n.a
reference to a system for determining the
position of a
vessel at
sea by determining its relationship to a heavenly
body and Greenwich,
England. In
order to locate things on the surface of the globe, the earth is divided into 360 equal sections by vertical
lines called meridians. These meridians are numbered in degrees, starting from Greenwich,
England (which is 0), and increasing in number as one travels westward (i.e., 1 degree, 2 degrees, 3 degrees, etc.). Using these meridians, one can
measure the angle in degrees of any heavenly
body in relation to Greenwich. Further, it has been observed that heavenly bodies moving westward in the sky (due to the earth's rotation), travel at a rate of 15 degrees per hour (360 degrees in 24 hours). Thus the
term Greenwich hour angle, as the two (hour and angle) are related because in
navigation the calculations can be
given in "
time" or "degrees." To
determine the
position of a
vessel at
sea, one first takes an exact reading of the angle of a heavenly
body (say the
sun) in relation to the
ship and
notes the exact
time of
day where one is. One then looks in a
navigation table to find what meridian the
sun is located over at that
time of
day in Greenwich. By knowing where Greenwich is (0 meridian) and by knowing where the
sun is in relation to Greenwich and by knowing where the
ship is in relation to the
sun, one can
determine the
position of the
vessel.