To get where they're going, navigators first need to know where in the world they are. And to help them find out, navigators calculate their latitude and longitude, which are imaginary lines around the globe.
Navigators sometimes found their latitude using an instrument called an astrolabe like the one above. Lines of latitude are the parallel lines north and south of the equator. The equator is at 0 degrees latitude. There are 90 degrees north and 90 degrees south of the equator.
In the northern hemisphere, navigators can use an astrolable and the North Star as a way to determine their latitude. The North Star is the first star in the handle of the Little Dipper. The angle that the North Star is above the horizon is equal to the latitude of the observer. For instance, if an observer using an astrolabe finds that the North Star is 20 degrees above the horizon, then the observer is at 20 degrees north latitude. Pretty nifty!
A compass is one of the oldest and still one of the most important navigational tools. A compass uses a magnetic needle with one end that always points to the north. Because the needle always points north, you can figure out in what direction you are traveling: north, south, east, west, or any direction in between.
How to make your own compass
The sextant like the one above is a tool developed in the year 1735. A sextant is kind of like a fancy astrolabe. It helps navigators figure out the angles between the sun and other heavenly bodies and the horizon. Then, by using the sextant, navigators can calculate their latitude and their position.
To find their longitude, navigators use clocks. Lines of longitude run up and down the world from the north pole to the south pole. A place called Greenwich, England, is at 0 degrees longitude. Greenwich is called the Prime Meridian. There are 180 degrees west and 180 degrees east of the Prime Meridian.
Here's the way navigators find their longitude:A navigator has a special clock set to the time in Greenwich, England. The navigator waits until it's noon (called local noon) wherever he or she is located. Then the navigator looks at the time in Greenwich and notes the difference in time between local noon and Greenwich time. The earth rotates on its axis 360 degrees in one day or 15 degrees in an hour. Knowing this, the navigator can figure out how many degrees east or west the ship is from Greenwich. For example, if the time is local noon on a navigator's boat and it's 3 p.m. in Greenwich, that three-hour time difference means the navigator is 45 degrees west of Greenwich, or 45 degrees west longitude. If the time is local noon on the boat and it's 9 a.m. in Greenwich, the boat is 45 degrees east of Greenwich, or 45 degrees east longitude.
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