Sidereal hour angle (SHA)

Since stars remain effectively fixed on the inner surface of the celestial sphere, their position in the sky can be described using two values: declination and sidereal hour angle. The sidereal hour angle is the angle measured from Aries along the plane of the Earth's equator to the point where the star is located directly above or below the equatorial plane.

Because Aries remains fixed on the celestial sphere and stars move extremely slowly relative to a human lifetime, a star’s sidereal hour angle does not change significantly.

If you know the Greenwich Hour Angle of Aries for a given moment and the sidereal hour angle of a star, you can easily calculate the star’s Greenwich Hour Angle.

Star’s Greenwich Hour Angle (GHA) = Aries’ Greenwich Hour Angle () + star’s sidereal hour angle (SHA). If the result is greater than 360°, 360° is subtracted.

Naturally, the Greenwich Hour Angle of Aries changes continuously, by 1° every four minutes, because the Earth rotates at exactly that rate.

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, the 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