Call this a question of ignorance but;
Isn't the sky in Sithicus different?
Seeing as there's only 1 moon in the sky there (Lunitari). And a few other things?
3e homogenized the night sky for the Core. The rule of thumb is always to consider it is the same all across unless noted otherwise in the rule.
The sky of Sithicus was canonically changed to match the rest of the Core after the events of
Specter of the Black Rose. Same sun, same moon, same stars. Sithicus has a single moon, which they call Nuitari. Gazetteer IV, p 120, states that "to the naked eye, Sithicus shares a single moon with the rest of the Core". For most it would look like exactly the same moon as anywhere else in the Core, although it may be felt that it is shedding its light in different hues while in Sithicus. (Something my traveling PCs would just ascribe to "something in the Sithican air")
The only other exceptions I can think of for the Core are Darkest Night in Darkon and the inconsistent sky in Vechor. Any AD&D 2nd Edition facts about the Core's night sky should just be ignored.
//Addendum, for obvious reasons the sun, moon and stars are never seen while on the Nocturnal Sea, but it is assumed they would be there above the clouds.