Sunset Longitude Fix

Same horizon timing as sunrise — shoot the half-set sun, save a Lon fix from the Sun Worksheet.

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When

About ±10 minutes around sunset on the Daily Schedule (classification window about ±20 minutes).

Use Solar Times → Sunset as your cue. Optional step — easy to combine with the PM twilight stars that follow.

In FairWinds (Guided)

  1. Open the Sky Tool → note Sunset under Solar Times
  2. Near sunset, Sextant the Sun with the disc half-submerged (Hs ≈ 0°)
  3. Sun tab → select the sight with the SET badge
  4. Sun Worksheet shows Sunrise / Sunset Fix: sun declination and GHA from the sky engine, Hs, Refraction (about 34′), Ho, the hour angle, and your longitude — a chronometer sight, as described on Sunrise Longitude Fix
  5. Tap Save to Log

Appears in Positions as a Lon fix. Compare it with your sunrise longitude if you took one — the difference is a quick read on how well your DR held through the day.

As at sunrise, the latitude for the solution is the one stamped on the sight from your position reference. By sunset this is where a static reference hurts most: a reference still sitting on a morning fix can be a day's run out in latitude, and that error comes through into the longitude. Keep the reference on GPS (Guided) or a projected DR mark, or take the sight right after a noon fix with a new DR laid from it. There is no alignment step under 5°, so Acquire is the mark; use the 1s sky clock. See Position reference, DR and the run.

Why it matters

A fast end-of-day longitude check before twilight. It will not replace a star fix, but it is cheap insurance on longitude before dark.

Expert mode

Same capture and the same chronometer sight as sunrise, with one change: at sunset the sun is west of you, so LHA = t (not 360° − t). Correct Hs first (IC, prefilled refraction of about −34′, parallax +0.1′; dip stays 0), then Longitude = LHA − GHA. Enter it with Enter Fix → Longitude Fix.

Optional — last sun observation before the stars come out.