For ancient mariners, determining latitude was fairly easy, but longitude was something else. Latitude can be determined by measuring the angle between the horizon and the pole star. Early navigators used a quadrant or an astrolabe. (Directions on how to build an astrolabe can be found in Year One, Unit Two.) Later the navigational cross staff was invented for even more accurate measurements. And then it too was replaced by the sextant. Today mariners use GPS to determine location.
Longitude was more difficult though. There is no fixed star that stays in either the east or west, since the stars rotate around the earth. Explorers had to guess based on direction (they did have compasses), currents, and the speed of the ship. A careful record was kept of the ship’s progress. The record became known as the log because of a navigation method called dead reckoning.
Sailors would toss a log overboard and see how quickly the ship pulled away from the log. That determined the speed the ship was sailing. It’s not terribly accurate since it doesn’t take into account currents and the new position reached depends on the previous position being accurate.
Make a Navigational Cross Staff
- Mark the center of a 1x2x5” piece of wood. Lay a ¼ x1 ½ x 36” piece of wood over the center mark on the 5” piece of wood. Draw a line along each side of the long piece of wood. Remove the long piece of wood and draw two dots centered and just outside each of the two lines you drew.
- Drill a hole through each of the dots, slightly larger than two carriage bolts.
- Center a 1x2x12” piece of wood under the 5” piece. Use a pencil to mark two dots through the drilled holes onto the 12” piece. Drill holes in the 12” piece at the dots.
- Test the assembly by placing the 36” inch of wood between the other two and slipping the bolts through the two pairs of holes. The wood should fit easily between the bots. Tighten the wing nuts to hold the bolts in place.
- If everything fits, you can mark the angles on the staff. Lay the cross staff on the floor. Loosen the wing nuts and slide the crosspiece to about 4” from one end of the long stick. Place a protractor along the opposite end of a long stick.
- Place the yardstick from the tip of the crosspiece to the end of the long stick over the protractor. Slide the cross piece so that the yardstick lies along the 10° line of the protractor.
- Draw a line across the flat surface of the long stick, using the side of the cross piece that is closest to the protractor as a guide. When the crosspiece is in this position the reading will be 10°.
- Now mark 15° following the directions from steps 6 and 7. Continue marking every 5° until you come to the end of the long stick. You should be up to 65° or 70°.
- Take the instrument completely apart to paint it. Paint one color between 0° and 10° and another between 10° and 15° and so on. After you have painted it draw over the numbers with black ink so you can read it.
- When completely dry, put the cross staff back together.
To Use the Navigational Cross Staff
Go outside on a clear night, hold the long piece level, with higher numbers toward you. Sight the pole star along the edge of the cross staff and the upper edge of the cross piece. Read the number closest to the cross piece to determine your latitude.
More From Layers of Learning