🔍 Key Concepts

• Celestial sphere model and how it represents the sky around an observer • Relationship between zenith (point directly above) and its opposite point • Difference between positions relative to the observer vs. fixed Earth locations like Greenwich

💭 Think About

• First, picture yourself standing on Earth and imagine the point in the sky directly above you. What is that point called, and what would be the opposite direction from it? • Which of the options describes a position that depends on where the observer is standing, rather than a fixed place on Earth? • Is the angle between the zenith and its opposite point greater than, less than, or equal to 90°?

✅ Before You Answer

• Identify which choices describe something observer-dependent rather than Earth-fixed (like a specific meridian). • Recall that zenith is the point directly overhead; the nadir is related to zenith by direction through the observer. • Check whether being 90° away from the zenith matches the idea of a point exactly opposite overhead on the celestial sphere.