Question 1

What is the purpose of a sextant in celestial navigation?

Accepted Answer

To measure a celestial body's altitude (angle above the horizon), used with tables and UTC time to calculate a position line. A sextant measures the altitude (angle above the visible horizon) of a celestial body, such as the Sun, Moon, a planet or a star. The sextant altitude is corrected for instrumental and observational effects. UTC time and astronomical data from the Nautical Almanac are then used with sight reduction tables or an equivalent calculation to derive a position line. This is a local approximation to a circle of equal altitude on the Earth's surface. Two suitably intersecting position lines obtained at effectively the same time give a fix. For a running fix, an earlier position line is advanced for the vessel's movement and intersected with a second position line from a later sight. Celestial navigation provides a backup if electronic position systems fail and forms part of Yachtmaster Ocean assessment.

Question 2

A noon sight (meridian passage of the sun) gives which position information?

Accepted Answer

Latitude, obtained from the sun's declination and zenith distance, which is 90° minus its corrected meridian altitude.. A noon sight gives latitude from the sun's corrected meridian altitude and its declination at the time of observation. Correct the sextant altitude for index error, dip and the appropriate solar altitude corrections to obtain the true altitude of the sun's centre. Zenith distance is 90° minus this altitude. Taking north latitude and declination as positive, latitude equals declination plus zenith distance if the sun bears south at upper meridian passage, or declination minus zenith distance if it bears north. Meridian altitude and declination do not by themselves give longitude. Longitude can be found if the UTC of meridian passage is also accurately determined.

Question 3

When is the best time to take star sights with a sextant and why?

Accepted Answer

At morning or evening nautical twilight, when stars and planets are visible and the sea horizon remains clear enough for sextant measurements.. Morning or evening nautical twilight is generally the best time for star sights because navigational stars and planets can be visible while the sea horizon is sufficiently distinct for sextant measurements. In the morning, the horizon becomes clearer as the sky lightens; in the evening, stars appear as the sky darkens. Star sights can also be taken outside twilight if both the selected star and a reliable horizon are visible. The useful observing window varies with latitude, season and conditions, rather than having a fixed duration of 15 to 30 minutes. Several stars can be observed in quick succession for a multi-body fix, using their positions from the Nautical Almanac and appropriate sight reduction methods.

Celestial Navigation, questions and explanations

What is the purpose of a sextant in celestial navigation?

A noon sight (meridian passage of the sun) gives which position information?

When is the best time to take star sights with a sextant and why?

Practise this properly

Celestial Navigation, questions and explanations

Questions

What is the purpose of a sextant in celestial navigation?

A noon sight (meridian passage of the sun) gives which position information?

When is the best time to take star sights with a sextant and why?

Practise this properly