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Intercept method
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Intercept method
In astronomical navigation, the intercept method, also known as Marcq St. Hilaire method, is a method of calculating an observer's position on Earth (geopositioning). It was originally called the azimuth intercept method because the process involves drawing a line which intercepts the azimuth line. This name was shortened to intercept method and the intercept distance was shortened to 'intercept'.
The method yields a line of position (LOP) on which the observer is situated. The intersection of two or more such lines will define the observer's position, called a "fix". Sights may be taken at short intervals, usually during hours of twilight, or they may be taken at an interval of an hour or more (as in observing the Sun during the day). In either case, the lines of position, if taken at different times, must be advanced or retired to correct for the movement of the ship during the interval between observations. If observations are taken at short intervals, a few minutes at most, the corrected lines of position by convention yield a "fix". If the lines of position must be advanced or retired by an hour or more, convention dictates that the result is referred to as a "running fix".
The intercept method is based on the following principle. The actual distance from the observer to the geographical position (GP) of a celestial body (that is, the point where it is directly overhead) is "measured" using a sextant. The observer has already estimated his position by dead reckoning and calculated the distance from the estimated position to the body's GP; the difference between the "measured" and calculated distances is called the intercept.
The diagram on the right shows why the zenith distance of a celestial body is equal to the angular distance of its GP from the observer's position.
The rays of light from a celestial body are assumed to be parallel (unless the observer is looking at the moon, which is too close for such a simplification). The angle at the centre of the Earth that the ray of light passing through the body's GP makes with the line running from the observer's zenith is the same as the zenith distance. This is because they are corresponding angles. In practice it is not necessary to use zenith distances, which are 90° minus altitude, as the calculations can be done using observed altitude and calculated altitude.
Taking a sight using the intercept method consists of the following process:
Suitable bodies for celestial sights are selected, often using a Rude Star Finder. Using a sextant, an altitude is obtained of the Sun, the Moon, a star or a planet. The name of the body and the precise time of the sight in UTC is recorded. Then the sextant is read and the altitude (Hs) of the body is recorded. Once all sights are taken and recorded, the navigator is ready to start the process of sight reduction and plotting.
The first step in sight reduction is to correct the sextant altitude for various errors and corrections. The instrument may have an error, IC or index correction (see article on adjusting a sextant). Refraction by the atmosphere is corrected for with the aid of a table or calculation and the observer's height of eye above sea level results in a "dip" correction (as the observer's eye is raised the horizon dips below the horizontal). If the Sun or Moon was observed, a semidiameter correction is also applied to find the centre of the object. The resulting value is "observed altitude" (Ho).
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Intercept method
In astronomical navigation, the intercept method, also known as Marcq St. Hilaire method, is a method of calculating an observer's position on Earth (geopositioning). It was originally called the azimuth intercept method because the process involves drawing a line which intercepts the azimuth line. This name was shortened to intercept method and the intercept distance was shortened to 'intercept'.
The method yields a line of position (LOP) on which the observer is situated. The intersection of two or more such lines will define the observer's position, called a "fix". Sights may be taken at short intervals, usually during hours of twilight, or they may be taken at an interval of an hour or more (as in observing the Sun during the day). In either case, the lines of position, if taken at different times, must be advanced or retired to correct for the movement of the ship during the interval between observations. If observations are taken at short intervals, a few minutes at most, the corrected lines of position by convention yield a "fix". If the lines of position must be advanced or retired by an hour or more, convention dictates that the result is referred to as a "running fix".
The intercept method is based on the following principle. The actual distance from the observer to the geographical position (GP) of a celestial body (that is, the point where it is directly overhead) is "measured" using a sextant. The observer has already estimated his position by dead reckoning and calculated the distance from the estimated position to the body's GP; the difference between the "measured" and calculated distances is called the intercept.
The diagram on the right shows why the zenith distance of a celestial body is equal to the angular distance of its GP from the observer's position.
The rays of light from a celestial body are assumed to be parallel (unless the observer is looking at the moon, which is too close for such a simplification). The angle at the centre of the Earth that the ray of light passing through the body's GP makes with the line running from the observer's zenith is the same as the zenith distance. This is because they are corresponding angles. In practice it is not necessary to use zenith distances, which are 90° minus altitude, as the calculations can be done using observed altitude and calculated altitude.
Taking a sight using the intercept method consists of the following process:
Suitable bodies for celestial sights are selected, often using a Rude Star Finder. Using a sextant, an altitude is obtained of the Sun, the Moon, a star or a planet. The name of the body and the precise time of the sight in UTC is recorded. Then the sextant is read and the altitude (Hs) of the body is recorded. Once all sights are taken and recorded, the navigator is ready to start the process of sight reduction and plotting.
The first step in sight reduction is to correct the sextant altitude for various errors and corrections. The instrument may have an error, IC or index correction (see article on adjusting a sextant). Refraction by the atmosphere is corrected for with the aid of a table or calculation and the observer's height of eye above sea level results in a "dip" correction (as the observer's eye is raised the horizon dips below the horizontal). If the Sun or Moon was observed, a semidiameter correction is also applied to find the centre of the object. The resulting value is "observed altitude" (Ho).