The Greenwich Meridian is an axial line drawn from North to South, passing through Greenwich, London, England. This is zero longitude and is the beginning of the first time zone (Western European Time Zone).
While we’re on the subject of meridians and such like, here’s an interesting definition for you to commit to memory: the difference between the true (also called real) solar day and the mean solar day.
The true solar day is the time it takes for the transit of the sun to pass travel from any given meridian back to the same point in the sky. The true solar day fluctuates in length because of the rotation and tilted ecliptic of the earth. The mean solar day is an average of the true solar day taken over a year, and works out at roughly 24 hours. The equation of time is the difference between the real solar day and the mean solar day at any point. At most the true solar day is 16:25 ahead of the true solar day, and at its furthest behind, it is 14:15 off the pace.
Some watches – very high-end watches – feature a complication displaying the difference between the mean solar day and the true solar day. Normally, this feature takes the form of a sub-dial that appears to be running either fast or slow depending on the time of year. It is functionally useless, but an unbelievably advanced complication for the purist to enjoy.
The equation of time can be mapped out on a graph, showing the position of the sun in the sky when viewed from a fixed point on every day of the year. If you had a camera trained on the heavens and took the same picture every day at the same time and then overlaid your results, you would see a bottom-heavy figure of eight pattern. In terms of complication manufacture, this pattern is (rather ingeniously) translated into a cam that mirrors the varying speed of the sun’s transit, resulting in an apparently simple display.