🔍 Key Concepts

• Universal Time (UT or UT1) and how it relates to the Earth’s rotation • The historical role of the prime meridian at Greenwich in defining world time • Differences between solar time, atomic time, sidereal time, and ephemeris time

💭 Think About

• Which of the choices is directly tied to the mean solar time at a specific reference meridian on Earth? • Which of these time scales was originally used as the worldwide standard for civil time before atomic time became common? • Think about what time scale navigators historically used when working with the standard meridian for longitude 0°.

✅ Before You Answer

• Identify which option is associated with the prime meridian (0° longitude) • Eliminate any options that are based on stellar positions rather than the Sun • Eliminate any options that depend on atomic clocks rather than Earth’s rotation