🔍 Key Concepts

• Zenith and nadir on the celestial sphere • Difference between a meridian and a vertical circle • What makes the prime vertical special among vertical circles

💭 Think About

• Ask yourself: a circle through the zenith and nadir is what general kind of circle on the celestial sphere? • Now, among the options, which term describes the vertical circle that goes specifically through the east–west points of the horizon? • Which option refers instead to circles through the celestial poles or the observer’s north–south line, and can therefore be eliminated?

✅ Before You Answer

• Confirm which option names a circle that MUST pass through zenith, nadir, and the east point of the horizon. • Eliminate any term that is associated with the observer’s meridian (north–south line) rather than the east–west line. • Make sure you distinguish between hour circles (related to celestial poles/right ascension) and vertical circles (through zenith–nadir).