Question

While deciding a country’s standard time, why it is necessary to choose longitude which is multiple of 15°? If you choose 7½°, what will happen?

Answer 1

While deciding a country’s standard time, it is necessary to choose a longitude that is a multiple of 15° because the Earth is divided into 24 time zones, each 15° of longitude apart, corresponding to one hour of time difference. This division aligns with Earth’s 24-hour rotation, making every 15° longitude represent one hour’s difference in time. Choosing a standard meridian that is a multiple of 15° allows the time difference with Greenwich Mean Time (GMT) to be expressed as whole hours, simplifying the calculation and uniformity of time within the country.

If you choose a longitude like 7½° (7.5°), which is not a multiple of 15°, the time difference with GMT would be expressed in half-hour increments, leading to non-integer time differences, such as 30 minutes. This is seen in countries like India, where the standard longitude is 82½° E, resulting in Indian Standard Time (IST) being 5 hours 30 minutes ahead of GMT. While this ensures a better fit geographically, it complicates time calculations because the difference is not in whole hours, but includes a half-hour offset.

Thus, choosing a 15° multiple longitude simplifies time zone calculations by keeping offsets in full hours, while choosing 7½° increments results in half-hour differences, which is possible but less common and more complex.