Instruction: answer questions 38 to 40 based on the following text.

How did local clock time in Greenwich, London change the world?

01 Greenwich Mean Time (GMT) is the yearly average (or ‘mean’) of the time each day when

02 the Sun crosses the Prime Meridian at the Royal Observatory Greenwich. Essentially, mean time

03 is clock time rather than solar (astronomical) time. Solar time varies throughout the year, but

04 each day measured by a clock has the same length. It’s a way of standardizing time so we can

05 all know exactly what time it is at any location.

06 But it wasn’t until the invention of the pendulum clock in the 1650s that it was possible

07 to work out the relationship between mean time and solar time, which was done by John

08 Flamsteed in the early 1670s. Until the mid-19th century, almost every town kept its own local

09 time, defined by the Sun. However, the need for a standard became imperative in the 1850s and

10 1860s with the expansion of the railway and communications networks. British railway companies

11 introduced GMT across their networks to make their timetables less confusing, and by the mid-

12 1850s, almost all public clocks in Britain were set to GMT. It finally became Britain’s legal

13 standard time in 1880.

14 In 1884 the Greenwich Meridian was recommended as the Prime Meridian of the World

15 for 2 reasons. The USA had already chosen Greenwich as the basis for its own time zone system,

16 and in the late 19th century, 72% of the world's commerce depended on sea charts that used

17 Greenwich as the Prime Meridian. The recommendation was based on the argument that naming

18 Greenwich as Longitude 0º would be of advantage to the largest number of people. From 1884

19 to 1972, GMT was the international civil time standard, and it has now been replaced by

20 Coordinated Universal Time (UTC).

Available at: https://www.rmg.co.uk/stories/topics/greenwich-mean-time-gmt