Question.
How are local time and standard time related to longitude?
( Chapter 1, Locating Places on the Earth)
Answer.
Local time and standard time are closely related to longitude because the Earth is divided into time zones, each of which corresponds to a specific longitudinal range.
Longitude and Local Time:
The local time of a location is based on its Longitude.
Each degree of longitude corresponds to a time difference of about 4 minutes. This is because the Earth rotates 360 degrees in approximately 24 hours, so each degree of longitude represents 1/360 of a full day or 4 minutes.
For example, the Longitude of New Delhi and Kolkata is about 77 East and 88 degrees east respectively, so the local time of New Delhi and Kolkata would be 5.13 minutes and 5.52 minutes ahead of London Time.
Standard Time and Longitude:
Standard Time was introduced to keep uniform time for regions. Local time of Delhi to Kolkata may vary, however standard time of both is the same ( that is based on Indian Standard Time).
To achieve standard Time, the time zone for London (Greenwich Mean Time or GMT) is centered on 0 degrees longitude, where the Prime Meridian is located.
The Standard Time of the region ( Country) is generally multiple of 7.5 degrees of longitude wide, corresponding to 30 minutes.
That is why, despite having a wide longitudinal difference in India, Indian Standard Time (IST) is based at 82.5 degrees ( 7.5 * 11=82.5) east longitude.
IST is 5 hours and 30 minutes ahead of London (Greenwich Mean Time Zone).
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