What is a circle of equal altitude?

A circle of equal altitude refers to a specific concept in celestial navigation where the position of a celestial object, such as a star or the sun, appears to be at the same angle above the horizon from all points on that circle. When a celestial body’s altitude is the same for all observers situated along this circle, it allows navigators to determine locations on the Earth’s surface based on the observed elevation of heavenly bodies.

This concept is crucial for celestial navigation as it simplifies the process of charting one’s position. By taking simultaneous altitude measurements of celestial bodies at known times, navigators can use these equal altitudes to draw lines of position on their charts, which helps in determining their location on Earth.

In contrast, the other options do not accurately describe a circle of equal altitude. A path followed by a ship at a constant speed pertains to speed and course rather than altitude measurements. A route defined by varying celestial altitudes does not conform to the idea of a circle where the altitude remains constant. The path of a star across the celestial sphere refers to its trajectory as viewed from Earth but does not denote a position of equal altitude.