The Compass Rose - Map of China: Decoding the Secrets of a Vanished Cartographer - page 104

passes, Heaven being situated at 315° from north. As the text explains, 'The man-
sion of Heaven is the chief of the twenty-four directions. Heaven governs the dis-
positionofcalamity,whichiswhyitisnecessarytostarttheneedlefromthere.The
needleshouldthenindicatethedirection,butdon'tletitsink.'Thepointofthisad-
viceseemstobetoavoidlayingtheneedleinthedirectionyouthinkyouaregoing.
Instead you should start off at an angle that is always random to the ship's bearing
butconsistenttothenorth-southaxis.Thisprocedureensuresthattheneedleisnot
preset but 'finds' that axis on its own.
Precisionincompassusewasessential.'Ifyoumakeanerrorofatinyfraction,'
the Laud editor declares a little pompously, 'you may miss your destination by a
thousand miles, by which time it will be too late for regret.' He provides a few
other snippets of information on the use of the compass. He lists the points of the
compass at which the sun and moon rise and set each month. He also includes four
methodsofcalculatingrelationshipsamongcompasspoints.Thiswasprobablyold
lorethatpilotsusedtotrainapprenticesinthementalagilitytheywouldneedwhen
calculatingpointsonthecompass.Ofthese,theonlymethodnavigatorslearntoday
is what the rutter calls the Four Directions Method. It consists of a simple set of
three four-character lines that reminds a pilot how to calculate reciprocals. These
arethecompasspointsontheoppositesideofthecircle(180°isreciprocalto360°,
150°to330°,120°to300°andsoon).PartofthechallengeoflearningtheChinese
compass is that points on the circle are not numbered but named. For a Chinese
pilot, 180° is called wu , and its reciprocal is zi , which we know as 360°. (These
twopositions havecounterparts intheorganisation oftimes ofday,' zi night'being
midnight and 'mid- wu ' being noon.) The reciprocals at right-angles to these are
mao and you , which for us are 90° and 270°. All we have to do is add or subtract
180 to calculate the reciprocal, whereas the Chinese pilot had to memorise their
names and relationships so that they can pop into his head without having to think
them through.
I haven't found a Ming source that explains how the naming system works, but
the eleventh-century author Shen Gua gives a good description in his monumental
compendium of notes and observations entitled Mengxi bitan or 'Pen Conversa-
tions from Dream Brook'. It comes up in the course of his describing a mapping
technique he calls As the Bird Flies. We attribute flying in a straight line to crows;
Shen thought it was true of all birds. As the Bird Flies was developed to com-
pensate for the problems of mapping an irregular surface that cannot be paced out.
We don't need to go through the entire method, which is in any case rather poorly