The values of the Greenwich hour angle and...

🔍 Key Concepts

• Greenwich hour angle (GHA) is an angle measured from the Greenwich meridian to the hour circle of the body • In the Nautical Almanac, GHA and declination are geometric positions used for ALL observers, not dependent on limb you sight • The difference between a body’s true center and what you actually measure (upper or lower limb) with a sextant

💭 Think About

• Ask yourself: when astronomers and almanac compilers describe the position of a celestial body in space, do they define that position by its limb or by a specific geometric point? • Think about how you handle a lower‑limb Sun sight in sight reduction: which part do you correct with semi‑diameter and which part is given directly in the almanac? • Consider whether the almanac would need separate GHA/Dec values for upper limb, lower limb, Sun, Moon, planets, and stars—or if one common reference point is more practical.

✅ Before You Answer

• Verify what GHA actually represents: an angle to the body’s hour circle associated with a single point on the body • Recall that declination is defined like latitude on the celestial sphere and is referenced to a specific point of the body, not its apparent edge • Confirm which corrections (e.g., semi‑diameter) are applied by the navigator, and which fundamental position is given directly in the almanac

The values of the Greenwich hour angle and declination, tabulated in all almanacs, are for the _________.

centers of the various celestial bodies

lower limb of a celestial body

upper limb of a celestial body

lower limb of the Sun and Moon; center of the stars and planets

🔍 Key Concepts

💭 Think About

✅ Before You Answer