🔍 Key Concepts

• Celestial equator and how stars appear to move across the sky due to Earth’s rotation • Definitions of diurnal circle, hour circle, altitude circle, and vertical circle • Which circles are fixed to the sky and which are fixed to the observer’s position

💭 Think About

• Ask yourself: as the Earth rotates, what kind of path does a star trace on the celestial sphere, and how is that related to the celestial equator? • Which of these circles run north–south through the celestial poles, and which run east–west parallel to the celestial equator? • Which terms are based on the observer’s local horizon, and which are based on the celestial coordinate system (declination/right ascension)?

✅ Before You Answer

• Identify which option describes a path that is parallel to the celestial equator everywhere on the sky, not just at your location • Separate circles that depend on your horizon (altitude/vertical) from those that depend on the celestial coordinate system • Make sure the term you pick is associated with the daily (24‑hour) apparent motion of a body caused by Earth’s rotation