Although the marine sextant is seldom used these days due to the introduction of satellite navigation systems (ie GPS), it still can be used for the following: Measuring the altitude of the stars, sun, moon and planets in order to determine geographical position out of sight of land (ocean navigation). Measuring horizontal sextant angles between…
Introduction. The marine sextant is a precision built optical instrument f which is used to measure altitudes and angles. It used to be an essential part of a ship’s navigation equipment. The sextant got its name from the fact that the length of its graduated arc is approximately one sixth of a circle. One of…
Explanation of twilight. Twilight is the period of approximately one hour’s duration before the sun rises in the morning and the period of approximately one hour’s duration after the sun sets in the evening, ie between the times when the sun is 12° below the horizon and sunrise in the morning and between the times…
Calculating times of sunrise and sunset. The times of the visible rising and setting of the sun are contained in the daily pages of the nautical almanac. They are tabulated for the middle day of the three on that page and provide the Local Mean Time (LMT) of the rising/setting of the sun on any…
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° – declination. When they have different names it is…
Since the declination gives us the “latitude” of the heavenly body we must now find it’s “longitude”. We do this by measuring the angle between the celestial meridian passing through the body and that running through the observer’s position. The hour angle is defined as “the angle between the meridian running through the observer’s position…
Declination. Declination corresponds to terrestrial latitude and is defined as the angular distance of a heavenly body north or south of the celestial equator, measured in units of arc (ie degrees, minutes and seconds). Declination is the result of the earth’s movement/orbit around the sun and the fact that the earth’s poles are not perpendicular…
Introduction. Astro-navigation is based upon the concept of the “celestial sphere”. What is the “celestial sphere”? If one looked up into the night sky one would see what would appear to be an enormous black dome with all the heavenly bodies stuck to the inside like thousands of diamonds. This is what we refer to…
Scale of a Mercator chart. Because the equator is shown on a Mercator chart as a straight line of definite length and the meridians appear as straight lines perpendicular to it, the longitude scale is fixed by that length and is constant in all latitudes. The latitude scale, on the other hand, is not constant.…
‘Sailings’ is a term given to various methods used to determine courses and distances to be steamed when travelling from one geographical position to another. There are six different “sailings”, ie Parallel, Plane, Middle Latitude, Mercator, Great Circle and a combination of Great Circle and one of the others. During this course, however, we will…
