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CHAPTER XXXL

ON  THAT   DIFFERENCE   OF  VARIOUS  PLACES  WHICH  W^E
CALL  THE   DIFFERENCE   OF   LONGITUDE,

He who aims at accuracy in this subiect must try to Onthe
determine the distance between the spheres of the meri- method of

-,.               (•   ,1                       1             -                    •              -\r      T                               determining

dians ot the two places m question. Muslim astrono- longitude.
mers reckon by equatorictl times corresponding to the
distance between the two meridians, and begin to count
from one (the western one) of the two places. The
sum of equatorial minutes which they find is called
the difference hetween the two longitudes; for they con¬
sider as the longitude of each place the distance of its
meridian from the great circle passing through the pole
of the equator, which has been chosen as the limit of
the olKovjievi^, and for this first meridian they have
chosen the western (not the eastern) limit of the oIkov-
pivi-j. It is all the same whether these equatorial times,
whatsoever their number for each meridian may be, are
reckoned as 360th parts of a circle, or as its 60th parts,
so as to correspond to the day-minutes, or as farsakh
or yojanct.

The Hindus employ in this subject methods which
do not rest on the same principle as ours. They are
totally different; and howsoever different they are, it is
perfectly clear that none of them hits the right mark.
As we (Muslims) note for each place its longitude, the
Hindus note the number of yojctnas of its distance from
the meridian of Ujain. And the more to the west the
position of a place is, the greater is the number of