Sunrise Longitude Fix

Sight the sun at Hs ≈ 0° as its center bisects the horizon — FairWinds turns that timing into a longitude fix.

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When

About ±10 minutes around sunrise on the Daily Schedule (sights within about ±20 minutes still classify as sunrise).

Check Solar Times → Sunrise for the predicted UTC (based on your position reference / AP). Timing is tight — a few seconds matter.

In FairWinds (Guided)

  1. Open the Sky Tool → Workbook → note Sunrise under Solar Times
  2. Near sunrise, tap Sextant and shoot the Sun with the disc half-risen (center on the horizon, Hs ≈ 0°)
  3. Open the Sun tab — your sight should show a RISE badge
  4. Select that sight so the Sun Worksheet switches to Sunrise / Sunset Fix
  5. Review the longitude calculation. It is a chronometer sight, the classic way to get longitude from a timed sun:
    • Sun declination and Greenwich hour angle from the sky engine at your sight’s UTC (the engine is the almanac)
    • Hs, Refraction (about 34′ at the horizon), and Ho — the sun’s geometric altitude, about −0°34′ for a sunrise sight
    • Hour angle from your latitude, the declination and Ho, then longitude
  6. Tap Save to Log

The fix is saved as a Lon fix and appears in Positions.

The latitude used comes from your position reference / AP, so a good noon or star latitude makes this longitude better.

Why it matters

This is a quick longitude check, not a full lat/lon fix. Use it to gut-check whether your DR longitude is still sane before the morning sun work.

Expert mode

The auto longitude worksheet is hidden. Reduce it as a chronometer sight:

  1. Correct Hs → Ho: IC, the prefilled refraction (about −34′ at Hs ≈ 0°), parallax (+0.1′); no SD for a center sight; dip stays 0. Ho comes out around −0°34′.
  2. From the almanac at the sight’s UTC: the sun’s declination (δ) and GHA.
  3. With your DR latitude φ: cos t = (sin Ho − sin φ sin δ) / (cos φ cos δ)
  4. Sunrise: the sun is east of you, so LHA = 360° − t.
  5. Longitude = LHA − GHA (east positive; add or subtract 360° to land between 180° W and 180° E).
  6. Fixes tab → Enter Fix → Longitude Fix.

Why not just compare with the almanac’s sunrise time? The printed sunrise is for the sun’s upper limb appearing, rounded to the minute, and one minute of time is 15′ of longitude. It is a fine rough check, but the chronometer sight is what gets you within a mile or two.

Refraction details: Altitude corrections and refraction.

Optional — pairs well with a later noon or star fix.