59th
The southernmost tip of the Big Island of the State of Hawaii is at roughly 18.9° north latitude. The US claim to that territory is not known to be in dispute.
latitude
The state extends westward from the "big island" of Hawaii, near 150°W longitude, to near 178° W longitude (Kure Atoll), and the associated island chain stretches northwest to the Aleutian Trench.
Only Hawaii.
The equator.
Western Australia, South Australia and New South Wales all have land area along the latitude line of 30.
The International Date Line runs primarily along the 180 degree longitudinal line. The Equator runs along 0 degrees latitude. The lines intersect along in the middle of the Pacific Ocean. The closest major land masses are the Solomon Islands and the Island of New Guinea, both to the south west.
The latitude line of 30°S passes through Western Australia, South Australia, and New South Wales.
Rovaniemi, Finland
-- Hainan Dao (China) -- Maug (Pacific) -- Hawai'i (big island) -- Cuba -- Île de la Tortue (Haiti)
The Island of Hawaii is the youngest of the "main" islands.
The Tropic of Cancer does.