The Sun takes four minutes to traverse one degree of longitude from East to West.
Sundials on the same line of longitude all tell the same time (shown on clocks for clarity).
Sundials on the 4° meridian are 16 minutes behind Greenwich time and those at 8° West are 32 minutes ‘slow’.
Example: Plymouth is 4° 08’ West of Greenwich. Sundials in Plymouth are always 16 minutes 32 seconds slow.
In 1880, to avoid chaos when the first railways gave rapid east/west travel,
Parliament established Greenwich Mean Time (GMT) so all UK clocks told the same time as Big Ben.