When faced with a wildfire massacre and a new King in the North, it's easy to overlook some of the smaller details in the Game of Thrones Season 6 finale. Even mid-sized moments such as Dany ditching Daario or Davos having a strop over a wooden stag were likely to overshadow the smallest scene in the episode: Sam looking happily around a library.
Still, that scene was a lovely nod to the rich core of Game of Thrones, and something that we've seen since the very beginning of the show: we finally saw the astrolabe in its home, the Maesters' library in The Citadel.
'The Maesters' Astrolabe' may sound like some sort of joke only a maester thirty years in the making would get, but it's really quite simple. The big, glowing globe / sphere thing with all of the complicated metal bits whirring around it is what we've been seeing in the Game of Thrones title sequence for the last five years, right the way back to Season 1.
You see it before every episode but may not have thought about it before: what the heck is this big astrolabe and what does it mean? without an advanced degree in Maestery, I don't know exactly how it works. Heavy have quite a nice explanation of what such a device does in our world:
The armillary sphere... has a skeleton made of graduated metal circles linking the poles and representing the equator, the ecliptic, meridians and parallels.
Usually a ball representing the Earth or, later, the Sun is placed in its center. It is used to demonstrate the motion of the stars around the Earth. Before the advent of the European telescope in the 17th century, the armillary sphere was the prime instrument of all astronomers in determining celestial positions.
This would certainly help the Maesters determine the seasons, but there is more to it. It seems to shine light on things below, a kind of mapping device, something which can be moved and manipulated to shine on specific areas. It is related somehow to the map and its moving pieces featured in the title sequence. Do the Maesters look at the map with funny glasses like the rude guy Sam meets at The Citadel's front desk? If you look closer at the credits, it gets interesting.
The title sequence for Game of Thrones was made by a company called Elastic, and show producer Greg Spence describes their vision for the credits thusly:
"a vision of a mad monk, in a tower somewhere who was somehow keeping track of all this action and creating as he went. He would then fashion little automatons out of the materials that would be available in his world. They would be stone, or tin, or wood, and everything would feel very hand-crafted."
This is very close to the truth of what the Maesters do. Remember, they're not only healers and raven keepers, but scientists, astronomers, accountants— anything you would expect of a learned man, the Maesters do it.
Vitally, the iron link in a Maester's chain is awarded for prowess in warcraft, so doesn't it make sense that the Maesters really do keep track of the world at war, its territories and battles, on a big, grand map at the heart of their center of learning? This makes even more sense when you realize that the Game of Thrones title sequence changes EVERY WEEK to reflect roughly where the major action takes place.
In this way, we can imagine that when we watch the Game of Thrones credits every week, we are actually seeing what the Maesters in The Citadel see as they keep track of the world around them. When we see the map change to reflect the Starks returning to Winterfell or Dany reaching Meereen, it's because the Maesters have shifted some pieces. It's all very meta, and a really nice little detail for die hard Game of Thrones fans.
TL; DR: The title sequence is an interactive map made by Maesters pushing pieces around so they know what's happening in Game of Thrones world. We finally saw it, in-world, in the Season 6 finale when Sam went to The Citadel.