What Are HO 249 Sight Reduction Tables?
HO 249 — formally titled Sight Reduction Tables for Air Navigation and published by the US National Geospatial-Intelligence Agency — is the most widely used set of pre-computed tables for turning a sextant observation into a line of position (LOP). Originally designed for rapid celestial fixes in the cockpit, the tables found a permanent home on offshore sailboats because they trade a negligible amount of accuracy (worst case ±1 arc minute) for a dramatic reduction in arithmetic.
The tables come in three volumes. Volume 1 covers selected stars and is used for star identification and reduction at twilight. Volumes 2 and 3 cover the Sun, Moon, planets, and any star — Volume 2 for declinations 0° to 29°, Volume 3 for declinations 30° to 90°. For the vast majority of Sun sights during an ocean passage, you will live in Volume 2.
Each page of Volumes 2 and 3 is entered with three arguments: your assumed latitude (whole degrees), the local hour angle (LHA, whole degrees), and the declination name (same or contrary to latitude). The table returns three values: Hc (computed altitude), d (a correction factor), and Zn (azimuth). That is the entire lookup. The rest is arithmetic you can do on one line.
The Setup: What You Need Before Opening the Table
Sight reduction does not begin with HO 249. It begins with two prior steps that produce the arguments you will enter into the table.
Step 1 — Observed altitude (Ho). You have taken a sextant observation of the Sun's lower limb, recorded the time in UTC, and corrected the sextant altitude (Hs) for index error, dip, and the altitude correction from the Nautical Almanac. The result is Ho, your observed altitude.
Step 2 — GHA and Declination from the Nautical Almanac. Using your UTC time, you look up the Sun's Greenwich Hour Angle (GHA) and declination (Dec) for the hour, then add the increments for minutes and seconds. You now have an exact GHA and Dec.
Step 3 — Choose an assumed position (AP). This is the critical bridge between the almanac and HO 249. You choose an assumed latitude equal to the nearest whole degree of your DR latitude. Then you choose an assumed longitude that makes the LHA come out to a whole degree. Since LHA = GHA - West Longitude (or GHA + East Longitude), you adjust your assumed longitude by the fractional part of GHA so the subtraction yields a round number.
At the end of this setup you have:
- Ho (observed altitude, corrected)
- Assumed Latitude (whole degrees)
- LHA (whole degrees)
- Declination (degrees and minutes, with name N or S)
Worked Example: Sun Observation in the North Atlantic
Let us work through a real observation. You are sailing from Halifax toward Bermuda. Your DR position is 43° 12' N, 062° 38' W. It is the afternoon of June 21. You take a Sun lower limb sight and, after all corrections, obtain:
- Ho = 61° 14.2'
- GHA Sun = 327° 46.3'
- Dec Sun = N 23° 26.1'
Choosing the Assumed Position
Assumed latitude: 43° N (nearest whole degree to 43° 12' N).
For LHA to be a whole number, you need the assumed longitude to absorb the 46.3' fractional part of GHA:
Assumed longitude: 062° 46.3' W
LHA = GHA - Assumed Longitude (West) LHA = 327° 46.3' - 062° 46.3' = 265°
Check: LHA is a whole degree. Good.
Entering HO 249 Volume 2
Open Volume 2 to the page headed Lat 43°. Run your finger down the LHA column to 265. Now look at the declination columns. Because your latitude is North and the Sun's declination is North, this is a "same name" case — you use the left-hand (Same) block of columns.
At the row for LHA 265 you find:
| Hc | d | Zn |
|---|---|---|
| 61° 03' | +48 | 237 |
These three values are your raw table output.
Applying the d Correction
The value d = +48 tells you that for each additional minute of declination beyond the tabulated whole degree, the altitude changes by that amount (in arc minutes, scaled by the fraction). Your declination is N 23° 26.1'. The whole-degree part is 23°. The minutes part is 26.1'.
