🔍 Key Concepts

• Greenwich Mean Time (GMT) and what location on Earth it is based on • The definition of a prime meridian in navigation and timekeeping • How the mean sun is used as a reference in determining time zones and standard time

💭 Think About

• Ask yourself: At what specific longitude is the world’s standard time reference established for GMT? • Consider which line is internationally agreed upon as the starting point (0°) for measuring longitude east and west. • Think about whether GMT is tied to the observer’s local position, a celestial reference point, or a fixed Earth-based meridian.

✅ Before You Answer

• Identify which choice corresponds to the prime meridian used for world timekeeping. • Confirm whether GMT is based on a fixed Earth meridian rather than on the observer’s changing position. • Be sure not to confuse a celestial reference point (used in celestial navigation) with the Earth-based meridian used to define GMT.