Certain spells and effects are defined in the rest frame of a certain unspecified observer which makes me wonder if it is the rest frame of the spellcaster or it is an absolute cosmological rest frame (like the hypothetical aether of our world). For example, is Leomund's Tiny Hut stationary with respect to the spellcaster's inertial frame of reference at the time of the casting of the spell or is it stationary with respect to an absolute frame? Another example would be the ambiguity in Glyph of Warding which specifies measurements from the position at which the spell is cast in order to determine the effective duration of the spell. Is that position relative or absolute?
If there is an absolute rest frame, do planets move with respect to this frame or are they stationary?