Last week's class described:
We also started to get some physical practice with the sextant, the tool of choice for measuring angles.
- how to use a bearing compass,
- how to plot the resulting measurement as a line of position (LOP) on a chart;
- how measuring the angle from bottom to top of a landmark of known height gives the distance from it,
- how that distance can be plotted as a circle of position (COP) on a chart,
- how 2 measurements can be combined to give a point of position.
The predicted weather for 2/22 is excellent for outdoor observations, and so the plan for class is to go to Pioneer, equip ourselves with sextants and bearing compasses, and start getting measurements, from on board, and from Pier 16. We can experiment with seeing magnetic influences, and establishing how (un)reliable the bearing measurements are. Most importantly, we can make a variety of measurements with the sextant, such as height of various nearby landmarks, angular distance horizontally between various pairs of landmarks, and perhaps after dark, some sky observations. Hand-eye coordination while using the sextant can only be learned by experience, so let's get lots of practice.
Plotting the results of the various measurements will also be an activity; you can print a copy of this chartlet (without the steeple marked), or this (new, with steeple) if you don't have a copy from last week. Depending on time, we can cover the plotting of another type of COP (or if not enough time, you can still read about it here):
All the chart plots so far depend on seeing landmarks. However, the most famous use of the sextant is at sea, out of sight of land, making measurements of objects in the sky - "celestial navigation". The sky on Saturday 2/22 wil start to get dark around 1800, and any hardy students interested in further practice using the sextant can try measuring the angles from a star or planet to the "horizon" or other visible object, and from star to star. Time will probably not permit covering the next topic below during Saturday's class, except for brief mention. Perhaps a 3rd class some time in the future will be possible. Meanwhile, the Web links below will serve as an introduction.
- Recipe to plot a circle of position from the angle between 2 visible landmarks
- Geometry behind the "recipe" above; navigation as practical geometry - wealth of ed program material
Basic Tools, Concepts, and Measurements of Celestial Navigation
- Basic tools: sextant, watch, almanac, Pub. 229 or equivalent; organized work sheet, plotting paper, plotting tools (straightedge, dividers, protractor)
- Geographical Position of a celestial object: changes with Earth's rotation; tabulated in Almanac; interpolation
- Zenith and horizon (ideal)
- Zenith distance of object, as seen from non-GP, and object's height above horizon
- Sextant measurement (noting exact time!) of object's height (ideal case: spherical Earth, no atmosphere, eye at sea level, object VERY far away)
- Ideal circle of position - plot on globe, with radius in angular form
- For fix, need 2nd COP from 2nd measurement