at least one who year of hte outer planet.
I once read that George R.R. Martin did write that there was an explanation for the suuper long winters and it would be given sometime in th future. I also hear it is taking him a long time to write the latest books in the series, and I suspect there is no guarantee that the explanation will be given before the last paragraph in the last book. And maybe not even then.
And I wonder whether George R.R. Martin aleady knew the explanation when he wrote that there was one, or if he is just stalling for time until he can think of one which will be good enough for him and for his fans!
I don't know whether Martin will use a scientific and astronomical explanation for the long and bad winters or some sort of magical explanation.
But I have an idea for a scientific and astronomical explanation.
The planet in ASOIAF, and any similar planet that KEY_ABRADE might be thinking of writing about in a series of their own, apparently has a fairly normal cycle of seasons.
At least the people there know of years, since the winter is said to last for years. I believe that in ASOIAF people's ages are given in years, and ages inASOIAF years seem to corerespond fairly well with the number of Earth years those characters seem to have lived.
Possibly there is a study somewhere about the possible range of ASOIAF year lengths compared to Earth year lengths. If so, that would be very helpful. For now I will assume that ASOIAF years are approximately the same length as Earth years.
I haven't read the books or seen the series, so I don't know if the world in ASOIAF does have a normal cycle of spring, summer, autumn, winter, and spring again, with occasional super winters which last for several years.
Not being familiar with the story I can't say for certain whether the planet does have normal seasons, including normal winters, betweent he terrible long winters that last for years. A world with little or no axial tilt - https://en.wikipedia.org/wiki/Axial_tilt - would have very mild seasons which people might not notice much.
In temperate zones the cycle of the seasons is very important, and people notice it, and that is the main evidence for the length of a a year.
But on Earth, people lived in tropical regions which the seasons were often much less important. Some tropical places have important and noticeable wet and dry seasons, but maybe some do not. So in some tropical regions people might not have noticed any vary important cycle of the years to show them how long a year is. Do those tropical cultures use years in their native calendars?
People can notice the length of a year without having a noticeable cycle of seasons.
A midnight the stars on the line between due north and due south through the zenith will be in the opposite direction from the direction to the Sun.
Over the course of a year, the celestrial sphere will seem to rotate, though actually the line between noon on Earth and midnight on Earth will be doing the rotating. After six months the stars seen on the midnight line will be 180 degrees, halfway across the sky, from the stars that were seen there six months earlier. And after another six months the original set of stars will be seen on the midnight line in the sky.
So that is one way which people could use to figure out the approximate length of a year.
Ancient Babylonian astronomers and astrologers were able to plot the postion of the Sun against the background of the stars despite not mbeing able to see the stars in the daytime sky. They knew the Sun at noon would be almost exactly opposite to the stars on the midnight line on the previous and following midnights. So ithey wound it i aeasy to plot the suns's position against eh unseen stars whose positions "on" the celstial sphere had long been mapped.
So it is possible that people know the length of a year, regardless of how noticeable the regular and ordinary seasons are on the ASOIAF planet.
My theory is that possibly the ASOIAF planet could sometimes be eclipsed for years by a planet in an interior orbit around their star.
The gravitational interactions between and star and its planets create forbidden zones around each planet. If an astronomical body enters the forbidden zone of a planet its orbit will be gradually perturbed until it takes a new orbit outside the forbidden zone, or crash into the star or antoher planet, or is ejected from the star system. Such things are believed to have happened more than once during the first few hundred million years of the solar system, ecjecting or colliding many asteroids, comets, planetesimals, moons, and even planets.
So the forbidden zones between neighboring planets make it hard to squeeze planetary orbits close enough for one planet to clipse all of another planet, or even any part of the other planet.
But there are theoreticall and real examples of astronomical bodies which share the same orbit, called a co-orbital configuration.
For example, there could be two planets in trojan orbit around their star, one planetbeig about 60 degrees ahead or behind the other planet in their mutual orbit around the star.
But a trojan planet wouldn't cast a shadow on its co-orbital planet.
Saturn has examples of moons in Trojan orbits with other moons, which are no good for this question, and it has anotherand more interesting set of co-orbital moons.
Epimetheus has dimensions of about 129.8 kilometers by 114 kilometers by 106.2 kilometers and its orbit around Saturn has a semi-major axis of about 151,410 kilometers.
Janus has dimensions of about 203 kilometers by 185 kilometers by 152.6 kilometers and its orbit around Saturn has a semi-major axis of about 151,460 kilometers.
