Celestial Navigation Basics Flashcards

A 'position line' from a celestial observation is:

Answer: A circle of equal altitude — a line on the chart on which the observer must lie, derived from the body's observed altitude

A celestial position line is theoretically a circle of equal altitude (a circle of position) centred on the geographic position of the body. For practical navigation, the relevant portion is plotted as a straight line on the chart.

The Yachtmaster Offshore oral exam may test a candidate's understanding of GPS limitations. Which of the following is a real limitation of GPS?

Answer: GPS can be jammed, suffer selective availability, show datum offsets from the chart, or fail due to power loss

GPS vulnerabilities include jamming, spoofing, datum differences between chart and GPS datum (e.g., chart on OSGB, GPS on WGS84), power failure, and antenna failure. Cross-checking with other methods is essential.

In basic celestial navigation, the 'assumed position' (AP) is:

Answer: A convenient whole-degree Lat/Long position near the DR position, chosen to simplify the sight reduction calculation

An AP near the DR position is chosen with whole-degree values to minimise the complexity of sight reduction tables (e.g., HO 249 or NP 303). The intercept is then plotted from the AP.

Which celestial body provides the most practical and frequently used sight for offshore passages in the Yachtmaster Offshore context?

Answer: The Sun — always visible in daylight, its Almanac data is the simplest to use

The Sun is the primary celestial body for Yachtmaster-level navigation: always visible in daylight, simple Almanac values, and noon sights provide direct latitude. Polaris latitude sights are also relatively straightforward.

What is 'Local Apparent Noon' (LAN) and how is it used?

Answer: The moment the Sun crosses the observer's meridian at its highest daily altitude — used to obtain a latitude by the noon sight method

LAN occurs when the Sun is due North or South (on the meridian), at its greatest altitude for the day. The altitude at LAN, corrected and combined with declination, gives latitude without knowing longitude.

A Yachtmaster candidate takes a sun sight and gets a position line running approximately east-west. The best time to take a second sight for a running fix would be:

Answer: In the morning or afternoon when the Sun bears significantly east or west — giving a position line at a good angle to the first

For a running fix, the two position lines should cross at a large angle (ideally 60–90°). An east-west noon latitude line crossed with a morning/afternoon position line from a different Sun azimuth gives a good angle of cut.