The Great Conjunction is an astronomical event where the three suns of Thra – the Greater Sun, Rose Sun and Dying Sun – appear to converge at a single point in the sky.
The First Great Conjunction
"At the hour of the Great Conjunction, the crystals sang out, calling each to each… and the star-beings opened a door in the light and stepped across the cold void of space."
The Second Great Conjunction
At the close of the Age of Harmony, the second Great Conjunction occurred and marked the beginning of the Age of Division. During this Great Conjunction, the urSkeks stepped into the light of the suns that filtered through the Crystal and refracted through the urSkeks' mirrors. Unintentionally, the urSkeks were split into two separate creatures: the Skeksis and the urRu.
The Third Great Conjunction
"Then with the shard restored the three suns stood above the glowing crystal and flooded it with light."