🔍 Key Concepts

• Horizon system of coordinates (altitude and azimuth) • How the Earth’s poles relate to “up-down” directions on a sphere • Difference between directions on the local sky and directions fixed in the celestial sphere (like celestial/ecliptic poles, nodes)

💭 Think About

• In the horizon system, what two opposite directions describe ‘straight up’ and ‘straight down’ from your position? • On a globe of the Earth, the poles are the ends of the Earth’s axis. In your local sky, what two opposite points play the same role of being the ends of a local ‘vertical’ axis? • Which of the options are tied to your local position on Earth, rather than being fixed by the Earth’s rotation axis or the ecliptic?

✅ Before You Answer

• Be clear on what the horizon system uses as its reference plane (your local horizon) and its vertical direction. • Identify which choice represents two opposite points aligned with your local vertical (not the Earth’s rotation axis or the ecliptic). • Eliminate options that are defined by the Earth’s axis or the ecliptic plane, not by the observer’s specific location and vertical.