A day's work by the sun: sun-run-sun

What you do on the bridge

1. Index error first
2. Morning sight
3. Work the morning sight
4. Plot the morning line
5. Noon latitude
6. Run up and 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 and apply the index error of a sextant?Often asked

   A good answer covers:

   • Set the arc near zero and look at the horizon, or a star, through the telescope
   • Turn the drum until the direct and reflected horizons make one straight line
   • The reading is the index error: on the arc means it reads high, so subtract it
   • Off the arc means it reads low, so add it
   • Check it before every set of sights, as it can change

   Show me on the bridgeClear sky and a good horizon. Open the sextant and find the index error first.

2. Talk me through working a morning sun sight by the intercept method.Often asked

   A good answer covers:

   • Note the deck watch time, correct it to UTC with the deck watch error
   • From the almanac, the GHA and declination for that UTC
   • Choose an assumed position, and find the LHA from the GHA and the longitude
   • Correct the sextant altitude for index error, dip, refraction and semi-diameter to get the true altitude
   • Calculate the altitude and azimuth for the assumed position
   • The difference is the intercept: towards the sun if the true altitude is greater, away if less
   • Plot from the assumed position along the azimuth, and draw the position line at right angles
3. How do you get latitude from the noon sight?Often asked

   A good answer covers:

   • Work out the time of meridian passage at the ship beforehand from the DR longitude
   • Start taking altitudes a few minutes before, and take the greatest as the sun stops rising
   • Correct it to the true altitude; the zenith distance is 90 degrees less the true altitude
   • Latitude is the zenith distance combined with the declination, by their names
   • No sight reduction is needed, which is why it is so useful
4. How do you run the morning position line up to noon, and what can go wrong?

   A good answer covers:

   • Move the line parallel to itself by the course and distance made good between the sights
   • Cross the transferred line with the noon latitude for the noon fix
   • Any error in the course or speed made good, from tide or leeway, goes straight into the fix
   • Best when the two lines cross at a good angle, so the morning sight is taken with the sun well round from south

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 offshore south of Rhode Island, making twelve knots west south west, with a clear sky and a good horizon. It's mid morning, so the sun is still well to the east.

   We'll check the sextant's index error, take a morning sight and plot it, take the meridian altitude at noon for latitude, then run the morning line up to cross it. That's the classic day's work by the sun.

2. Index error first

   Open the sextant from the chart table locker. Before any sun sight, choose Index error, and turn the drum until the reflected horizon runs straight on from the direct one. Read it to within a third of a minute of arc, and note whether it is on or off the arc.

3. Morning sight

   Now choose Sun, lower limb. Bring the sun down until its lower edge just kisses the horizon, use Rock to make sure you're measuring straight down, and press Mark. The reading needs to be within one minute of arc.

4. Work the morning sight

   Open the Sight form. Correct the deck watch to UTC with its error from this morning, then find the GHA and declination in the almanac, and the LHA from your assumed longitude.

   Correct the sextant reading for index error, dip and the main correction to get the true altitude, then work the calculated altitude, the azimuth and the intercept. Every one of those must be right, and the form tells you the first step that is not.

5. Plot the morning line

   Go to the chart table and use the Astro LOP tool: the assumed position, the azimuth, and the intercept towards or away. Set the time it was taken. The line should pass within a mile of where we were at the sight.

6. Noon latitude

   Work out when the sun crosses our meridian, and be on the sextant a few minutes before. The sight counts within four minutes of meridian passage, so follow the sun up and take it as it stops rising.

   On the sight form, enter the declination, the true altitude and the noon latitude, and say whether the sun bears north or south. The latitude needs to be within one and a half minutes of arc.

7. Run up and fix

   At the chart table, draw the noon latitude with the Latitude tool. Then use Running fix to carry the morning line forward by the course and distance made good since the sight. Cross the two with the Fix tool. It should be within two miles of the ship.