The minimum number of stars required to obtain a reliable position
- A minimum of three stars to be used to get a good angle of cut.
- The stars should be well spread around the horizon, and hence, the fix will be inside the cocked hat; otherwise, the position will be outside the cocked hat.
- Altitudes of the stars should be between 30-60 degrees for a good fix, and where possible, with approximately the same altitude.
- Generally, four stars are preferred, if possible, 90 degrees apart in azimuth, because any error due to abnormal refraction will be eliminated by using opposite horizons.
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The problems involved in making stellar observations in the Red sea.
- Abnormal refraction causes an angle between the true direction and apparent directions of the body as a result of which the body could appear higher than their actual altitude.
- As temperatures around the Red sea are very high, the altitudes of a body can be affected by refraction as it depends upon temperature and pressure of the atmosphere.
- Nautical tables have table giving mean refraction based on standard sea level and pressure values .
- Refraction changes values of “dip” as refraction can also alter a visible horizon.
- Possible dust can create incorrect visible horizon.
- Low coastline can be mistaken as visible horizon.