Over in the Star Trek 'verse, Vulcan has no moon.
Now I understand why JJ Abrams turned Vulcan into Alderaan in nuTrek.
Someone forgot to tell Robert Wise that. 😉
Supposedly Douglas Trumbull didn’t think the original test matte (with orange sky) looked alien enough, so he designed that larger and more complex matte with the moons/planets in the night sky. Not sure why they thought it was a good idea to change it to night though, as Spock is clearly shielding his eyes from the sun when he stands up.
That said, a planet-sized moon also appears in the Vulcan sky in the animated series, so The Motion Picture wasn’t the first thing to make that creative choice.