rhitidiadelphus: hy or hi in the first syllable?
heard as rhitidiadelphus
spelled rhytidiadelphus
It is hy. Rhytidiadelphus.
What the ear did
Greek upsilon becomes y in Latin, and the y survives here even though the syllable is said as a long i, the sound in rye. The same y does the same work in rhythm and rhyme.
R-H-Y-T-I-D-I-A-D-E-L-P-H-U-S
The full entry for rhytidiadelphus covers the name itself.