Although tefillin is technically the plural form (the singular being tefillah), it is loosely used as a singular as well. The arm-tefillin, or shel yad, is placed on the upper arm, and the strap wrapped around the arm/hand, hand and fingers; while the head-tefillin, or shel rosh, is placed above the forehead.
Definition of declination. 1 : angular distance north or south from the celestial equator measured along a great circle passing through the celestial poles. 2 : a turning aside or swerving. 3 : deterioration moral declination. 4 : a bending downward : inclination.
The mariner's compass consists of a magnetic needle freely suspended so that in the earth's magnetic field it turns until aligned with the magnetic north and south poles. Declination is the angle between the magnetic needle and the geographical meridian. Use of the compass by the early Chinese is probably legendary.