🔍 Key Concepts

• Definition of a vertical circle on the celestial sphere • Relationship between the zenith (point directly overhead) and great circles • Difference between circles that pass through the poles versus those that are parallel to the horizon

💭 Think About

• Ask yourself: A vertical circle must start at the zenith; what other point must it reach so that it becomes a great circle rather than a small circle? • Which option describes a circle that is truly vertical, i.e., rising straight up from the horizon through the zenith, instead of running along or parallel to the horizon? • Do all great circles on the celestial sphere have to pass through the poles, or only some special ones (like meridians)?

✅ Before You Answer

• Verify which choice describes a great circle, not a small circle • Confirm that a principal vertical circle must pass through the zenith and also intersect the horizon at right angles • Eliminate any option that describes a circle that is parallel to the horizon or that only involves the poles and a terrestrial reference (like Greenwich)