Vertical and horizontal sextant angles

What you do on the bridge

1. Vertical sextant angle
2. Distance off as a range circle
3. Two horizontal angles
4. Station pointer fix

The oral questions, and what a good answer covers

The key points are in our own words. In the course, these questions come once you have done the job.

1. How do you find your distance off a lighthouse with a vertical sextant angle?Often asked

   A good answer covers:

   • Measure the angle between the light and the shoreline or sea directly below it
   • Correct the reading for index error
   • Use the charted elevation of the light with tables or the formula to get the distance
   • Plot it as a range circle round the light, and cross it with a bearing for a fix
   • Good for distance off a danger: set a danger angle and keep the angle below it

   Show me on the bridgeStopped east of Sandy Hook. Find a lighthouse with a charted height.

2. Explain a fix by horizontal sextant angles. When does it fail?Often asked

   A good answer covers:

   • Two angles between three charted marks, with a mark shared in the middle
   • Each angle puts the ship on a circle through its two marks
   • Set the angles on a station pointer, or plot the circles, and the ship is where they cross
   • No compass is needed, so compass error does not come into it
   • It fails when the ship and the three marks lie on one circle: the circles coincide and there is no cut
3. What are the sources of error in a vertical sextant angle?

   A good answer covers:

   • The base of the light is not always visible, especially when the shore is far off
   • Height of eye matters little when close, but adds error when the object is near the horizon
   • Index error not applied

How the lesson runs

The tutor's narration from the lesson, step by step. On the bridge, each control lights up as it is named.

1. Brief

   We're lying stopped east of Sandy Hook, with plenty of charted marks in sight. The sextant isn't just for the sun. Turned on its side or held upright, it measures angles between things on the shore far more finely than any compass.

   We'll measure the vertical angle of a lighthouse to find our distance off, then two horizontal angles between three marks for a fix. Check the index error first: this sextant reads low.

2. Vertical sextant angle

   Open the sextant and choose Vertical angle. Pick a light with a charted height, then bring the top of the light down to the shoreline beneath it and press Mark. The reading should be within one and a half minutes of arc.

3. Distance off as a range circle

   Correct the angle for index error, then work the distance off from the light's charted height. At the chart table, use the Range tool, click that light, and enter the distance. It needs to be within a tenth of a mile.

4. Two horizontal angles

   Now choose Horizontal angle. Pick two marks, left one first, and bring one image onto the other, then Mark. Then do a second pair that shares the middle mark with the first.

   Choose marks well spread, and avoid three that lie on a circle through the ship, or the two circles won't cut. Each angle needs to be within one and a half minutes of arc.

5. Station pointer fix

   At the chart table, use the HSA tool for each angle: the left mark, the right mark, then the sea on our side, and the angle corrected for index error. Then use the Fix tool, tick both, and click where they cross. It should be within a tenth of a mile.