# MIT 12 215 - Modern Navigation (20 pages)

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Pages:
20
School:
Massachusetts Institute of Technology
Course:
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12 215 Modern Navigation Thomas Herring Review of Last Class Almanacs Paper and electronics Paper Almanacs Nautical Almanac Electronic Available on many web sites Homework 1 solutions 10 17 2006 12 215 Modern Naviation L07 2 Today s Lecture Dead reckoning and conventional navigation Magnetic North pole Distance measurements Use of Sextant Principles of instrument Vernier Scale reading ex Pierre Vernier 1580 1637 French mathematician Corrections needed for sextant measurements Next class we make sextant measurements 10 17 2006 12 215 Modern Naviation L07 3 Direction determination Before GPS it was difficult to determine direction of motion relative to true north From latitude and longitude determinations at two points the direction between them could be determined Geodetic maps show the directions between monuments in the ground but these monuments need to be found Some geodetic monuments triangulation stations have vanes on top that can be seen from a long distance 10 17 2006 12 215 Modern Naviation L07 4 Magnetic North Pole For navigation most common method of finding direction is relative to the magnetic North Pole The Earth magnetic field is largely a dipole field but it has higher order variations as well Notice that the direction of the dipole axis does not align with the rotation axis A compass needle tries to align with the field lines 10 17 2006 12 215 Modern Naviation L07 5 Compasses Since a compass wants to align with the field lines it will point either up or down relative to gravity when it is away from the magnetic equator True North Magnetic North Field Lines Direction compass needle points 10 17 2006 To compensate for the tendency to point up or down good compasses have weights to make the needle lay flat 12 215 Modern Naviation L07 6 Dip or Inclination The angle that the needle wants to tilt to is called the dip or inclination of the needle Normally a compass designed for use in the Northern Magnetic hemisphere can not be used in the Southern

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