Pub. 249 Vols. 2 and 3: the drill, and the traps

Finding the right figure is five choices, always in this order:

1. Volume and latitude: the section for your AP latitude (Vol. 2 to 40°, Vol. 3 from 39°).
2. Declination group: the 0°–14° or the 15°–29° pages.
3. Same or contrary name: is the declination named the same as your latitude?
4. Column: the whole degree of declination below the body’s.
5. Row: LHA down the left side if it is 180° or less, down the right if more.

Then read Hc, d and Z; correct Hc with Table 5; turn Z into Zn with the rules on the page.

Try it

Each case below is a sight; make the five choices, then check them. Repeat until it is automatic.

The traps

• Contrary name, read from the wrong end. The contrary-name tables run upward; read downward and you land on the wrong LHA without noticing. Check the LHA in the margin of the very row you read.
• The sign of d. d is printed with its sign, and the Table 5 correction takes that sign. The book’s own advice is a good habit: glance at the next column to see whether Hc goes up or down with declination.
• Interpolating Z. No need: Z to the nearest degree is plenty for plotting.
• Using the DR. The tables are entered with the AP, a whole degree of latitude and a longitude that makes LHA whole. The intercept is plotted from that AP, not from the DR.

What to leave alone at sea

Volumes 2 and 3 carry some small tables for aircraft: corrections for the aircraft’s own motion and the body’s motion in the minutes between sights, refraction for an aircraft’s altitude, and a correction for Coriolis acceleration at aircraft speeds. A sailor needs none of them. Correct your sextant altitude with the altitude correction tables of the Nautical Almanac, as in the course, and use the running fix for the boat’s motion.