We have three tags that should be synonyms: moon, the-moon, and lunar. Tags related to the Sun (the-sun and solar) have been marked as synonyms of sun, which is the master tag for this group. In keeping with this, tag:moon should be the master tag for questions related to the Moon.
There are a number problems with this, however. The correct tag for the Sun should be Sun rather than sun, and for the Moon, Moon rather than moon. This is not possible as SE tags are lowercased. Related to this is that there's a big difference between a moon (with a lowercase m) and the Moon (with an uppercase M). Despite the verbiage, there are already some questions tagged as "moon" that pertain to moons other than the Moon.
Another problem is that the tag "the-moon" has many more questions (247) than do the tags "moon" (32 questions) and "lunar" (10 questions). Making "moon" the master (without a capital M) will invite continued mis-tagging.
- Make "the-moon" the master tag for questions pertaining to the Moon.
- Make "lunar" tag a synonym of "the-moon".
- Add a new tag "moons", a synonym of "natural-satellite", and retag the few questions tagged as "moon" that don't belong in that category as "moons".
- Retag as "the-moon" all the remaining questions tagged as "moon" that do indeed pertain to the Moon.
- Lock the tag "moon" as ambiguous given the lowercase nature of SE tags.
I can't do the retagging presently; work beckons. I can undertake this effort later this evening, about 12 to 13 hours from now.