🔍 Key Concepts

• Meridian transit – when a celestial body crosses the observer’s local meridian (the north–south line through the observer) • How altitude and declination are shown in a meridian altitude diagram • Difference between a diagram based on a global reference (like Greenwich) and a local reference (the observer)

💭 Think About

• Ask yourself: At meridian transit, are we mainly concerned with a global reference meridian (like Greenwich), the celestial equator from a pole, or the observer’s own north–south line? • Which choice best describes the geometric plane that contains both the observer’s zenith and the celestial poles? • When you draw the altitude of the Sun or a star at LAN (local apparent noon), what line on your sketch does the body cross?

✅ Before You Answer

• Identify which option refers to the observer’s own meridian, not a distant or global reference meridian • Confirm that meridian transit is about the body crossing the local north–south great circle, not the equator itself • Eliminate options that only describe a viewpoint (e.g., "as observed from above a pole") rather than the geometric plane used for the altitude diagram