Eta Carinae
Halley gave an approximate apparent magnitude of 4 at the time of discovery, which has been calculated as magnitude 3.3 on the modern scale. The handful of possible earlier sightings suggest that Eta Carinae was not significantly brighter than this for much of the 17th century. Further sporadic observations over the next 70 years show that Eta Carinae was probably around 3rd magnitude or fainter, until Lacaille reliably recorded it at 2nd magnitude in 1751. It is unclear whether Eta Carinae varied significantly in brightness over the next 50 years; there are occasional observations such as William Burchell's at 4th magnitude in 1815, but it is uncertain whether these are just re-recordings of earlier observations.