Or a part eclipse? The problem is that partial is derived from the noun 'party' in the sense of 'one side in a dispute' so an ambiguity is introduced. If an arbiter (judge, referee) made a partial decision I would take that to mean that the decision was one-sided but the loose way it's used could take the statement as meaning that only a part of the decision had been made.
Widespread use doesn't make it correct; English usage changes and that's the sign of a living language but loose use creating ambiguity is another thing.
My version of Fowlers (1997) doesn't agree with your etymology at all:
Partially, partly 1. These two long-established adverbs-partially first recorded in the 15c and partly in the 16c.-have shared the sense 'in part' from the beginning.
Whenever these sort of debates come up, it's often forgotten that..
1. Many of the so-called 'rules' of English were invented by the Victorians based on what sounded nice in Latin, which isn't even the right language family for English development.
2. Many of the so-called "Americanisms" have their origin in earlier British spellings (e.g. 'gotten').
3. If language never changed, we'd still decline nouns, and have separate forms of the second person, be that singular/plural or polite/friendly (or both!).
4. The key thing in any communication, written or spoken, is the potential for confusion. I estimate the number of people to be confused by 'alternate' versus 'alternative' in this message to be in the region of zero.
It's rather like my company insisting on a differentiation between an em-dash, an en-dash, and a hyphen (especially space-hyphen-space, which Word autocorrects to an en-dash). Last time I looked, there is only one UK publisher who still uses the em-dash. Does anyone really get 'confused' by the length of a dash?
to hear the first person using 'of' as a verb on its own. The (perhaps U.S. led) conversion of nouns to verbs grate on me.- 'medaling' at the olympics, for example. 
