Global Positioning System Reference
In-Depth Information
with the orbital axis? In that case we would have no seasons, no variation in
day length. The height of the sun in the sky, and the intensity of sunlight,
would vary with latitude, but the days would all be the same length at all
latitudes, and for every day of the year. At the other extreme, we might ask
what the seasons would be like if our planet's rotation axis were tilted over
90\ (that of Uranus is actually more than that: it is 97.86\). At the poles,
the sun would be below the horizon for nearly six months of the years,
during what would surely be a very deep winter, and above the horizon for
the remaining time—a broiling summer. At latitude 45\ north or south, the
sun would remain below the horizon for nearly three months of the year,
and for a slightly longer period it would not set. At the equator, the sun
would vary in height from directly overhead on midsummer's day to skirt-
ing the horizon in midwinter. The world would be a very di√erent place.
Magnetic Mysteries
The Song-dynasty Chinese scientist and statesman Shen Gua wrote about
the magnetic compass in 1088 CE; from his and other writings it is clear
that the compass had been in use by Chinese mariners for some centuries.
The earliest written reference to the compass in Europe dates from the
thirteenth century, so it seems likely that the magnetic compass originated
in Asia. These compasses made use of
lodestone,
a naturally magnetic mate-
rial; with lodestone it proved possible to magnetize an iron needle which,
when attached to a low-friction bearing (or floated in a little boat in a bowl
of water), would orientate itself along the geomagnetic field lines, thus
telling a navigator which way is north.
18
Let's back up a little. Why should there be a geomagnetic field at all?
Given that one exists, why is it more or less aligned north-south? The
physics of planetary magnetism is very complicated and is still an area of
active research. The phenomenon has been well studied for centuries
because of its importance for maritime navigation, but an understanding of
the underlying causes began only in the 1950s. Basically, the magnetism
arises from the liquid metal of the earth's outer core: the turbulent convec-
tion of the rotating fluid generates electrical currents (the outer core is a
good conductor of electricity) which in turn produce a magnetic field.
19
18. See Needham (1959, pp. 26-27) or Selin (1997) for more on the Chinese origin of
magnetic compasses.
19. Researchers suggest that an initial magnetic field would induce a current in the
conducting outer core, and this current would in turn induce a magnetic field. Depending
