What features does an observer's meridian include?

The observer's meridian is a crucial concept in astronomy and navigation, particularly in the context of celestial observations. It is defined as a great circle that passes through the observer's zenith (the point directly above them) and both celestial poles. This positioning allows for precise measurement of celestial objects' altitudes and azimuths.

In celestial navigation, the observer's meridian is used to determine local noon, which is the moment when the sun is at its highest point in the sky relative to the observer's position. This is essential for accurate timekeeping and navigation as it relates to the earth's rotation and the sun's apparent motion.

Thus, option C correctly identifies the essential characteristics of the observer's meridian. It encapsulates the meridian's function as a reference line for astronomical observations, illustrating how it connects the zenith with the celestial poles. The other options do not accurately represent the definition or characteristics of an observer's meridian, focusing instead on different celestial or geographical features that do not pertain directly to this concept.