🔍 Key Concepts

• Horizon system of coordinates (altitude, azimuth, zenith distance) • Celestial equator system (declination, hour angle, right ascension) • Relationship between hour angle and directions measured along the horizon

💭 Think About

• In the horizon system, which quantity is measured around the horizon in a way that is comparable to how local hour angle is measured around the celestial equator? • Which options are vertical (up–down) measurements, and which are horizontal (around-the-horizon) measurements? • How is local hour angle defined: is it an angle above/below the horizon, from the zenith, or around from a reference meridian?

✅ Before You Answer

• Identify which choices are measured from the horizon upward versus around the horizon • Recall that local hour angle is measured westward from the local meridian along the celestial equator • Match the type of angular measurement (direction around vs height above/below) between the two systems before choosing.