The "u" at the end is actually silent or at least very close to silent, the pronunciation is incorrect. "A-MA-TE-RAS" would be more accurate, if still not perfect. A̠ma̠te̞ɾa̠sɨᵝ in IPA.
It's important to realize that Japanese is spoken in many different dialects, and so, the exact pronunciation of a word or name in the language can sound different depending upon dialect or even context. Therefore, "Amaterasu" has a couple of different pronunciations. However, the most accurate pronunciation (Tokyo-dialect) would be "ah-MAH-teh-RAH-soo." The "rah" could also be said "lah" (southern dialects), and the "soo" could be dropped so that the final syllable is "rahs" or "lahs."
― Anonymous User 4/12/2015
8
The "u" at the end of Amaterasu is actually silent.