USCG Celestial Navigation Practice Exam

When determining a line of position from a celestial body with an observed altitude, the correct method involves plotting the position as an arc around the geographic position (GP) of the celestial body. This is because the altitude measurement represents the angle above the horizontal plane (the observer's horizon) to the celestial body. If the observed altitude is 88°45.0', it indicates that the celestial body is very close to being directly overhead. When you plot this line of position, it isn't a straight line or a point; rather, it will be a circle (or arc) with the GP of the celestial body at its center. The arc will represent the locations from which the celestial body could be observed at that altitude. As you get closer to the zenith (which occurs at 90° altitude), the arc becomes smaller, but it will always be an arc, indicating the area of potential positions of the observer consistent with that altitude measurement. This method provides a more accurate representation of the navigator's position in relation to the celestial body.