Celestial Navigation Basics Flashcards

What is the purpose of celestial navigation in the context of the Yachtmaster Offshore syllabus?

Answer: To provide a backup position-fixing method when GPS is unavailable, using the Sun's altitude and the Nautical Almanac

The Yachtmaster Offshore syllabus introduces celestial navigation as a backup system and develops understanding of its principles. In practice, the Sun sight is most commonly used offshore.

The 'sextant' is used in celestial navigation to measure:

Answer: The altitude of a celestial body above the visible horizon

A sextant measures the angle between a celestial body (e.g., the Sun) and the visible horizon. This altitude, combined with the time (GMT) and the Nautical Almanac, allows calculation of a position line.

The Nautical Almanac provides which information for celestial navigation?

Answer: The Greenwich Hour Angle (GHA) and Declination of celestial bodies for every hour of every day

The Nautical Almanac (published annually by UKHO/USNO) provides GHA and Declination of the Sun, Moon, planets, and 57 navigational stars for every hour of every day, enabling celestial position calculations.

What is 'declination' of the Sun as used in celestial navigation?

Answer: The angular distance of the Sun north or south of the celestial equator — equivalent to latitude on the celestial sphere

Declination is the celestial equivalent of latitude — the Sun's angular distance north or south of the celestial equator. It ranges from 23.5°N at summer solstice to 23.5°S at winter solstice.

A noon sight of the Sun gives a latitude line because:

Answer: At local apparent noon, the Sun is on the observer's meridian — its maximum altitude gives latitude directly

At local apparent noon (LAN), the Sun transits the observer's meridian at its highest altitude. Latitude = 90° − altitude + declination (or − if same hemisphere). This is the simplest and most reliable celestial observation.

The 'intercept method' (Marcq St. Hilaire) in celestial navigation involves:

Answer: Comparing the calculated altitude of a body from an assumed position to the observed altitude, to find the distance and direction of the position line from the assumed position

The intercept method calculates what altitude a body SHOULD have from an assumed position (AP), then compares this to the observed altitude. The difference (intercept) in nautical miles gives the distance of the actual position line from the AP.