Calculating twilight times

In the Nautical Almanac, for each day there are sunrise and sunset times for different latitudes on the Greenwich meridian. It also lists when the Sun’s center is 6° and 12° below the horizon. The same times can also be used elsewhere than the 0° meridian, as long as you first calculate the local mean time.

C o n t e n t s

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