Instructions for use
This GeoGebra animation replaces the reduction tables used in astronomical navigation.
Step 1: Enter your estimated position
Use the first two sliders:
- Latitude (degrees, minutes and negative, if southern hemisphere), and
- Longitude (degrees, minutes and negative, if east of Greenwich).
=> Point A moves on the Earth’s sphere to represent the estimated position of the navigator.
Step 2: Position the substellar point (or Ground Point) at the time of observation
To find out these values, we use an almanac or an astronomical software.
Use the last two sliders:
- Declination (degrees, minutes and negative, if the Sun is in the South)
- Greenwich Hour Angle (GHA)
=> The GP point moves on the Earth’s sphere.
Result
Three data are obtained:
- The calculated height (Hc): this is the calculated height that is compared to the height observed (at the sextant), to know the distance of the line of position from point A.
- The azimuth: this is the direction in which the Sun is in relation to the observer. The line of position must be drawn perpendicular to this azimuth, at the distance calculated just above.
- Local hour angle (LHA): this is not a necessary parameter with this method. However, it is an input data in the reduction tables, so it is necessary to verify the calculation, if we have a table. Please note: if the LHA is more than 90 degrees (in absolute value), the Sun is not visible to the navigator.
Click on the sphere to rotate it, to see A and GP at the same time. If this is not possible, it is because it is night where the observer is. You have to choose another star to take stock!