Highest Astronomical Tide (HAT) and Lowest Astronomical Tide (LAT). These levels are predicted to occur under average meteorological conditions and under any combination of astronomical conditions. They will not be reached every year. They are also not the extreme levels that can be reached, as storm surges and other meteorological conditions can cause considerably higher…
The Hydrographer of the SA Navy is responsible for producing a table of tides covering the South African coast and that of Namibia. These tables contain: The predicted times and heights of low water and high water in all South African and Namibian ports, as well as the phases of the moon for every day…
Tides can be defined as the periodic rise and fall of the level of the sea caused by the combined solar and lunar gravitational forces acting on the earth. There are generally two high waters and two low waters occurring at any place during a period of 24 hours and 50 minutes. This strange period…
The procedure laid out below should be followed to find the latitude of the observer at meridian passage. A pro forma format for working the problem out is also provided and should be followed in conjunction with the procedure. A good navigator will start preparing for the midday sight an hour or two before the…
When the sun and the observer’s zenith are on the same celestial meridian, then there is a simple relationship between declination, zenith distance and the latitude. All three are arcs of the same great circle and their values give the distance of each point from the celestial equator (equinoctial) and the distance of one from…
The local mean time (LMT) of mer pass on any meridian is obtained from the Nautical Almanac. At the bottom of the right hand page of the three-daily tables is a small table entitled “sun-mer pass”. The time of mer pass for the three days covered by the page is contained in this table. Remember…
Introduction Meridian passage of an observed heavenly body, ie the sun, occurs when the body is directly over the observer’s meridian and the local hour angle (LHA) is zero. During this course we will only be covering the meridian passage (mer pass) of the sun. Those who wish to become navigating officers will cover the…
Total correction tables. The altitude correction tables for the sun are given at the beginning of the Nautical Almanac. They cover all altitudes from 0° to 90° and the heights of eye from 1 to 48 metres. The tables provide corrections to apply to both the upper and lower limbs of the sun and are…
The last correction to be applied is parallax. So far the observer has been able to find the heavenly body’s centre above the horizontal plane. All that is left is to allow for the earth’s radius. In the drawing parallax is the angle OXC. As you can see the true altitude (angle XCR) is equal…
Because of their relative sizes and nearness to the earth, the sun and moon are not merely seen as points of light but as spheres with sizeable diameters. Therefore, in order to take a sextant altitude, the observer needs to measure the angle between the horizon and their upper or lower rims. This angle then…
