Treat each practice sight as one unbroken chain: record, correct, reduce, plot, advance. The skill the Ocean Yachtmaster level of work builds is keeping every assumption in that chain consistent, so one error never hides inside a plausible-looking plotted position.
Why a celestial fix inherits your dead reckoning assumptions
A single sight gives one line of position, not a fix. The fix appears only where lines cross, and each line is only as trustworthy as the dead reckoning and corrections behind it. Learn to audit assumptions before trusting any plotted position.
Distinguish three named concepts from the outset. Dead reckoning (DR) is the position projected from the last known fix using course and distance run, ignoring current. The estimated position (EP) adjusts that DR for estimated tidal stream or ocean current. A line of position (LOP) is the locus on which the vessel lies after one sight. A running fix combines an earlier LOP, advanced along the vessel's track, with a later one. Naming these precisely prevents the classic error of treating a DR plot as if it were evidence.
Practice the audit habit: after plotting any fix, write beside it what it assumes — the sight time accuracy, the horizon and index error used, the current applied between sights. Compare this with a coastal-style GNSS fix, which arrives as a position without your having to declare assumptions. Ocean work inverts that: the plotted fix is only as good as the declared inputs, so your study should train you to state and check inputs, not just to produce a dot on the chart.
The intercept method end to end: Ho, Hc, and the toward/away decision
The intercept method compares observed altitude (Ho) with computed altitude (Hc) from an assumed position. The difference and its direction set the intercept. Master the comparison rule mechanically so it never depends on intuition about how the body looked.
Trace the full sequence: take the sight time and apply it to the nautical almanac to get the body's Greenwich hour angle and declination; choose an assumed position near the DR that makes the local hour angle a whole number of degrees; work the sight reduction tables to obtain Hc and the azimuth (Zn); correct the sextant altitude through dip, refraction and body-specific corrections to get Ho. The intercept is the numerical difference between Ho and Hc, drawn along the azimuth from the assumed position.
Fix the direction rule with the standard mnemonic: Ho Mo To — if Ho is More than Hc, the intercept goes Toward the body's geographical position; if Ho is less, it goes Away. The logic is physical: a larger observed altitude means the body appeared higher than predicted from the assumed position, so you are closer to the point below it than assumed. Rehearse that logic aloud until the direction decision is automatic, because intuition about whether the sun 'looked low' is unreliable at sea and in exam conditions alike.
Altitude corrections in the right order, and where sun, moon and stars differ
Corrections follow a fixed order — index error, dip, refraction, then body-specific terms such as semidiameter and parallax. Signs matter more than memorised values. Compare the correction sets across bodies so you know which terms switch on and off.
Index error is read from the instrument itself and applied with its observed sign (off the arc positive, on the arc negative, or consistently by whichever convention your tables state). Dip depends on height of eye and is always negative. Refraction is always negative and grows near the horizon. Semidiameter applies only to sun and moon because their discs have measurable radius: add for the lower limb, subtract for the upper. Parallax in altitude applies mainly to the moon, whose proximity makes it significant, and is positive.
Build a comparison table in your notes: stars need only index error, dip and refraction; the sun adds semidiameter; the moon adds both semidiameter and parallax. Drill the order until it is reflex, because reversing, say, dip and semidiameter in a lower-limb sun sight produces a small but real error in Ho, and an error made early in the chain propagates into the intercept and then the plot. Practise with pre-printed sight proformas so the sequence is identical every time.
Worked scenario: the intercept drawn the wrong way
A sun sight taken near 40 degrees south shows Hc greater than Ho by 10.0 minutes. The intercept is 10 nautical miles away from the sun's geographical position. Drawing it toward shifts the whole line by 20 miles and contaminates every later fix.
Scenario: your DR is 36°10'S, 174°45'E, bound south. You reduce the sight and find Hc 44°22.6' and Ho 44°12.6'. Hc is larger, so by Ho Mo To the intercept runs 10.0 minutes away from the body along azimuth 028°. The plausible mistake is to reason that the sun looked low, so the boat must be nearer the sun than assumed, and draw the line toward. That reasoning reverses the physics: a lower-than-predicted altitude means you are farther from the geographical position, not nearer.
The better decision is mechanical: compare Ho with Hc, apply the mnemonic without consulting instinct, and plot. Mark the assumed position, lay the azimuth 028°, measure 10 nm away, and draw the LOP perpendicular to the azimuth. Why it matters: the two possible plots sit 20 nm apart, and any second sight crossed against the wrong one yields a fix that is confidently wrong rather than obviously wrong. Make the comparison step a written line in your reduction — 'Ho 44°12.6' < Hc 44°22.6', intercept 10.0 Away' — so the decision is auditable.
