USCG Exam Prep
Celestial Navigation Practice Questions
Sextant sights, the Nautical Almanac, sight reduction, and the celestial line of position — required on 200-ton and MMC exams.
158 Celestial Navigation questions are in the Binnacle School bank (2,256 total across all 12 USCG categories). Here are 5 to try right now — answers and explanations included.
1. The Nautical Almanac tabulates the positions of celestial bodies against which time standard?
- A.Universal Time (UT1), formerly called Greenwich Mean Time✓
- B.Local apparent time at the observer's meridian
- C.Zone time of the vessel's time zone
- D.Local mean time at 180° longitude
Why: The Almanac is entered with UT (UT1, the modern successor to GMT). The navigator must convert the watch/zone time of an observation to UT before extracting GHA and declination.
2. An azimuth observation determines compass error by:
- A.Measuring the altitude of the body and comparing it to Hc
- B.Timing how long the body takes to set
- C.Counting the number of stars visible at twilight
- D.Comparing the body's computed true azimuth with its observed compass bearing✓
Why: The true azimuth (Zn) of a body is computed (or taken from azimuth tables) for the time of observation. The difference between Zn and the compass bearing of the body is the compass error — east if the compass reads low, west if it reads high.
3. An observer at longitude 120°W observes a star. GHA Aries = 200°, and the star's SHA = 146°. What is the star's LHA?
- A.226°✓
- B.346°
- C.106°
- D.466°
Why: GHA star = GHA Aries + SHA = 200° + 146° = 346°. LHA = GHA − West longitude = 346° − 120° = 226°. This two-step process (GHA star first, then LHA) is standard for star sights.
4. An observer at longitude 75° 30.0' W measures the sun's GHA as 310° 15.0'. What is the LHA of the sun?
- A.234° 45.0'✓
- B.385° 45.0'
- C.25° 45.0'
- D.130° 15.0'
Why: LHA = GHA − West longitude = 310° 15.0' − 75° 30.0' = 234° 45.0'. Since GHA exceeds the west longitude, the result is positive and valid without adding 360°.
5. Which error in a running fix cannot be eliminated by retaking the celestial observations?
- A.Error in the course or speed used to advance the first LOP✓
- B.Index error of the sextant
- C.Incorrect dip correction
- D.Misidentification of the celestial body
Why: Errors in the DR track used to advance the LOP—whether from leeway, current, helmsman's error, or inaccurate log reading—accumulate with time and cannot be corrected by improved celestial observations; only accurate ship's movement data can improve the running fix.
Drill all 158 Celestial Navigation questions
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