SELECT "Diameter (mi)" FROM table_39991 WHERE "Longitude" = '79.8° e'
What is the Diameter (mi) when the Longitude is 79.8 e?
CREATE TABLE table_39991 (
    "Gill" text,
    "Latitude" text,
    "Longitude" text,
    "Diameter (km)" text,
    "Diameter (mi)" text
)