You can actually straddle the Greenwich meridian at the Royal Observatory - there's a metal line in the courtyard. The zero is a line of longitude, arbitarily set to go through Greenwich (the French tried pushing for a Paris-centred system, sorry Gebe). The equator is zero degrees of latitude. Daylight saving is also needed because day length (local noon to local noon) is not constant throughout the year, because the earth's orbital velocity varies with its distance from the sun.