Note that the difference between their orbits is less than their sizes. That seems like a recipe for a collision, even though they have existed in a co-orbital configuration for thousands, millions, or billions of years.
If perfectly circular, the orbit of Epimetheus would have a circumference of about 951,336.28 kilometeers, and the orbit of Janus would have a circumf951,650.44 kilometers.
Epimetheus has an orbital period, during which it travels 360 degrees around Saturn, of about 0.694333517 Earth days, while Janus has an orbital period of about 0.694660342 Earth days.
Epimethmeus travels about 518.4828201 degreees along its orbit every Earth day, while Janus travels about 518.2388834 degrees along its orbit every Earth day. So Epimetheus gets about 0.2439376 degrees ahead of Janus every Earth day. So after 1,475.78251 Earth days, or about 4.040485285 Earth years Epimetheus gets a full circle ahead of Janus and so catches up with Janus from behind.
So Epimetheus should have smashed into Janus after no more than four years of sharing an orbit.
Epimetheus's orbit is co-orbital with that of Janus. Janus's mean orbital radius from Saturn is, as of 2006 (as shown by green color in the adjacent picture), only 50 km less than that of Epimetheus, a distance smaller than either moon's mean radius. In accordance with Kepler's laws of planetary motion, the closer orbit is completed more quickly. Because of the small difference it is completed in only about 30 seconds less. Each day, the inner moon is an additional 0.25° farther around Saturn than the outer moon. As the inner moon catches up to the outer moon, their mutual gravitational attraction increases the inner moon's momentum and decreases that of the outer moon. This added momentum means that the inner moon's distance from Saturn and orbital period are increased, and the outer moon's are decreased. The timing and magnitude of the momentum exchange is such that the moons effectively swap orbits, never approaching closer than about 10,000 km. At each encounter Janus's orbital radius changes by ~20 km and Epimetheus's by ~80 km: Janus's orbit is less affected because it is four times more massive than Epimetheus. The exchange takes place close to every four years; the last close approaches occurred in January 2006, 2010, 2014 and 2018. This is the only such orbital configuration of moons known in the Solar System (although, 3753 Cruithne is an asteroid which is co-orbital with Earth)
So could an Earth like planet and another planet be co-orbital like Epimethmeus and Janus?
If the inner planet was moving 0.2439379 of a degree farther than the other planet every Earth day, it would travel about 89.098317 degreees past the outer planet in one Julian calendar year of 365.25 days.
If the outer planet has an orbital period of about one Earth year, or about 365.25 Earth days, and orbits its star at at a distance of 1 AU like the Earth does, its orbit shoudl have a circumferance of about 939,950,349.2 kilometers, and each degree of arc along its orbit would be about 2,610,973.192 kilometers, and a planet 89.098317 degrees wid would be about 232,633,317.2 kilometers in diameter.
That would be too big to fit inside the orbit of the Earth. It would also be about 1,626.988 times the the 142,984 kilometer equatorial radius of Jupiter, and planets and brown dwarfs can not get much larger in diameter than jupiter. Adding more mass just makes them denser, not wider.
So let me design an extreme example of co-orbital planets that almost touch.
Earth has a radius of about 6,400 kilometers and thus a diameter of about 12,800 kilometers. Suppose there is a planet of that size in Earth's orbit, and another planet in an almost identical orbit which is about 20,000 kilometers smaller than Earth's orbit.
Earth has a semi-major axis of about 149,598,023 kilometers, so if its orbit was perfectly circular the orbit would have a circumference of 939,951,306.2 kilometers. Earth has an average orbital speed of about 29.78 kilometers per second. So it would take Earth about 31,563,173.48 seconds to complete one orbit.
The sidereal year of Earth, the time it takes to complete one orbit as seen from the distant stars, is 31,558,149.504 seconds, the difference being explained by Earth's
elliptical orbit and varying orbital speeds.
So the orbit of the inner planet in my example would have a semi-major axis of about 149,578,023 kilometers and thus if it was perfectl circular it would have a circumference of 939,825,642.6 kilometers. It would have a slightly faster orbital speed than Earth, but I will make it about 29.78 kilometers per second, just like Earth's.
So the inner planet would take about 31,558,953.75 seconds for each orbit.
One degree of Earth's orit would be 2,610,975.851 kilometers, and one degree of the inner planet's orbit would be 2,610,626.785 kilometers.
One arc minute of Earth's orbit would be 43,516.26418 kilometers, and one arc minute of the inner planet's orbit would be 42,510.44642 kilometers.
One arc second of Earth's orbit would be 725.2710696 kilometers, and one arc second of the inner planet's orbit would be 725.1741069 kilometers.
