Question: Where is the starting meridian for the longitude system on the world map?

Knowledge 1: Meridian (geography)
A (geographical) meridian (or line of longitude) is the half of an imaginary great circle on the Earth's surface, terminated by the North Pole and the South Pole, connecting points of equal longitude. The position of a point along the meridian is given by its latitude indicating how many degrees north or south of the Equator the point is. Each meridian is perpendicular to all circles of latitude. Each is also the same length, being half of a great circle on the Earth's surface and therefore measuring 20,003.93 km (12,429.9 miles). [SEP] Knowledge 2: Prime meridian
On Earth, starting at the North Pole and heading south to the South Pole, the IERS Reference Meridian (as of 2016) passes through:
Knowledge 1: Meridian (geography)
A (geographical) meridian (or line of longitude) is the half of an imaginary great circle on the Earth's surface, terminated by the North Pole and the South Pole, connecting points of equal longitude. The position of a point along the meridian is given by its latitude indicating how many degrees north or south of the Equator the point is. Each meridian is perpendicular to all circles of latitude. Each is also the same length, being half of a great circle on the Earth's surface and therefore measuring 20,003.93 km (12,429.9 miles). [SEP] Knowledge 2: North Pole
The North Pole is the northernmost point on the Earth, lying diametrically opposite the South Pole. It defines geodetic latitude 90 ° North, as well as the direction of true north. At the North Pole all directions point south; all lines of longitude converge there, so its longitude can be defined as any degree value. Along tight latitude circles, counterclockwise is east and clockwise is west. The North Pole is at the center of the Northern Hemisphere.
Knowledge 1: Greenwich Mean Time
As the United Kingdom grew into an advanced maritime nation, British mariners kept at least one chronometer on GMT to calculate their longitude from the Greenwich meridian, which was by convention considered to have longitude zero degrees, internationally adopted in the International Meridian Conference of 1884. Synchronisation of the chronometer on GMT did not affect shipboard time, which was still solar time. But this practice, combined with mariners from other nations drawing from Nevil Maskelyne's method of lunar distances based on observations at Greenwich, led to GMT being used worldwide as a standard time independent of location. Most time zones were based upon GMT, as an offset of a number of hours (and possibly a half - hour) ``ahead of GMT ''or`` behind GMT''. [SEP] Knowledge 2: Prime meridian
A prime meridian is a meridian (a line of longitude) in a geographic coordinate system at which longitude is defined to be 0 °. Together, a prime meridian and its antimeridian (the 180th meridian in a 360 ° - system) form a great circle. This great circle divides the sphere, e.g., Earth, into two hemispheres. If one uses directions of East and West from a defined prime meridian, then they can be called the Eastern Hemisphere and the Western Hemisphere.

Answer: latitude 90 ° North