Which of the following would reckon noon from the upper transit of the mean sun on the Greenwich meridian?

• A. Local Mean Time
• B. Coordinated Universal Time
• C. Universal Time
• D. Greenwich Mean Time

Answer: (C)

Noon is defined by the Sun crossing the local meridian, and when you set that reference at the Greenwich meridian, the time scale tied to that solar event is Universal Time. UT, specifically UT1, is the time derived from Earth's rotation and aligns with the mean solar day; the moment the mean Sun makes its upper transit on the Greenwich meridian corresponds to noon on UT.

Greenwich Mean Time is the historical solar time at Greenwich, which is closely related to UT but today we use UTC for civil time, based on atomic time with leap seconds to stay aligned with Earth's rotation. Local Mean Time would be the mean solar time for a local longitude, not a standard reference at Greenwich.