What does 'one semi-diameter' indicate?

One semi-diameter refers specifically to the distance from the center of a celestial body, such as the sun, to its edge as viewed from Earth. In practical terms, when navigating or observing the sun's position, 'one semi-diameter' is crucial for calculating the sun's apparent altitude at true sunrise. True sunrise occurs when the upper limb of the sun just begins to illuminate the horizon.

In navigation, understanding this concept ensures that navigators can accurately determine their position relative to the sun's actual position in the sky, accounting for the apparent distortion caused by the sun's shape. Knowing the semi-diameter allows navigators to make precise adjustments in their calculations related to celestial navigation, ensuring more accurate readings.

Thus, the correct choice highlights the sun's position in relation to the horizon at true sunrise, placing importance on how celestial measurements are taken and used in navigation.