Hour angle, Greenwich Hour Angle (GHA)

Now you have some idea of how latitude can be determined when a star is at its upper culmination. It is based on the fact that every star has a specific declination, the angle between the star and the plane defined by the equator. Since stars are effectively fixed in place, their declination does not change significantly. The declination indicates the latitude above which the star is always located and onto which its projection always falls.

As the Earth rotates, the projection point of a star moves along a latitude circle that is exactly equal to the star’s declination. This projection completes one full rotation around the Earth every 24 hours.

To determine the point above which a star is located at any given moment, both declination and hour angle are needed. There are three types of hour angle; first we consider the Greenwich Hour Angle (GHA).

The zero meridian, or 0° longitude, is defined to pass through a specific location on Earth: Greenwich in England. The hour angle is the angle between this meridian, the Earth's center, and the star, measured along the equator.

In the image above, the Greenwich Hour Angle of Alnilam is about 270°, and that of Diphda about 310°, etc. As the Earth rotates, the hour angle changes continuously, at a rate of 1° every 4 minutes.

C o n t e n t s

• Position fixing using stars
• The night sky
• The celestial sphere
• What you see from the North Pole as the Earth rotates
• How the starry sky changes when traveling to the equator
• Effect of Earth's rotation on the sky at the equator
• Celestial meridian, upper culmination and lower culmination
• Change in star altitude at the North Pole, equator, and between them
• Declination
• Example of determining latitude
• Hour angle, Greenwich Hour Angle (GHA)
• Aries, Greenwich of the celestial sphere
• 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
• Relationship between angles, degrees and nautical miles on Earth’s surface
• Position circle
• Azimuth (A, Azimuth)
• True bearing to a star
• Nautical triangle
• Solving the nautical triangle (altitude calculation)
• Celestial navigation using 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