The most important device for knowing directions on the ocean is a compass. A compass is a device containing a metal object that points toward the magnetic north pole. This shows navigators the direction of north, and therefore also south, east, and west. But sailors need more information to sail safely on the open sea.
STEVE EMBER: Most maps of the world show lines that are not on the Earth's surface. One line is the equator. It is an imaginary line around the widest part of the Earth.
There are similar lines both north and south of the equator. These circles become smaller and smaller toward the north pole and the south pole.
These lines, or circles, are parallel - meaning that they are equally distant from each other at any point around the world. These lines show what is called latitude.
A navigator can know the latitude of his ship by observing the location of stars, where the sun rises in the morning and sets in the evening, and what time of year it is. With this information he knows where his ship is in relation to the north or south pole and the equator.
BOB DOUGHTY: Still, there is one more important piece of information necessary for safely sailing the oceans.
For many centuries, scientists, astronomers and inventors searched for a way to tell longitude. The lines of longitude go the other way from latitude lines. They stretch from the north pole to the south pole, and back again in great circles of the same size. All of the lines of longitude meet at the top and bottom of the world.
最新
2013-11-25
2013-11-25
2013-11-25
2013-11-25
2013-11-25
2013-11-25