Navigation ( Part 3 )

 

Navigation ( Part 3 )

 

Parallel Sailing

Sailing between two positions on the earth’s surface involves calculating the course and distance between them . the shortest distance between any two points on the earth is the shortest arc of the great circle through that points. The great circle track crosses the various meridians at different angles .Thus a ship following a great circle track would have to continuously alter her course throughout the passage . Therefore in navigating from one place to another the usual method is to sail along a rhumb line track.

 

Rhumb Line

A Rhumb line is a line on the earth’s surface crossing all meridians at the same angle . it can thus be seen that the rhumb line is the most convenient track to follow as the course of the ship remains constant for the entire passage .

On Mercator chart however , a rhumb line appears as a straight line as the meridians on a Mercator chart are represented as straight lines parallel to each other.

 

Plane Sailing

Plane sailing is sailing along a rhumb line from one position to another , which are not situated on the same latitude.

Dep / d’long = Cos lat

 

Dep / d’lat  = Tan .course

 

Distance = d’lat X Sec. Course

 

Departure 

On a Mercator chart , the E – W distance between two places , measured along the latitude scale ( in minutes of latitude ) is the departure between them in nautical miles . but if measured along the longitude scale it is the d’long.

 

DMP or d’ long – Difference in Meridian latitude

On a Mercator chart the N – S diatance between two places , measured  along the latitude scale , is the d’lat between them ( in minutes or nautical miles ) but if measured along the longitude scale ( in minutes of longitude ) it is the DMP between them.

 

Meridional Parts

Meridional parts for any latitude is the arc of the meridian , or the angle at the center of the earth , contained between the equator and that parallel of latitude expressed in minutes of longitude is called the meridional parts for that latitude.

 

Natural Scale

The Natural scale of a chart is the ratio that the distance between two points on the chart bears to the actual distance between them on the earth.

Natural scale = chart distance / earth distance

 

  Mercator sailing

  Where the distance exceeds 600 miles. It is recommended that calculation be done by Mercator sailing in order to get more accurate results than by plane and parallel sailing methods .

D’long / DMP  = Tan course

 

Gnomonic charts – Great circle chats

If a navigator is to follow the shortest route between two positions , he must sail along a great circle . it would therefore be convenient to have charts on which great circles are represented by straight lines.

 

Plan chart

Plan charts are representations of very small areas of the earth’s surface .such as an  anchorage , a port or a harbour

 

Different types of charts

Star charts

Magnetic variation charts

Co – Tidal and Co – range charts

Instructional charts

Time zone charts

 

Ecliptic

Is a great circle on the celestial sphere in the same plane as the plane of the earth’s orbit around the sun . Thus the sun’s apparent annual path on the celestial sphere is the ecliptic . It is so called because the sun , moon and earth must must be on this plane for a solar or lunar eclipse to occur.

 

Ellipse

The sun’s apparent orbit around the earth is an ellipse. Ecliptic is a projection of this ellipse on to the celestial sphere.

 

Obliquity of the Ecliptic

Is the angle between the plane of the equinoctial and that of the ecliptic . It’s value is approximately 23 ½ degree .

 

  Zodiac

                       Is a belt on the celestial sphere extending 8 degree on each side of ecliptic within which the sun , the moon and the planets are always found . The belt of the zodiac is divided into 12 equal parts of the length 30 degree each . These are named after groups of stars or constellations within them . they are Aries , Taurus , Gemini , Cancer , Leo , Virgo , Libra , Scorpio , Sagittorius , Capricorn , Aquarius and Pisces.

 

Observer’s zenith

Is the point on the celestial sphere vertically above the observer ie) the point at which a straight line from the center of the earth through the observer meets the celestial sphere.

 

Observer’s Nadir

Is the point on the celestial sphere vertically opposite his Zenith.

 

Vertical Circles

Are great circles on the celestial sphere passing through the observer’s Zenith and Nadir.

 

Position on the Celestial sphere

A position on a sphere may be defined by the angle at the center of the sphere or the great circular co ordinates of that position with respect to two reference great circles which are at right angles to each other.

Position on the earth’s surface for instance are defined by stating such angles or co ordinates with respect to two reference great circles the equator from which latitudes are measured and the prime meridian from which longitude are measured . 

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Will upload Part 4 shortly 

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