How to Calculate the LHA of the Sun: MCA Oral Exam Guide for OOW and Chief Mate

Direct Answer: Calculating the Local Hour Angle (LHA)

In the context of the MCA oral exam, if you are asked to calculate the Local Hour Angle (LHA) of the sun given a Greenwich Hour Angle (GHA) of 315°24.2' and an assumed longitude of 048°18.8'W, the answer is 267°05.4'.

The calculation follows the standard rule for westerly longitudes: LHA = GHA − West Longitude. Therefore, 315°24.2' − 048°18.8' = 267°05.4'.

Why LHA Calculation Matters for Your MCA Oral

For an Officer of the Watch (OOW) or a Chief Mate candidate, celestial navigation remains a fundamental pillar of professional competence. Even in an era dominated by GNSS, the MCA expects candidates to understand the principles behind a celestial position line.

The examiner asks this question not just to test your arithmetic, but to make sure you understand how a body's position is measured relative to the observer's meridian. GHA comes from the Nautical Almanac; LHA (with declination and latitude) is what you use to enter the sight reduction tables, such as NP 401, or to solve the spherical triangle by calculation. An error here produces a wrong calculated altitude and azimuth, and therefore a wrong position line.

Step-by-Step Breakdown for the Examiner

When you are sitting in front of the examiner, clarity is key. Talk through your logic:

1. Identify the GHA: State the GHA of the body (here, the sun) for the time of the sight.
2. Apply the Longitude Rule: State whether the longitude is east or west. Remember: "Longitude east, LHA greatest; longitude west, LHA least." In other words, add east longitude to GHA and subtract west longitude.
3. Perform the Subtraction: Align your degrees and minutes carefully.
   • GHA: 315° 24.2'
   • Minus west longitude: 048° 18.8'
   • LHA: 267° 05.4'
4. Final Check: The LHA must be between 0° and 360°. If your result is negative, add 360°. If it exceeds 360°, subtract 360°.

A quick sense check: an LHA between 180° and 360° means the body is east of your meridian (morning, for the sun). An LHA of 267° means the sun is about 93° east of the meridian, so this is a morning sight, which matches a GHA of 315° at a longitude of 48°W.

Common Pitfalls and How to Avoid Them

• Mixing up east and west: This is the most common failure point. Visualise your position relative to Greenwich. West of Greenwich, the LHA is the GHA minus your longitude; east of Greenwich, it is the GHA plus your longitude.
• The borrowing error: If you have to subtract more minutes than you have, borrow 1° (60') from the degrees column. Practise this under pressure.
• Using the wrong GHA: In practice, the GHA must be taken from the Nautical Almanac for the correct date and UT, with the increments for minutes and seconds applied.

Practical Tips for the OOW Oral Exam

• Show your work: Write the formula clearly. Even if you make a minor arithmetic slip, a sound method shows the examiner you understand the steps.
• Use the right notation: Use degrees (°) and minutes ('), and keep your columns aligned to avoid confusing decimal minutes with seconds.
• Be ready for variations: The examiner may ask, "What if the longitude was east?" Answer: "Then I add the longitude to the GHA, and subtract 360° if the result exceeds 360°."

Conclusion: Mastering Celestial Fundamentals

The ability to work out the LHA accurately is a basic skill for any professional mariner. Whether you are aiming for your OOW or progressing to Chief Mate, getting it right every time shows the examiner you have the foundation for safe, independent navigation.

Key Takeaways:

• Identify your longitude direction first.
• Longitude east, LHA greatest; longitude west, LHA least.
• Keep the answer between 0° and 360°.
• Explain your logic clearly to the examiner.

Updated 25 October 2026: corrected the longitude memory aid and clarified that the LHA is used to enter sight reduction tables, not the Nautical Almanac.