Greenwich Time is equal to the local time plus the ______ of the place.

• A. latitude
• B. altitude
• C. longitude
• D. declination

Answer: (C)

Time difference around the globe is governed by longitude because the Earth completes one rotation in 24 hours, which means 15 degrees of longitude correspond to one hour. Greenwich Time serves as the reference time at the prime meridian, so the local solar time at any place differs from Greenwich Time by that longitude difference converted to hours. In practice, you add the place’s longitude (expressed in time, with east positive and west negative) to local time to get Greenwich Time. The sign convention is what keeps the relation consistent: places east of Greenwich yield a positive adjustment, while places west yield a negative one. This is why longitude is the quantity that links Greenwich Time to local time. Latitude, altitude, and declination don’t influence this time difference. For example, at 90 degrees west, longitude/15 is -6 hours, so Greenwich Time is six hours behind local time; at 30 degrees east, longitude/15 is +2 hours, so Greenwich Time is two hours ahead when viewed through the same signed convention.