Earth's longest mountain range is underwater. Called
the Mid-Ocean Ridge system, it is over 56,000 kilometers
(35,000 mi) long and snakes around the globe like
a zipper or the stitching on a baseball.
East Pacific Rise is part of this system. This ridge
is in a geologically active area, where the Earth's
tectonic plates are moving apart and new seafloor
is being born, giving rise to hydrothermal vents,
volcanoes, and earthquakes.
East Pacific Rise was discovered during the Challenger expedition
in 1875. Over 100 years later, in 1979, the submersible Alvin made
its first trip to the East Pacific Rise and revealed "black
smokers" spewing super-heated water.
East Pacific Rise is one of the fastest-spreading
sections of the Mid-Ocean Ridge system. While many
tectonic plates move about as fast as your fingernails
grow, the plates here move an average of 14.2 cm
(5.6 inches) each year.
do mariners find their way on the high sea? Modern-day sailors
rely on several high-tech tools such as the Global Positioning
System (GPS), a satellite-based radio navigation system initiated
in 1973 by the U.S. Department of Defense for military applications.
Today, the GPS system is widely available for civilian use. It
can provide users with accurate information about their location
at any time, in any weather, anywhere
in the world.
GPS satellites circle the Earth twice a day, transmitting
signal information. GPS receivers compare the time a
signal was transmitted by a satellite with the time it
was received. The time difference tells the GPS receiver
how far away the satellite is. By taking distance measurements
from several more satellites, the receiver can compute
latitude, longitude, altitude, and time.
U.S. Coast Guard requires mariners to know celestial navigation
to operate a vessel more than 200 miles offshore. By using
a traditional navigational tool such as a sextant (represented
in the graphic above), mariners can determine their location
by measuring the vertical angles between two or more stars
and the horizon line. This sighting process is called "shooting