🔍 Key Concepts

• Celestial equator system vs. terrestrial (Earth) coordinate system • Relationship between latitude / declination and longitude / hour angle or right ascension • Which quantities measure angles east-west along an equator-like great circle

💭 Think About

• On Earth, longitude is measured east or west along the equator from a reference meridian. In the celestial system, what is the equivalent reference circle and starting point? • Which of the options represents an angular distance measured along the celestial equator, not north-south from it or along the horizon? • Look at each choice and decide: does this quantity describe a position on the celestial sphere relative to the celestial equator, or relative to the observer’s horizon?

✅ Before You Answer

• Identify which option is measured along the celestial equator from the Greenwich celestial meridian or similar reference • Eliminate any option that is clearly related to north-south position (like latitude) instead of east-west (like longitude) • Make sure the quantity you pick is used to define the hour angle or right ascension of a body, not its altitude or direction from the observer