This is what the sails on a spelljammer ship catches to provide movement. It can only be detected by very specialized magic. Mostly the magic used by spelljamming helms. While using a spelljamming helm a magic-user can sense the flows and texture of the Aether around the ship for a good distance. This is how the major currents and moods of the Aether are first mapped out in unexplored space. You can also judge them by how they are pulling on the ship’s sails, but that is a lot less accurate and much harder.
The flows of the aether are influenced by gravity of the planets, all the asteroids and comets whizzing around, and the turbulent energies coming from the elemental maelstrom. This creates chaotic, but somewhat predictable patterns and paths for ships to follow.
The City of Doors 