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102 OUR ANCESTORS CAME FROM OUTER SPACE
Still, fiow exactly did they do it? By what means were they
able to determine in open sea the two co-ordinates of longitude
and latitude in order to know where they were and where to go?
The theory that all ancient ocean crossings were accidents caused
by wind or current could not be true. Too many legends report
of planned voyages and of heroes who visited distant lands and
returned to tell about their adventures.
When ancient Roman coins were found in Venezuela, Sumerian
mining colonies discovered in Peru and Bolivia, and
Hindu cotton and jute plantations traced in Mexico, I began
wondering and searching again for the answer to the riddle of
ancient navigation and now I believe that I have found the answer.
Instead of chronometers, they used sunset and moonrise
tables that were skillfully calculated for every day of the year in
the case of long voyages and for many years to come. Thus at
any time a navigator of a ship on open sea could determine his
location by comparing the actual times of sunset and moonrise to
those charted for the same day for his home port.
Since latitude is easily determined by an astrolabe or primitive
sextant, the difficult part was the determination of the exact time
span between the moments when the sun disappeared and the
moon showed up. It was done by a battery of hourglasses set to
measure a fraction of a minute. Let me explain with an example
from our recent past what the sunset-moonrise timing has to do
with the geographical longitude that has to be found.
All astronomical phenomena of short duration, like an eclipse,
are never seen simultaneously at difiFerent points of the globe. So,
for instance, during the total solar eclipse of June 30, 1973, that
lasted up to 7 minutes 3 seconds, the shadow of the moon was
running eastward over oin: earth in a narrow band 240 km wide
at a speed of 2,150 km per hour. In order to observe the ecHpse a
little longer, seven astronomers from England, France, Scotland,
and the United States used a supersonic Concorde equipped
with aU necessary instruments to fly east at an altitude of 17,-
000 m (56,000 ft) so they could watch the eclipse for 84 minutes.
To enable them to do this, the plane naturally had to move with
the same speed as the shadow of the ecHpse—2,150 km ph—
which it did easily. (This eclipse came exactly nineteen years af-