London and St. Louis (in America) are 90 degree of longitude-apart Therefore,

The Earth rotates 360° in 24 hours, which translates to a rate of 15° per hour or 1° every 4 minutes [c1][c2]. A longitude difference of 90° therefore results in a time difference of 6 hours (90 divided by 15) [c3][t5]. The Earth rotates from west to east, meaning places located to the east see the sun earlier and are ahead in time, while places to the west see the sun later and are behind [c1][c2]. London is located at approximately 0° longitude (Prime Meridian), while St. Louis is located at approximately 90°W [t6][t10]. Since London is 90° east of St. Louis, it is 6 hours ahead in local solar time [t9]. Consequently, solar events such as sunrise and sunset occur 6 hours earlier in London than in St. Louis [c1][t3].

Sources

1. [1] Certificate Physical and Human Geography , GC Leong (Oxford University press 3rd ed.) > Chapter 2: The Earth's Crust > Longitude and Time > p. 11
2. [2] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 18: Latitudes and Longitudes > Longitude and Time > p. 243
3. [3] Exploring Society:India and Beyond. Social Science-Class VI . NCERT(Revised ed 2025) > Chapter 1: Locating Places on the Earth > Understanding Time Zones > p. 20