Publications

Here we describe the outstanding contribution of D João de Castro, an educated Portuguese sailor from the early 16th century. João de Castro was probably the first person to perform robust geomagnetic declination observations on a large spatial scale. He was the first to detect geomagnetic anomalies for closely-spaced sites and to acknowledge the role of metallic objects near compass devices. This sensibility led Castro to adopt precautionary approaches, namely using many different compass makers, repeating observations and using different observers. His data allowed him to demonstrate no correlation between geographical longitude and geomagnetic declination.