Notes from A Song of Fire and Ice and SCIENCE: A panel on George R.R. Martin’s Game of Thrones series. Saturday, July 5, 2014, at Convergence 2014.

One of the distinctive features of this world is that the seasons last for multiple years, and the length of each season is unpredicable. How can this happen? Not by axial tilt, as on our Earth. Nicole Gugliucci said it could be achieved by planetary dynamics and referred us to “Winter is coming”. This paper (and I strongly recommend a look at the full paper as well as the abstract) suggests that the planet is orbiting a double star, in which case the orbit could be chaotic, leading to unpredictable seasons. Apparently (I have not finished reading the series) GRRM never explicity wrote that there was only one sun. In any case, one of the stars could be a roughly solar mass black hole, and hence invisible. Contrary to popular culture, black holes do not suck up everything around them. If you are in an orbit that stays well outside the event horizon you will never fall in. I asked about the long term stability of such a system. You want to be sure your planet stays in the “Goldilocks zone” — not too hot and not too cold. From “Winter is coming”:

To study the long-term stability of the CBP, we embarked on a long voyage of integration.
The orbit was stable after integrating for one million days. On Day 1,000,001 the Andals invaded. Funding cuts to all research projects promptly followed. We were unable to continue our work. Those of us who expressed strong opinions against the cuts were sent to the Wall.

You have to use days as your unit of time here because the years would vary in length. In winter they would be longer. I think these would also have to be sidereal days, certainly if both stars of the binary system are visible suns.