Details and practical considerations

The sections above explain how position fixing using stars works. Unfortunately, that is not the whole story — a few practical details complicate the otherwise elegant trigonometry and time calculations, and these must be understood. They are explained from here onward.

Since the previous sections have given you an understanding of the overall concept, it should now be easier to see where the following details fit in. They already have a place to attach themselves in your memory.

C o n t e n t s

• Position fixing from stars
• Night sky
• Celestial sphere
• What you see from the North Pole as Earth rotates
• How the night sky changes when traveling to the equator
• Effect of Earth’s rotation on the sky at the equator
• Celestial meridian, upper and lower culmination
• Change of star altitude at the poles, equator, and between
• Declination
• Example of determining latitude
• Hour angle, Greenwich Hour Angle (GHA)
• Aries, celestial Greenwich reference
• Sidereal hour angle (SHA)
• Local hour angle (LHA)
• Relation between hour angle and time
• Example of longitude and latitude from the Sun at noon
• Relation between angles, degrees, and nautical miles on Earth
• Position circle
• Azimuth
• True bearing to a star
• Nautical triangle
• Solving the nautical triangle (sight reduction)
• Position fixing using Marcq Saint Hilaire’s intercept method
• Star identification
• Details and practical considerations
• Time
• Time difference (ΔT)
• Altitude measurement
• Example of position fixing
• Twilight
• Calculation of twilight time
• Navigation exam