Introduction
Today, 2/15/2014, is the first of 2 classes examining the usefulness of various angle measurements from shipboard, and the tools for these measurements, such as the protractor, pelorus, bearing compass, and especially the sextant. We'll start with the bearing compass, easiest to get a measurement with, and easiest to represent the result of the measurement on a chart. We'll then briefly use a protractor, which makes angle measurement conceptually clear, but is inaccurate and awkward in practice. We'll then focus on the sextant, which is so central to traditional navigational methods that it has become a universally recognized icon for the whole maritime industry.
Measuring Angles Associated with Landmarks
- The bearing compass:
- what it looks like, and how it works
- how to get the angle between a line of sight and Magnetic North (giving us a "Line of Position" LOP)
- how to plot the LOP on a chart - use compass rose and parallel rules to draw a line through the landmark, with same angle to magnetic north as measured; extend in opposite direction, where observer must be
- Crude horizontal angle measurement with a protractor (or on board ship, a pelorus) - physically difficult, and not accurate
- Measuring a vertical angle with a protractor and plumb line - lucky to get within .5 degree
- Circle of Position (COP) from angle of elevation of top of object of known height (good measurement of angle is assumed)
- Generalized Concept of LOPs and COPs - position in 2D requires 2 measurements
- Plotting position from 2 LOPs, 2 COPs, or a COP and a LOP
- Better angle measurer than a protractor - a sextant!
The Sextant
- Its anatomy and how it works (including parts vulnerable to damage, and precautions to be taken)
- eyepiece (telescope), shade glasses, horizon glass, index mirror
- index arm, arc (index scale), limb, limb teeth, tangent screw, release; IMPROPER USE OF RELEASE CAN DAMAGE GEAR TEETH!
- micrometer drum, vernier
- How to use it (general concepts)
- focus the telescope
- determine if shades will be needed, and put in place if so
- unclip the release for changing the angle by a large amount - MAKE SURE TO PINCH IT TIGHTLY ENOUGH THAT THE WORMGEAR DOESN'T DRAG ON THE LIMB TEETH!
- hand-eye coordination is now needed to move the index arm as you slowly change the aim of the eyepiece - PRACTICE!!
- when visually at desired angle, reclip release to reengage wormgear teeth with limb teeth - GENTLY! and MAKE SURE THEY MESH!
- if expected measured value is known approximately, can preset index arm to that value
- turn micrometer drum while looking through telescope, until exact lineup achieved
- How to use it (horizontally):
- set to zero
- hold in right hand, with hand below sextant, and sextant plane horizontal
- aim at rightmost landmark
- slowly move aim of telescope to leftmost object, while moving index arm to keep right object in field of view (unclip release, move arm)
- when nearly there, clip in release and switch to micrometer drum; proceed as above
- How to use it (vertically):
- "pendulum-swinging" a/k/a "rocking the sextant" for shortest distance from point to line
- brief geometry explanation
- hand-eye like that above
- How to read results: index arc, micrometer drum, vernier; instrument errors
- Adjustments (e.g. mirrors) and calibrations (including parallax vs. index error)
- Proper stowing: fold up filters, set arm to about 65, shorten telescope, wrap up lanyard, carefully fit into box