Navigation in the Singapore Navy
Distinction between the visible horizon and the celestial horizon
The visible horizon is the small circle on the Earth’s surface where the sky and sea appear to meet. Whereas the celestial horizon is the great circle on the celestial sphere which passes through the centre of the earth, every point of which is 90o from the observer’s zenith. This is shown in the figure below.
Sidereal Hour Angle (SHA)
Greenwich Hour Angle (GHA)
Local Hour Angle (LHA)
As it is physically impossible to tabulate the local hour angles of every celestial body for all minutes of longitude, a method linking LHA and GHA has been devised to circumvent this problem. To get the LHA of any celestial body for any time at any longitude, the following step is needed.
LHA = GHA + East Longitude
LHA = | GHA – West Longitude |
W LHA E W LHA E
In addition, the GHA of a particular star can be found by:
GHA star = GHA aeries + SHA star
The Relationship Between the Hour Angles:
These equations are essential in converting data on a ship and they are needed in calculating the exact position of the star.
Navigational officers must know the time of the rising and setting of the sun as this is important in determining twilight- the period of day when even though the sun is below the horizon, the observer stills sees light due to refraction by the upper atmosphere.
Twilight is the time when the altitudes of the stars can be taken because that is when one can see both the horizon and the stars at the same time. This is due to the fact that it is extremely dark at sea at night and so it is difficult to see the horizon even though the stars shine brightly because of minimal light pollution. Conversely, though it is easy to see the horizon in the day, it is impossible to see the stars due to the glare of the sun.
Merpass is when the celestial body is over the observer's meridian.
The true bearing of the celestial body will be either 000 or 180 and this depends on 3 factors:
- Latitude of observer
- Declination of the celestial body
i.e. In the Northern hemisphere, when latitude and declination have opposite names, the bearing is 180.
However in the Southern hemisphere, when latitude and declination have opposite names, the bearing is 000.
The exact time of the merpass can be calculated from the nautical almanac.
The exact latitude of the observer at the time of merpass can be easily found out by observing the Sun's altitude at the time of merpass.
Latitude = 90o - Altitude + Declination
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