🔍 Key Concepts

• Astronomical triangle (PZX triangle) and what its three vertices represent • Definition of a celestial meridian vs. prime meridian • Which great circle always passes through both the celestial pole and the zenith of the observer

💭 Think About

• Identify the three key points that make up the astronomical (PZX) triangle, then think about which great circle must always connect two of those points • Compare which of the listed circles depends on the observer’s position on Earth versus which are fixed by the Earth’s rotation or coordinate system • Ask yourself: which of these circles is defined for every observer, at every location, and is always part of the PZX triangle?

✅ Before You Answer

• Be clear on what the P (pole), Z (zenith), and X (celestial body) points are • Verify which option is a great circle that can pass through both P and Z for a given observer • Eliminate any circles that are fixed by longitude or latitude and do not necessarily go through the observer’s zenith