Morrowind Mod:GetMoonPhase

GetMasserPhase
GetSecundaPhase

                GetMasserPhase
                GetSecundaPhase

        Type:   Misc

     Returns:   short
                        0 = New moon or Interior Cell (this is the default)
                        1 = Waxing or Waning Cresecent
                        2 = Waxing or Waning Half
                        3 = Waxing or Waning Gibbous
                        4 = Full Moon

     Example:   if ( GetMasserPhase == 1 )
                set sValue to GetSecundaPhase

     Scripts:   Not Used

Returns the current phase of the two moons of Tamriel. Note that the GetSecundaPhase function is incorrectly referred to as GetSecundusPhase in the Construction Set help. Another thing that should be pointed out that the moons return whether or not they're full even during the day time, when logically the moons are invisible.

This isn't confirmed, but in practice, experimenting with both functions almost always resulted in both moons being full immediately upon loading a saved game, and both moons remaining at the precise same phase as one another throughout their rotation.

When the player is in an interior these functions will return the phase when you were last in an exterior cell.