Worked scenario: advancing a sun line without losing the current
A running fix advances the morning LOP along the vessel's track — including current — to the later sight time. The common slip is transferring the line from the DR run alone, which quietly misplaces the crossing by the current's set over the interval.
Scenario: a 0900 sun sight gives an LOP; by 1500 the boat has steamed 48 nm on course 080°T, and the tidal stream or current is estimated at 1.5 knots setting 040°. Over six hours the current contributes about 9 nm toward 040°. The advance is therefore the resultant of the 48 nm steam and the 9 nm drift: about 55.2 nm on a course of roughly 074°T. The plausible mistake is advancing the first line by the 48 nm run alone, treating the current as someone else's problem for the fix.
The better decision is to advance the entire first LOP, parallel to itself, by the combined motion vector — steam plus current — drawn from the 0900 DR position, then plot the 1500 LOP and read the crossing as the running fix. Why it matters: the 9 nm of unapplied current moves the advanced line bodily, and because LOPs from the same body at different times are usually nearly parallel, a small transfer error produces a poorly conditioned crossing with no visual warning. Practise drawing the two-component advance triangle explicitly every time.
Route selection: great circle, rhumb line, and when composite sailing earns its complexity
Ocean route choice is a distance-versus-course trade. Great circles shorten distance but change course continuously; rhumb lines hold a steady course but run longer; composite sailing caps the track at a limiting latitude. Choose per voyage, not per habit.
Apply the table with a concrete comparison: compute the great circle initial course, distance and vertex for a sample voyage, then compare that distance with the rhumb line distance on the same chart. The saving grows with latitude difference and east-west extent, which is why the decision belongs to the voyage, not to a rule of thumb. Practise both computations for identical endpoints so you can justify a choice with numbers rather than preference.
Connect route work back to the fix work above: a great circle track changes course daily, which means the DR advance between sights changes with it, and a running fix computed on a stale course inherits that error. In mixed practice scenarios, deliberately couple the two skills — plan a great circle leg, then advance a sun line along its changing track — because ocean passage-making rarely presents them separately.
| Criterion | Great circle | Rhumb line | Composite great circle |
|---|---|---|---|
| Distance | Shortest surface path between points | Longer than the great circle | Nearly great circle distance, with a constraint |
| Course behaviour | Course changes continuously along the track | Single constant course | Great circle legs joined at a limiting latitude |
| Chart workload | Plotted as a curve on Mercator; needs vertex and initial course work | Straight line on Mercator; simplest plotting | Most involved: two arcs plus a parallel-sailing leg |
| Typical choice | High-latitude or long east-west passages where savings dominate | Short legs or trades where steady steering outweighs distance | Passages that would carry the great circle into undesired high latitudes |
A daily reduction exercise with a self-check rubric, plus an adaptable preparation sequence
Reduce one full sight daily under time pressure and score it against a written rubric. Then sequence your weeks: tables fluency, full reductions, plotting and advancing, route computation, mixed scenarios, timed review with an error log.
Exercise: take three simulated sun sights at two-hour intervals with a stated assumed sight accuracy of about 2 nautical miles. Reduce all three, advance the earlier lines by a declared run and current, and plot a running fix. Self-check rubric, scored as pass or revise per item: (1) corrections applied in order with correct signs; (2) assumed position chosen so the local hour angle is a whole degree; (3) intercept direction matches the Ho-versus-Hc comparison; (4) LOP drawn perpendicular to the azimuth within about 1 nm of plot precision; (5) the three resulting lines are nearly parallel and their mutual scatter does not exceed roughly the stated sight accuracy; (6) the advance includes current. Expected observation: parallel lines with small scatter mean your reductions are internally consistent; a rogue line flags a sign or transfer error, not bad luck.
Adaptable sequence: week one, almanac and sight reduction table fluency until lookups stop being the bottleneck; weeks two to three, one end-to-end reduction daily using the rubric, logging every error category; week four, plotting and advancing drills including two-component current advances; week five, great circle and rhumb comparisons for sample voyages; week six, mixed scenarios coupling route and fix work, then timed sets. Readiness checks before you consider the topic closed: reduce a sun sight to a plotted LOP with no notes; state aloud the sign of every correction; produce a current-corrected running fix; compute a great circle initial course and distance; and show an error log whose recent entries show your recurring categories shrinking rather than repeating.
References and further reading
Use these references to explore the concepts and check the latest information from the relevant organizations.