So Earth would travel 2,572,992 kilometers in 86,400 seconds or one day. That would be 59.12713438 arc minutes per day. And the inner planet would also travel 2,572,992 kilometers in 86,400 seconds or one day. That would be 60.52611103 arc minutes or 1.008768517 degrees per day.
So the inner planet would go 1.39897665 arc minutes farther than the outer planet each Earth day. Since there are 21,600 arc minutes in a full circle it would take 15,439.85741 Earth days or about 42.2720 years for the inner planet to gain a full circle on the outer planet.
The inner planet would have an angular diameter of about 27.74 degrees or 1,664.50416 arc minutes as seen from the outer planet. If the star has an angular diameter of about 0.5 degrees or about 30 arc minutes, the planet will travel a total of about 55.98 degrees or about 3,359.00832 arc minutes from first contact to last contact during its eclipse of the star.
If the inner planet gains 1.39897665 arc minutes on the outer planet each Earth day, the inner planet will take 2,401.046737 Earth days or 6.5737 Earth years for the eclipse of the star. And according to my rough calculations about 3,299.00832 of those arc minutes will be during a total eclipse of the star, with all of the star covered, which should take about 2,358.158244 Earth days or about 6.4562 Earth years.
So that imaginary outer planet should spend about one seventh of its time in eclipse.
Except that during what would otherwise be the middle of the eclipse, the two palnets will be at their closest and will switch orbits, thus putting the former outer planet into the inner orbit and into the bright sunlight.
But then, 42 years later, when the former outer planet is eclipsing the former inner planet, they will switch places again and the former outer planet will be the outer planet again and will be eclipsed again.
So each planet should experience about 3 years of eclipse every 42 years, one fourteenth of the time.
I note that:
In addition to swapping semi-major axes like Saturn's moons Epimetheus and Janus, another possibility is to share the same axis, but swap eccentricities instead.
I am not certain how that would work or whether that would allow one planet to always be in the outer orbit.
I note that this is a very extreme example of two co-orbital planets being very close together. If the fictional Earth like planet is supposed to have a moon in an orbital period about the same as Earth's, that orbit would be many times as wide as the distance between the planet's orbits.
For a planet to have a moon at the distance of Earth's Moon with a stable orbit, the inner planet would probably have to be at least 1,000,000 kilometers closer to the star. If the inner planet's orbit gets too much smaller than that of the outer palnet, they may cease to be co-orbital. And then they would have separate forbidden zones, and they would have to get much farther apart to be out of each other's forbidden zones.
Another possibility would be to make the inner planet a bit closer to the star, and make it a giant planet with a ring system. All four giant planets in the solar system have ring systems, though only Saturn has spectacular and easily seen rings.
Jupiter has an equatorial diameter of 142,984 kilometers. And no giant planet can have a diameter more than about 20 percent larger than Jupiter's, so no more than about 171,580 kilometers. Saturn is much smaller than Jupiter, but has a large ring system. The A ring, the outermost bright ring, has a radius of 136,775 kilometers and thus a diameter of 273.550 kilometers, and there are fainter rings much farther out than that.
Suppose that the inner planet has a ring system with a diameter of about 2,700,000 kilometers, about 10 times that of Saturn. Suppose that it orbits around the star 5,400,000 kilometers closer to the star than the outer planet, and thus at a semi-major axis of 144,198,023 kilometers, and an orbital circumference, if the orbit was perfectly circular, of about 906,022,134.2 kilometers.
The inner planet with the rings would have an orbital period of about 0.946185 Earth year or about 345.59407 Earth days according to this orbital calculator.
So the inner planet would travel about 1.0416845 degrees along its orbit during one Earth day, while the outer planet in the position of Earth would travel about 0.9856262 degree during one Earth day. So the inner planet would gain 0.0560583 degree on the outer planet during one Earth day, and it would take it 6,421.8857 Earth days or about 17.582164 Earth years to gain 360 degrees on the outer planet.
When the inner planet with the ring system passes closest the ring system will have an angular diameter of about 22.918332 degrees as seen from the outer planet. With the star having an angular diameter of about 0.5 degree, the ringed planet will travel a total angle of 46.336664 degrees during the eclipse. That will take it about 826.57989 Earth days or about 2.263 Earth years.
So I think that thes rough calculations show that it might be possible to design a system where there are two co-orbital planets with very similar orbits, and where every few decades the outer planet suffers and eclipse from the inner planet lasting for at leas tone who year of hte outer planet.
Of course it would take more expert calculations than mine to work out the details fo such a system and make certain it was possible.