First of all: not the way that I’ve been saying it for the past few weeks. Ed, rogue classicist that he is, opened up his bag of clues and set me straight. Here’s his explanation:

Closest pronunciation to actual ancient Greek would be:

poLYtropos

where “o” is always short as in “hot” and the “y” is a French u, and the “LY” is accented — if you want to be really strict, it’s pitch-accented, not stress-accented.

“pu-LI-tru-puss” would be a slightly Anglicized approximation of that — perfectly correct.

“Polly-TRO-pos” is right out. Even though Greek is often pronounced with Anglo-Latin stresses and phonemes instead of the Greek (this is called the Henninian system), even under that system, the accent would fall on the antepenult, because the “o” in “tro” is short, making it a light syllable.

I suspect that the reason that “TRO-pos” sounds so “right” is that an accented “tro” appears in words like “psychotropic” and “heliotropic.” Alas, this is not due to the length of the o, as you’d think, but the loss of an extra syllable — these in their Latinate/Greek form would have been psychotropicus/os.

Indeed, if you wanted to really be correct about them, you wouldn’t pronounce them “helio-TROH-pic” but “helio-TROP-pic” as in Tropic of Cancer (not “Troh-pic of Cancer”).

So, you assumed wrong, but there’s a reason you assumed wrong.

No room for disagreement, however. “polly-TROH-pos” is just plain wrong.

Apologies to everyone who’s been saying it wrong in their head because of me. I kind of like the right way better, anyway.