זוהר שמות — דף ריב ע”ב

A second gate is marked on the northern side, and in it stands the letter Dalet. This letter stands and scintillates, ascending and descending, blazing in that gate. Sometimes it sparkles brilliantly, and sometimes that light is hidden. The letter Dalet is connected to the secret of the moon, which sometimes shines and sometimes is hidden. The northern gate and the letter within it are part of the secret of the Garden gates and the sacred letters that govern the worlds.