Mastering Sight Reduction
A comprehensive guide to the H.O. 229 Sight Reduction & Plotter. Learn how to transform a sextant angle into a line of position using modern web tools.
Powerful Features
Everything you need to navigate by the stars, offline in your browser.
Built-in Almanac
Automatically calculates GHA and Declination for the Sun, Moon, Planets, and 57 Navigational Stars. No physical books required.
Table Simulator
A unique H.O. 229 Table Simulator that helps students understand the manual lookup process by visualizing the actual table pages.
Interactive Plotter
Visualize Lines of Position (LOPs) and perform running fixes graphically on a digital plotting sheet.
The Mathematics of Navigation
Sight reduction involves two main steps: correcting the raw sextant altitude to a true observed altitude, and solving the navigational triangle to compare it with a calculated altitude.
1. Altitude Corrections
How we get from Sextant Altitude (Hs) to Observed Altitude (Ho).
2. The Navigational Triangle
The relationship between the Pole, your Assumed Position (AP), and the Body (GP).
The H.O. 229 method solves this spherical triangle. Given LHA (angle at Pole), Latitude (side AP-Pole), and Declination (side GP-Pole), we calculate:
- Hc (Calculated Altitude): 90° - Distance(AP to GP).
- Zn (Azimuth): The bearing of the body from the AP.
3. The Intercept (a)
The "Intercept" is the difference between what you saw (Ho) and what you calculated (Hc).
Intercept = (Ho - Hc) × 60
* We plot the LOP perpendicular to the Azimuth line, offset by this distance.
Application Workflow
A step-by-step guide to using the H.O. 229 App.
Observation & Ship State
Enter your raw data. Use the "Open Almanac" button to fetch astronomical data for the exact second of your sight.
The Worksheet & AP Selection
The app calculates GHA and Corrections. Crucially, you must select an Assumed Position (AP) compatible with the tables.
Simulation & Lookup
Once your LHA is an integer, the "TABLE" button activates. This opens the H.O. 229 Simulator.
- Verify the Page matches your LHA.
- Find the intersection of your Latitude (Column) and Declination (Row).
- Review the interpolation math at the bottom of the simulator.
Plotting
Click "ADD TO PLOT". In the "Saved LOPs" table, select your sights and click "PLOT SELECTED". The app draws the AP, the Azimuth line, and your LOP. Running fixes are calculated automatically if Speed/Course are provided.
H.O. 229 App Documentation
Quick Reference
This application runs entirely in your browser. Data is saved to Local Storage. Use Night Mode for evening sights to preserve dark adaptation.
Key Features
- Integrated Nautical Almanac: Sun, Moon, Planets, 57 Stars.
- Automated Sight Reduction: Full spherical trig calculations.
- H.O. 229 Table Simulator: Simulates physical book lookup.
- AP Assistant: Auto-calculates Assumed Longitude for integer LHA.
- LOP Plotter: Visualizes intercepts and running fixes.
Usage Details
1. Data Entry
Enter Sextant Altitude (Hs), Index Error (IE), and Height of Eye (HE). For the Moon, the app automatically applies Parallax corrections.
2. The "Set AP" Button
H.O. 229 tables require an integer LHA. Your DR position likely results in a decimal LHA. Clicking SET AP FOR TABLE adjusts your Longitude slightly (minutes only) to force the LHA to the nearest whole degree.
3. The Plotter
Select multiple sights to see them on one sheet. Use pinch-to-zoom or mouse wheel to navigate the plot. Advanced LOPs (running fixes) are displayed in a lighter shade to distinguish them from simultaneous sights.