Turn to the "Table 5" correction table at the back of HO 249 (or the interpolation table printed on the inside cover). Enter with d = 48 and declination increment = 26'. The correction is +21'.
The sign of d was positive, and the declination name is the same as latitude, so the correction is added:
Hc (corrected) = 61° 03' + 21' = 61° 24'
Computing the Intercept
The intercept is the difference between what you observed and what the tables predict:
Intercept = Ho - Hc = 61° 14.2' - 61° 24.0' = -9.8 nautical miles
The classic mnemonic is "Computed Greater Away" (CGA) — or equivalently, "Ho Mo To" (Ho more, toward). Since Hc is greater than Ho, you plot the intercept 9.8 NM away from the Sun's azimuth.
The Azimuth
The table gave Zn = 237°. This is the true bearing from your assumed position to the Sun's geographical position (GP). Your line of position is drawn perpendicular to this azimuth, passing through the intercept point.
Understanding "Same Name" and "Contrary Name"
HO 249 splits each page into two blocks: Same and Contrary. "Same name" means your latitude and the body's declination are both North or both South. "Contrary name" means one is North and the other is South.
This matters because it changes the geometry of the celestial triangle. When the Sun is on your side of the equator (same name), altitudes are generally higher and the d corrections behave differently than when the Sun is on the opposite side.
Getting this wrong is the single most common HO 249 error. Before you touch the table, write down: Lat N, Dec N → Same. Or Lat N, Dec S → Contrary. Make it a habit.
Handling the d Correction Sign
The d value from the table can be positive or negative. A positive d means altitude increases with increasing declination (for same-name cases at moderate hour angles, this is typical). A negative d means altitude decreases. The sign is printed in the table — do not guess it.
The interpolation table (Table 5) gives an unsigned correction. You apply the sign of d from the main table. Then you add or subtract from Hc. This is straightforward but demands attention; a sign error flips your intercept direction by double the correction value.
Plotting the Line of Position
On your plotting sheet:
- Mark the assumed position (43° 00' N, 062° 46.3' W).
- From the AP, draw a line in the direction of Zn (237° true).
- Along that line, measure 9.8 NM away from the azimuth direction (because Hc > Ho).
- At that point, draw a line perpendicular to the azimuth. This is your LOP.
Your true position lies somewhere along this line. Combine it with a second sight (taken at a different time or from a different body) and you have a fix.
Common Mistakes and How to Avoid Them
Wrong volume. If the declination is 23° N, you need Volume 2 (0°–29°). If it were 31° N, you would need Volume 3. Check before opening.
Wrong LHA. Verify that GHA minus assumed longitude equals a whole number. If it does not, your assumed longitude is wrong.
Same vs. Contrary. Write it down explicitly before entering the table.
Forgetting the d correction. Without it, your Hc can be off by up to 30' or more. That is 30 nautical miles of error.
Plotting toward instead of away. Use the CGA mnemonic. If in doubt, ask: is my observed altitude higher or lower than predicted? Higher means you are closer to the body's GP; lower means you are farther.
Why HO 249 Endures
Despite the availability of calculator-based methods and apps, HO 249 remains the standard for offshore certification exams worldwide. The RYA Yachtmaster Ocean uses AP 3270 (the UK reprint of HO 249). Transport Canada and the US Sailing Offshore certifications expect proficiency with these tables. They require no batteries, no software updates, and no screen. A waterproof copy of Volume 2 and a Nautical Almanac will outlast every electronic device on your boat.
More importantly, working through HO 249 by hand forces you to understand the geometry of the celestial sphere. You learn what declination means physically, why LHA determines which side of the sky a body appears on, and how altitude and azimuth relate to your position on the surface of the earth. That understanding makes you a better navigator even when you do have electronics.
Ready to Practice?
Reading about sight reduction is useful. Doing it repeatedly is what builds competence. Sailcasted generates realistic Sun, star, and planet observations along actual offshore routes — complete with almanac data, plotting sheets, and step-by-step feedback.