Celestial Navigation: User Manual
This app helps to calculate the line of position for a sextant
reading. I assume, that you are familiar with the use of a
sextant and knows the basic calculations to reduce the sextant
sight.
Entering a Sextant Sight
This app calculates the line of position based on the Marcq-St.-Hilaire method.You will need the following information for a sight:
- Assumed Position
- The Saint-Hilaire method needs an assumed postion since it gives basically the difference of the assumed position to the real position. The result will be the distance from the assumed position to the actual line of position, and the angle of the line of position.
- The assuemd position has not to be very accurate, taking last days position is usually good enough, but of course the more decent way is to calculate the dead reckoned position.
- If you don't care much about this position, the app allows to just use the current GPS position.
- Course and Speed
- Several sights are usually not taken at the same time. To correct for the movement of the ship between the sights you can give a course and the speed of the ship. This effects only the plotting of the lines of position on the map.
- Celestial Body
- Which object was measured? Use the star finder if not sure...
- Sextant reading
- This is the basic input for the calculation. Be sure to divide it by two if you use a artifical horizon.
- Index error
- The index error is the missreading of the sextant due to misalignment of the mirrors. It is determined e.g. by putting the sextant to zero degrees and matching the image of the horizon with both mirrors. Index errors up tu 10' are usually considered as acceptable, above that you should put some work into aligning the mirrors. Be sure to determine the SIGN of the error!
- Time of measurement
- The accurate time of the measurement is neccessary. The earth rotates with 1/4 nm per second at the equator, so ususually you should measre the the time with this accuracy. The app offers the option to set to the current time for a fast measurement.
- Conditions of reading
- Most important is the height of your eye. An error of one meter can cause a nautical mile difference in position! You can also give temperature and air pressure to improve the calculation of the athmospheric refraction, but this is only a small effect for small measured altitudes. It's better to restrict yourself to altitudes above 15°.
Calculation
This tab cacluates the line of position. It is given as- Intercept
- The distance of your line of position and the assumed position in nautical miles. "Towards" means, the difference is towards the assumed position, "Away" is away from the assumed position.
- Azimuth
- The angle pointing to the celestial body you have measured. The sight can be saved and plotted on the map.
Star Finder
Just point your device to the sky to identify stars.Manage your Records
In the list of Sights you can chose with the slider, which sight is plotted on the map. Tap on the sight to change the input values. If you don't change the body or the time, saving the values will overwrite the old sight, otherwise you will generate a new entry. You cna delete them again, of course.For further support contact celestialnavigation@merkel.de