I started watching Game of Thrones-- in that alternate universe, or whatever it is, a season can last years-- when the HBO series starts, a ten year long Summer is just ending.
This got me thinking, we typically think of a year by one complete revolution around the Sun. But seasons, with which our calendar is carefully calibrated to stay in synch, are caused by axial tilt, the Earth “wobbling” on its axis. So when the northern hemisphere of the Earth where I live is tilted toward the Sun, I experience Summer.
Wikipedia tells me that there are three astronomical definitions of a year: the sidereal and anomalistic year, which are basically different ways of tracking Earth’s orbit around the Sun, and the tropical year, which tracks the equinoxes, and therefore is the basis of most calendars. The sidereal year is 20 minutes longer than the tropical year, so there is a difference, but it’s pretty darn close. Wikipedia doesn’t explain why this is, unless I missed something. and I haven’t been able to Google whether this is coincidental or related.
So is it an astronomical coincidence that one orbit and a complete cycle of seasons are almost exactly the same? Could the cycle of axial tilt slow down to the point where four seasons could last significantly more than a year? Or is the cycle of axial tilt somehow connected to the Earth’s orbit?
The erath does not wobble on its axis. OK, it does, but with a 26,000 year cycle, so effectively the axis stays pointed at the same spot over the last millenia, roughly the star Polaris.
the catch is the axis is pointed at a tilt to the sun. It keeps this same direction all the time. So the axis (imagine a line coming out of the north pole) at one point in the cycle (winter) points about 25 degrees away from the earth - winter - and at the opposite side of the orbit, pointing in the same direction, the north pole is tilted slightly toward the sun.
hence, from our North American perspectve, in summer the sun is high in the sky, in winter lower. Higher, more sunlight, more direct; winter less sunlight because it is at a lower angle. More sunlight - hot; less sunlight -cold.
The four seasons last a year because it takes a year for the earth to revolve around the sun. I guess if the earth did have some sort of precession (think of a wobbling top) you could get a divergence between the seasons and the solar year, but that seems like an unstable situation to me (but I could be wrong).
The Earth’s wobble is so slow it doesn’t matter in the short perspective of a year. Maybe you’re thinking that whitout it a constant axis tilt should be, for instance 23 degrees with the North pole pointing away fron the Sun at all times, but what physics demands is that a constant tilt is constant with respect to the universe as a whole. So as we orbit the Sun, the plane the axis is tilted in is only perpendicular to the orbit twice a year, the event we call a solstice.
The wobble just means that over time this event moves along the orbit and the tilt of the axis changes. Currently we have a tilt of close to 23.5 degrees, solstices on June 20/21 and December 21/22 and perihelion and aphelion in early January and July respectively. So in the Northern hemisphere we’re actually closer to the sun during winter, but axial tilt is the more significant factor.
The slow cycle of the wobble (26 000 years) is why the sidereal year is only 20 minutes longer than the tropical year.
Also Game of Thrones doesn’t take place on Earth. But it is an Earth-like planet so I have to assume it has a similar mass, gravity and orbit around it’s star.
Because their “seasons” are long and unpredictable, I have to assume they are caused by a mechanism closer to that which causes ice ages on Earth.
The four seasons last a year mainly because we define a year to help track the seasons. The Calendar with the current leap year rules was created before most people understood that the earth goes around the sun. The Jewish and Chinese lunar calendars add leap months to keep alignment with the seasons. So matching the seasons to years is a common desire across many cultures.
With our understanding of how the solar system works there are 3 obvious was to define the year that are slightly different. For our calendar we use the tropical year because it follows the seasons.
Tropical year it the time it takes to have the tilt of the earth relative to the sun make one cycle.
Sidereal year time it takes to see the same stars rising behind the sun.
Anomalistic year time between closest approaches to the sun.
You make it sound as if seasons are caused by axial tilt as opposed to revolution around the Sun. In reality, of course, the two factors work together. The reason that an axial tilt induces alternating seasons is that the earth revolves around the Sun. If we imagine a stationary Earth with an axial tilt, there would be no alternating seasons, only a hot hemisphere (permanently tilted toward the Sun) and a cold hemisphere (permanently tilted away from the Sun).
Instead of asking why the sidereal year and the tropical year are so similar, it’s more helpful to ask why they are different. The default situation, with no precession, would be that the two were identical. The slow cycle of precession, superimposed on the annual revolution, induces the twenty-minute difference.
OK, so I had a basic misunderstanding of how seasons happen. It is closely related to the Earth’s orbit-- I get it now. Thanks!
If GoT is supposed to take place on another planet, not an alternate universe, then I have questions about how another planet can be so earth-like, to the point a society almost exactly like Medieval England can develop, right down to the accent.
I suppose the old probability thing saying if a roomful of monkeys bang on typewriters long enough, they would eventually reproduce the works of Shakespeare, would apply here. So with enough habitable planets in the universe, a scenario like GoT could happen. Riiiight…?
But I guess that would be a question for Cafe Society.
Technically, it’s a function of the Earth’s tilt on its axis, and the fact that it maintains this tilt (in the same relative direction) throughout its orbit.
Obviously it isn’t - just like the people in Star Wars didn’t speak English either. But for the convenience of the viewing audience, everything is translated into Earth terms and cultural references.
Just be grateful we have a nearly circular orbit, so that summer in the northern hemisphere gives winter in the southern, and vice versa. If the earths orbit was somewhat more eccentric, there would be a long global winter and short global summer.
As for the world on which A Game of Thrones takes place, there may be a number of possibilities (although the author has stated that there is a supernatural cause for the seasons).
The planet’s sun could be somewhat variable in output (as per our sun’s 11ish year sunspot cycle). A period of diminished solar output probably caused the “little ice age” from 1250 - ~1900 (dates vary depending on criteria).
There could be climatic system similar to the Atlantic multidecadal oscillation, with an impact on warm water circulation (like the gulf stream on our world).
I am discounting multistar systems - the inhabitants of Westeros would spot an additional sun during summer, and that would not be erratic.
Well there’s a theory floating around that the planet is an Earth colony (possibly terraformed) and the colonists lost (or abandoned) advanced technology, and so many millenia have past they’ve fortgotten everything about their origins. As for the accents; the characters aren’t really speaking English, they’re speaking Common Tongue (aka Andalish) and thank’s to the Translation Convention the audience here’s it as English. For what it’s worth George R. R. Martin insists that the explanation for the strange seasons will be magical, not scientific.
Because it’s made up. Not everything has to be historically plausible. For all intents and purposes, it’s a planet of midevil humans that isn’t Earth. Unless it somehow ties into the story, it doesn’t matter if they are decended from a lost Earth colony, are a product of convergent evolution or even some other species that are portrayed as human simply for convenience (translation convention).
Marge: “You…you speak English!”
Kang and Kodos: “I am actually speaking Rigelian. By an astonishing coincidence, both of our languages are exactly the same.”
