🔍 Key Concepts

• Horizon system of coordinates vs the equatorial system • What lines on a globe run from pole to pole (north–south) and what their equivalent is in the sky • The meaning of vertical circle and what it passes through

💭 Think About

• First, recall what a meridian is on the Earth: what two points does it always connect, and what does it cross at your position? • In the horizon system, think about lines on the sky that start at the zenith (point directly overhead) and go down to the horizon. What are those lines called? • Contrast that with the term hour circle: which coordinate system is that associated with, and what reference line (equator or horizon) does it relate to?

✅ Before You Answer

• Be sure you are using the horizon system, not the equatorial system • Confirm which option describes great circles that pass through the zenith and nadir of the observer • Eliminate any option that is defined relative to the celestial equator rather than the local horizon