{"id":211,"date":"2020-10-20T10:01:21","date_gmt":"2020-10-20T10:01:21","guid":{"rendered":"https:\/\/maritimesa.org\/nautical-science-grade-11\/?p=211"},"modified":"2020-10-22T08:29:17","modified_gmt":"2020-10-22T08:29:17","slug":"geographical-position","status":"publish","type":"post","link":"https:\/\/maritimesa.org\/nautical-science-grade-11\/2020\/10\/20\/geographical-position\/","title":{"rendered":"Geographical position"},"content":{"rendered":"

Polar Distance.<\/strong>
\nThis is the angular distance of a body from the elevated pole (the pole that is above the celestial horizon). This is the distance PX in the sketch. When the elevated pole and the declination have the same names, the polar distance is 90\u00b0 – declination. When they have different names it is 90\u00b0 + declination.<\/p>\n

\"\"

Polar distance and geographical position.<\/p><\/div>\n

Geographical position.<\/strong>
\nIf a line drawn from a heavenly body to the earth\u2019s centre, the point where this line cuts the earth\u2019s surface is called the geographical position of the body, ie XC.\u00a0 The geographical position of a celestial body is very important to the navigator when using celestial bodies for navigation.<\/p>\n

The Observer\u2019s Zenith.<\/strong>
\nThis is the point where a straight line from the earth\u2019s centre passes through the observer\u2019s position and cuts the celestial sphere.\u00a0 When drawing a sketch on the plane of the celestial horizon,it is the centre of the circle marked with the letter \u201cZ\u201d.<\/p>\n

\"\"

The observer’s zenith.<\/p><\/div>\n

The celestial or rational horizon.<\/strong>
\nThe great circle formed by the celestial sphere, every point of which is 90\u00b0 from the observer\u2019s zenith, is known as the celestial or rational horizon.\u00a0 It divides the sphere into two hemispheres, the upper one being the visible hemisphere.<\/p>\n

The Observer\u2019s Meridian.<\/strong>
\nThis is the celestial meridian which passes through the observer\u2019s zenith from one celestial pole to the other, ie PZQSP\u2019.\u00a0 The points N and S, in which the two meridians cut the celestial horizon are the north and south points, the north point being the one nearest the north pole.\u00a0 The east and west points (E and W) lie on the celestial horizon midway between N and S.<\/p>\n

Plane of the celestial horizon.<\/strong>
\nWhen it is necessary to show the whole of the visible sky, the figure must be drawn on the plane of the celestial horizon, as if the sphere were seen from a position directly above the observer\u2019s zenith.\u00a0 The zenith (Z)is therefore in the centre of the circle that is the celestial horizon, the north-south and east-west lines divide the circle into four parts and the equator appears as a curve stretching from W to E through the point Q on the N-S line.\u00a0 ZQ is the observer\u2019s latitude, P is the elevated pole.
\nPZ + PN = 90\u00b0 = PZ + PQ
\ntherefore PN = ZQ<\/p>\n

\"\"

Sketch of the plane of the celestial (rational) horizon.<\/p><\/div>\n

Vertical Circles.<\/strong>
\nAll great circles passing through the observer\u2019s zenith are perpendicular to the celestial horizon and are known as vertical circles.<\/p>\n

Prime Vertical.<\/strong>
\nThe particular vertical circle passing through the east and west points is called the prime vertical circle. In the above sketch this will be line WZE.<\/p>\n

Principal Vertical Circle.<\/strong>
\nThe observer\u2019s meridian is sometimes called the principal vertical circle because it provides a fixed direction in the celestial sphere just as the observer\u2019s terrestrial meridian provides one on the earth\u2019s surface. In the above sketch this is line NPZQS.<\/p>\n

