Midday GMT is defined to be when the mean sun is directly over the prime meridian. ("Mean sun" as opposed to "apparent sun" because of the Earth being a natura

Midday GMT is defined to be when the mean sun is directly over the prime meridian. ("Mean sun" as opposed to "apparent sun" because of the Earth being a naturally occurring celestial object, rather than an idealised mathematical model. See http://en.wikipedia.org/wiki/Equation_of_time http://en.wikipedia.org/wiki/Equation_of_time .)