Longitudes are related to local time and standard time as they help us to calculate time.
• Time is measured by the movement of the earth.
• The earth spins on its axis West to East making a full turn of 360° in 24 hours (rotation).
• This means 15° per hour (15° x 24 = 360°) or say 4 minutes for one degree of longitude.
• Accordingly the earth has been divided into 24 time zones of one hour each.
• Moving eastwards from the Prime Meridian we get 12 time zones, 15° apart example: 0°, 15°E, 30°E etc. and likewise westwards (example 0°, 15°W, 30°W).
• Each place has different time of sunrise and sunset.
• Places east of Prime Meridian/ Greenwich Meridian experience sunrise (day) earlier than places lying west of the Meridian.
Local time of a place is based on the longitude of that place i.e. local time varies when places are situated on different meridians.
• To solve confusion in a country every country fixes its central meridian, passing through them as the standard meridian. The standard times are organised in time zones of 15° or 7.5°.
• The local time of the meridian is considered standard for the whole country. It is called standard time.
• Example: In India the longitude of 82 ½° E is treated as the standard meridian. The local time of the Meridian is known as Indian Standard Time (IST).
•Through an international, agreement, the local time of all places is linked to the Greenwich Mean Time. The IST is 5 hours 30 minutes ahead of GMT.
• Both local and standard time are related to longitude.
Local time refers to the time of a place as defined by the apparent motion of the Sun. Standard time is agreed time of places along the same meridian established by law in the same country.
While local time changes continuously as longitude changes, the standard time stays-the same for a specific nation.