🔍 Key Concepts

• Principal vertical circle vs prime vertical – understand the difference • Relationship between a body's hour circle, vertical circle, and the observer's meridian • What it means for the Sun to be in upper transit vs lower transit

💭 Think About

• Think about which vertical circle passes through the observer’s zenith and the observed body when we say the Sun is on the principal vertical circle. • Ask yourself: when would the Sun’s position line up with an important great circle that is defined by the observer’s horizon and meridian? • Consider which of the listed options describes a condition of transit across the meridian rather than a specific azimuth like N 135° E or W.

✅ Before You Answer

• Be clear on the definition of principal vertical circle (how is it oriented relative to the observer’s meridian?). • Verify whether azimuths like N 135° E/W correspond to any special named circles (meridian, prime vertical, etc.). • Check which event, upper or lower transit, involves the Sun crossing the observer’s meridian, and how that relates to the principal vertical circle.