Magnetic declination
You ask me as well how I passed beyond the equator by compass or needle without lodestone. I answer you that the needle still pointed to the north, except that it moved one quarter to the northwest; and this we knew because the tail of the needle always pointed to the Antarctic Pole, although it took a quarter of the southeast, and we always sailed with the same needle: and in conclusion, the lodestone does not vary in the south more than in the north.
Ridolfi Fragment (Formisano 1992, pp. 43-44)