John Anderson

I am searching on information on my great grandfather. The only information I have is was born about 1858 in Chritiansand and was a mariner. His mother's name was Asper don't know his father's name but he was a farmer. In about 1878 he came to Australia and in 1884 married a Agnes Mary Watson. There was a story he may have changed his name from Andersen . I have found a mariner John Anderson b 1858 in Christiana. This maybe the same but I don't knowif Christiansand and Christiana are the same place. any help in this matter would be appreciated.