TECHVEINNATARAJAMerchant Navy LMS
OP-10 practice — not an approved assessment

The altitude-correction laboratory

Learning outcomes & permitted supports
  • Apply IC, dip, refraction, semidiameter and parallax in the correct order, each with its physical sign.
  • Correct Sun, Moon and star sights — and say WHY Moon corrections are different (parallax, augmented SD).
  • Diagnose a wrong correction from its size and direction, and state the LOP impact (1′ = 1 nm).

Supports in this lab: reveal-one-cause-at-a-time stack with real ephemeris SD/HP by date, live refraction curve.

Glossary: Hs · Ha · Ho · index error · refraction · semidiameter · HP · augmentation · dip

Build the observation

From the ephemeris for 16 Aug 2026: SD 15.79′, HP 0.15′ — the Sun barely has any.

10′20′30′0°15°30°45°60°75°90°1.6′ at Ha
Why navigators avoid low sights: refraction is ~34′ at the horizon, uncertain with weather — and only ~1′ by 45°.

The correction stack — reveal one cause at a time

Sextant altitude (Hs)32° 30.0′
Index correction −2.0′32° 28.0′

Index correction: Instrument zero offset: error ON the arc is taken OFF (IC = −IE). Sign belongs to the instrument, not the sky.

My notebook — OP-10 (0)

All notes & standing →

Every interaction here is recorded as an ordered evidence trail — 0 events this attempt.