
Measurements and Analysis Using GIS 219
movement is called secular variation. Also known as declination, it is a change
that occurs over long periods and is probably caused by convection in the
material at the Earth’s core.
Daily variation is probably due to the effect of the solar wind on the
Earth’s magnetic field. As the Earth rotates, a particular place alternately
moves toward and away from the constant stream of ionized particles from
the sun. Therefore, it is understandable that the daily variation swings from
one side of the mean declination to the other over the course of a day. For
example, if the mean declination at a place was 15
◦
west of