The Sky Tool
Your in-game sextant for taking celestial fixes.
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Overview
The Sky Tool is your on-board sextant and navigation station. It renders a real-time star field powered by the Stellarium engine and lets you take sights of the sun, moon, planets, and 57 navigational stars.
Opening the Sky Tool
From any race, tap the Sky button in the bottom navigation bar. The Sky Tool opens full-screen with the star field centered on your current heading. Your boat's position and the current UTC time are shown in the top bar.
Taking a Sight
- Find a body — pan the sky or use the Best Bodies panel to see which stars, planets, or the sun are well-placed right now.
- Enter sextant mode — tap the Sextant button in the top right to open the sight-taking view.
- Acquire — center the reticle on your target and lock it.
- Align — use the drum to bring the body down to the horizon line. For a sun sight, bring the bottom edge of the orange disc (the lower limb) to just touch the top of the blue horizon line. Your alignment accuracy determines the error on your observed altitude (Hs).
- Mark and save — confirm the sight. It records Hs (in DMS), UTC to the second, your GPS position (Guided mode) or DR position (Expert / No GPS mode), and environmental parameters.
Sight and Fix Quality Ratings
After every interaction you get a quality rating. There are two:
Sight quality is shown immediately after you mark a sight. It measures how accurately you brought the body down to the horizon line — the alignment error in arcminutes. One arcminute of error equals roughly one nautical mile of position error in your LOP.
Fix quality is shown when you cross two or three LOPs to compute a fix. It measures the crossing angle between the LOPs — a shallower crossing stretches the uncertainty ellipse and makes the fix less reliable.
| Rating | Sight (alignment error) | Fix (LOP crossing angle) |
|---|---|---|
| Excellent | < 2 arcmin | ≥ 70° |
| Good | 2 – 4.9 arcmin | 40° – 69° |
| Fair | 5 – 14.9 arcmin | 20° – 39° |
| Poor | ≥ 15 arcmin | < 20° |
For the best fix quality, choose bodies whose azimuths are 60–120° apart. Two bodies at right angles give a near-ideal 90° crossing; three bodies spread ~120° apart form a tight triangle fix.
Why doesn't an Excellent fix show my exact position? The FairWinds drum reads to 0.1 arcminutes — matching a real vernier micrometer sextant. A perfect alignment scores 0.0', and an Excellent rating requires less than 2'. In Guided mode your GPS position is used as the assumed position, so the only error in the fix comes from your sight alignment. An Excellent sight with a Good or Excellent crossing angle will typically land within 1–2 nautical miles of true position. That's not a FairWinds limitation — in real life, a 3-body star fix with a quality sextant and sharp technique producing a triangle under 1 NM is considered exceptional. Elite offshore navigators shooting twilight stars would be thrilled with that result. In No GPS / Expert mode your DR accumulates error on top, so a stale DR will shift the fix further. If your fix is off by more than a few miles, check your azimuth spread and make sure your DR is recent.
Navigation Modes
FairWinds offers two modes that control how much of the math the game does for you. Switch anytime from the Guided / Expert badge in the top bar, or from Settings.
| Guided | Expert | |
|---|---|---|
| Who does the math | FairWinds computes Hc, Zn, intercept, LOPs, and fixes | You reduce externally and enter the fix manually |
| Assumed position | Boat GPS | Your AP / DR choice (GPS hidden) |
| Sky time | Scrub freely | Locked to real time |
| Sextant mag limit | Yes — dimmer stars are blocked | Same |
| Instrument precision & index error | Ignored | Live — see below |
| Altitude corrections (Hs→Ho) | FairWinds applies them | You enter them (Correct Hs worksheet) |
| Sight reduction UI | Worksheets, Compute Fix, plot | Copy sight data; reduce externally |
Guided — FairWinds does the math
In Guided mode the sextant still teaches acquire / align / mark, but the instrument catalogue’s precision and drift do not affect the reading. FairWinds applies the altitude corrections and computes Hc, Zn, intercept, LOPs, and fixes for you. Pick any sextant for flavor and star mag limit; the fix quality is driven by your alignment and LOP geometry.
Expert — live instrument + chart-table workflow
In Expert mode FairWinds acts as your instrument suite: it records raw data; you apply corrections and reduce the sight.
Choose a sextant (picker in the sky UI). Mag limit always applies. In Expert, two more stats matter:
- Precision — marked Hs is quantized to that instrument’s least count
- Drift — creates a persisted index error (IE) that wanders over time (′/week). Instruments that “hold” (e.g. FairWinds default, C. Plath) stay at IE = 0′; others (Davis, Mark II, Astra) seed and drift
Measuring index error and applying IC
IC is tuned on the sextant (one place), not retyped on every sight.
- Open the sextant picker — each instrument has a Last IC field (Expert only), or enter Sextant mode → Check IE.
- For Check IE: coincide the two images on the horizon → Mark! → Save to sextant. That writes IC = −IE into that instrument’s Last IC.
- Or type Last IC directly on the picker row.
- Take normal sights (Acquire → align → Mark! → Save). Saved Hs is the raw drum reading (includes hidden IE + alignment, quantized). The sight’s env IC is stamped from Last IC.
- Open Correct Hs — IC is prefilled from Last IC. Add dip / other lines as needed → apply to write Ho. Applying the worksheet also updates Last IC.
- Copy Hs/Ho, UTC, and AP into your external reduction tool; enter the fix via Enter Fix / Set DR.
If Last IC is wrong or unset, LOPs shift by roughly the residual IE in arcminutes (≈ NM). Re-check after long gaps or when switching to a drifting instrument.
Navigation Modes — full comparison →
Workbook
Tap Workbook to open the Cel Nav Workbook drawer:
- Solar Times — Sunrise, Sunset, and Mer. Pass. for your position reference / AP
- Daily Schedule — checklist of observation windows (optional steps labeled optional; never blocks play)
- Tabs: Sun (sun sights + Sun Worksheet), Stars (star sights, Saved LOPs, Compute Fix), Fixes (Enter Fix / Enter DR)
Sun sights get badges when they fall in special windows: AM, PM, NOON, RISE, SET. Selecting a sight drives the Sun Worksheet (noon lat/lon, sunrise/sunset longitude, or running fix).