Fatto generale: 1842 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Fatto matematico: 1842 is the number of rooted trees with 11 vertices.
Trivia: 1842 is an uninteresting number.
Data storica: January 15th is the day in 69 that Otho seizes power in Rome, proclaiming himself Emperor of Rome, but rules for only three months before committing suicide.