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.

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.