The measurement of local time is based on...

🔍 Key Concepts

• local time vs. Greenwich Mean Time (GMT) – which one is based on the observer’s position? • What a meridian is – a line of longitude passing through a specific location on Earth • Difference between upper transit (Sun on your meridian at local noon) and lower transit (Sun on the opposite side of Earth)

💭 Think About

• Ask yourself: when we say "local noon," are we talking about the Sun being directly over your meridian or over the Greenwich meridian? • Think about whether the daily time scale we use is tied to when the Sun crosses above your horizon (upper branch) or when it is under your feet (lower branch). • Which crossing of a meridian by the Sun would be observable and practical for determining time on board a vessel?

✅ Before You Answer

• Confirm which meridian is used for local time—the observer’s meridian or the Greenwich meridian. • Decide whether upper transit (meridian passage at noon) or lower transit (midnight) is the standard reference for measuring local time. • Eliminate choices that combine Greenwich with local time, and focus on which branch (upper or lower) makes sense for a practical time measurement.

The measurement of local time is based on the passage of the Sun over the __________.

lower branch of the Greenwich meridian

upper branch of the Greenwich meridian

lower branch of the observer's meridian

upper branch of the observer's meridian

🔍 Key Concepts

💭 Think About

✅ Before You Answer