Page 128 - Solar Power in Building Design The Engineer's Complete Design Resource

98 INTRODUCTION TO SOLAR POWER SYSTEM DESIGN

Figure 4.5 U.S. magnetic declination map. Photo courtesy of Solar Pathfinder.

direction that a compass needle points is referred to as magnetic north and the angle
between magnetic north and true north is called the magnetic declination. They are also
sometimes referred to as the variation, magnetic variation, or simply compass variation.
The magnetic declination is not a constant figure and varies in an unpredictable
fashion over time. Earth’s magnetic field is a result of complex movements of fluid
magnetic elements such as iron, nickel, and cobalt that flow in the outer core of Earth,
which lies about 2800 to 5000 km below Earth’s crust. The poles resulting from the
rotation of the molten matter generate a magnetic field that surrounds Earth. The poles
of Earth’s magnetic field do not coincide with true north and the south axis of rotation
of Earth. The resulting changes to the magnetic field are referred to as secular varia-
tion. Figure 4.5 depicts the U.S. magnetic declination map.
Local magnetic material inconstancies in the upper crust of Earth’s mantel or surface
also cause unpredictable variations and distortions of the magnetic field. Some of the
anomalies of Earth’s magnetic field can be attributed to ferromagnetic ore deposits,
volcanic lava beds with high concentrations of iron, and human-made structures (such
as power lines, pipes, railroad railings, and building metallic structures; small ferrous
manufactured items such as iron stoves; and even very small metallic objects like
kettles, knifes, and forks). These items can induce a small percentage of error in mag-
netic field measurements.
Movements of the tectonic plates causes Earth’s magnetic field to constantly
undergo a gradual displacement, and over millennia cause a reversal in Earth’s north
and south poles, which is evidenced in the planet’s paleomagnetic records.
So when orienting the Solar Pathfinder, keep the instrument away from metallic
objects such as air-conditioning equipment, metallic ducts, and pipes.
After the magnetic declination angle from the given Web site is determined, turn the
platform that holds the solar shading graph clockwise or anticlockwise accordingly,
which is in turn lined up against a marked white dot located on the outer rim of the
device. As an optional item Solar Pathfinder also provides software that allows insertion