The Azimuth of a Heavenly Body.<\/strong>
\nThis is the angle at the zenith between the observer\u2019s meridian and the vertical circle through the heavenly body and it is measured east or west from this meridian from 0\u00b0 to 180\u00b0 and named north or south from the elevated pole. The azimuth is not always the same as a true bearing since the latter is measured from 0\u00b0 to 360\u00b0. The azimuth can never be greater than 180\u00b0. In the above sketch it is the angle PZX.<\/p>\n

Altitude.<\/strong>
\nAlthough the position of a heavenly body is fixed in the celestial sphere, in order for an observer to point out a heavenly body, he utilizes his own meridian and the visible horizon. He will use a bearing from his own meridian (azimuth) and an altitude of the body above the horizon. He will also make use of these to find his own position. As a result, he is not only interested in the altitude of the body above the visible horizon but also the altitude above the celestial horizon. Having measured the one, he will apply certain corrections until it refers to the other.<\/p>\n

The True Altitude of a Heavenly Body.<\/strong>
\nThis is the angular distance of the body above the celestial horizon, measured along the vertical circle through the body and the observer\u2019s zenith. The true altitude of X is AX in the sketch below:<\/p>\n

\"\"

The true altitude of a heavenly body.<\/p><\/div>\n

Zenith Distance.<\/strong>
\nOnce the true altitude of a celestial body is obtained, the distance of the body from the observer\u2019s zenith can be found, ie ZX.\u00a0 It is (ZA \u2013 XA) or (90\u00b0 \u2013 altitude).\u00a0 The distance ZX is known as the zenith distance and it is important because it is the third side of the spherical triangle PZX.<\/p>\n

True Altitude of the Pole. <\/strong>
\nThe true altitude of the pole is the latitude of the place from which the observation was made.<\/p>\n

The Observer\u2019s Sea or Visible Horizon.<\/strong>
\nThis is the horizon above which the observer actually measures the altitude of the heavenly body.\u00a0 It is a small circle drawn around the observer where the sea and sky appear to meet.\u00a0 A tangent from the observer to the earth\u2019s surface decides the position of this circle, but refraction alters it slightly because a ray of light from the horizon is not a straight line.<\/p>\n","protected":false},"excerpt":{"rendered":"

Polar Distance. This is the angular distance of a body from the elevated pole (the pole that is above the celestial horizon). This is the distance PX in the sketch. When the elevated pole and the declination have the same names, the polar distance is 90\u00b0 – declination. When they […]<\/p>\n","protected":false},"author":6,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[4,9,19],"tags":[],"class_list":["post-211","post","type-post","status-publish","format-standard","hentry","category-11-1-navigation","category-11-1-2-astro-navigation","category-11-1-2-4-geographical-position"],"_links":{"self":[{"href":"https:\/\/maritimesa.org\/nautical-science-grade-11\/wp-json\/wp\/v2\/posts\/211"}],"collection":[{"href":"https:\/\/maritimesa.org\/nautical-science-grade-11\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/maritimesa.org\/nautical-science-grade-11\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/maritimesa.org\/nautical-science-grade-11\/wp-json\/wp\/v2\/users\/6"}],"replies":[{"embeddable":true,"href":"https:\/\/maritimesa.org\/nautical-science-grade-11\/wp-json\/wp\/v2\/comments?post=211"}],"version-history":[{"count":4,"href":"https:\/\/maritimesa.org\/nautical-science-grade-11\/wp-json\/wp\/v2\/posts\/211\/revisions"}],"predecessor-version":[{"id":313,"href":"https:\/\/maritimesa.org\/nautical-science-grade-11\/wp-json\/wp\/v2\/posts\/211\/revisions\/313"}],"wp:attachment":[{"href":"https:\/\/maritimesa.org\/nautical-science-grade-11\/wp-json\/wp\/v2\/media?parent=211"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/maritimesa.org\/nautical-science-grade-11\/wp-json\/wp\/v2\/categories?post=211"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/maritimesa.org\/nautical-science-grade-11\/wp-json\/wp\/v2\/tags?post=211"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}