The Dragon's Tail was a poetic term used by the Pai to describe the celestial phenomena in which the Dragon Nebula effected its planets. Apparently according to the Lord High Celestial Mechanic of the First Rank and Grand Astronomical Savant, every century or so, Pai and the rest of the Dragon Empire’s core system passed through one of the nebula’s trailing clouds of gas. The gas interacts with the solar and atmospheric gases to produce a display of lights like auroras that lasts about a year. The gases develop rippling effects, like scales in green and pink, and flat planes of color that beat against each other. La Forge, who was charged in creating a spectacle to commemorate the Dragon Heir's wedding and the Empire's initiation into the Federation, had to scramble to find a suitable event that would be on par if not surpass the Dragon's Tail. (TNG novel: Dragon's Honor)