USCG Exam Prep · ☀️ Interactive Trainer
Celestial Navigation & Sight Reduction
Sight reduction is the skill most candidates dread, because a written question can't show you the procedure. This walks the whole chain on a fresh sight every time.
Tested on 200-Ton Master and the MMC (deck officer) endorsements. Binnacle School has 158 Celestial Navigation questions in the bank.
What it is
Celestial navigation fixes your position from the observed altitude of a celestial body. You correct the sextant reading (Hs) for index error, dip, refraction, semidiameter, and parallax to get the observed altitude (Ho); compute the altitude and azimuth the body should have from an assumed position (Hc and Zn) by the law of cosines — the same answer HO 229 and HO 249 tabulate; then compare Ho to Hc to lay off the Marcq St Hilaire intercept and draw a celestial line of position.
How the USCG tests it
On the 200-Ton and MMC deck exams, celestial questions test each link in the chain: applying almanac corrections, reading the sign of index error, computing local hour angle (LHA), choosing the right azimuth quadrant, and knowing the intercept rule — 'Ho Mo To' (Ho More means Toward the body). The interactive trainer drills exactly these steps rather than asking you to memorize a single number.
Key concepts
Sextant altitude corrected for index error and dip (→ apparent altitude Ha), then refraction, semidiameter, and parallax (→ observed altitude Ho).
Local Hour Angle = GHA of the body ± your longitude (east added, west subtracted), normalized to 0–360°.
Calculated altitude Hc = asin(sin·L·sin·D + cos·L·cos·D·cos·LHA); the azimuth Zn comes from the law-of-cosines Z plus the N/S, LHA>180° quadrant rule.
a = Ho − Hc in nautical miles. Toward the body if Ho is greater (Ho-Mo-To), away if less; the LOP is perpendicular to Zn.
Worked example
You shoot the Sun's lower limb: Hs = 20°07.2′, index error 2.0′ off the arc, height of eye 20 ft. Find Ho.
Dip ≈ 0.97·√20 = 4.3′. Ha = 20°07.2′ − 2.0′ (IE) − 4.3′ (dip) = 20°04.9′. Sun lower-limb correction ≈ −refraction (2.6′) + semidiameter (16.0′) + parallax (0.1′) = +13.5′. Ho ≈ 20°18.4′ (20.31°).
Practice Celestial Navigation hands-on
The interactive trainer generates a fresh problem every time and checks your work against exam-accurate math. Free with a Binnacle School account, on web and iOS.
Frequently asked questions
Is celestial navigation still on the USCG exam?
Yes — celestial navigation and sight reduction are tested on the 200-Ton Master exam and the MMC deck-officer modules. It is not on the OUPV (Six-Pack) exam.
What is the Marcq St Hilaire (intercept) method?
You assume a position, compute the altitude and azimuth a body would have there (Hc and Zn), then compare it to your observed altitude (Ho). The difference is the intercept, laid off along the azimuth toward or away from the body to draw your line of position.
What does 'Ho Mo To' mean?
A memory hook for the intercept direction: if Ho is MOre than Hc, plot the intercept TOward the body's geographic position; if less, plot it away.