Some studies of the Solar Quiet Day variation, Sq.

If a compass needle is weighted so as to swing horizontally it takes up a definite direction at each place, usually not agreeing with the geographical or true north. The deviation from true north is called the declination, D, if eastward is reckoned as positive, if westward as negative. The end of the needle which, over most of the earth points nearly northward, is called the north-seeking or positive end; the compass direction from the centre to this end is called the Eggggtic north, and the vertical plane through this direction is called the local magnetic meridian plane