If it is 12 noon in a city located on 90°W longitude, what would be the time in a city located on 105° W longitude?

The Earth rotates 360° in 24 hours, which equates to 15° of longitude per hour or 1° every 4 minutes [c1][c2]. Time is advanced when moving eastward and retarded (subtracted) when moving westward [c2][t4]. In this scenario, the difference between the two locations is 15° (105°W - 90°W = 15°). Since 105°W is further west than 90°W, the time at 105°W will be behind the time at 90°W [c1][t4]. A 15° difference corresponds exactly to a 1-hour time difference [c3][t9]. Therefore, if it is 12:00 noon at 90°W, the time at 105°W (which is 15° further west) will be 12:00 minus 1 hour, resulting in 11:00 hours. This calculation aligns with standard time zone divisions where 90°W represents Central Time and 105°W represents Mountain Time [t1].

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