Star identification

When using the table:

1. If you are in the Northern Hemisphere, use your measured true bearing as the value for the table’s local hour angle (LHA). If you are in the Southern Hemisphere and the true bearing is greater than 180°, use true bearing − 180° as the LHA value. If the true bearing is less than 180°, use 180° − true bearing as the LHA value.
2. Use the measured altitude of the celestial body as the table’s declination.

The altitude (Hc) you read from the table gives the correct declination of the celestial body. If you end up in the CONTRARY area, the declination is southern if your latitude is northern, and vice versa. The azimuth value read from the table corresponds to the correct local hour angle (LHA) after this conversion.

If your latitude and the declination read from the table (to which you use the measured altitude) fall in the SAME area of the table, use the following formulas:

• If the true bearing is between 0°–180°, the correct LHA = 360° − azimuth
• If the true bearing is between 180°–360°, the correct LHA = azimuth

If you are in the Northern Hemisphere and end up in the CONTRARY area of the table, use these formulas:

• If the true bearing is between 90°–180°, LHA = 180° + azimuth
• If the true bearing is between 180°–270°, LHA = 180° − azimuth

If your latitude is in the Southern Hemisphere and you end up in the CONTRARY area of the table, use the following formulas:

• If the true bearing is between 0°–90°, LHA = 180° + azimuth
• If the true bearing is between 270°–360°, LHA = 180° − azimuth

This is possible because the table contains precomputed solutions of the nautical triangle. When identifying a star, the problem is still solving a spherical trigonometric triangle. In principle, it would be possible to create a separate table book only for star identification. However, it would contain exactly the same numerical values as the position-fixing tables, only with the rows, columns, and labels renamed according to the rules above. Instead of saving shelf space and money, the navigator’s brain is used with the rules above, and the same table book can be reused for star identification.

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 practice
• Time
• Time difference (ΔT)
• Altitude measurement
• Example of position fixing
• Twilight
• Calculation of twilight time
• Navigation exam