The local time and standard time on Earth is different : local time is determined by the sun’s motion, but standard time is the time defined by a nation for a particular region. While local time varies with longitude, standard time always stays the same no matter where you are. Other differences between local time and standard time include their meaning, changes, longitudinal features, units of measurement, and accuracy.

Local Time

The time at a particular place with reference to the overhead position of the sun in the sky is considered as the local time of that place.

• Local time of a particular place is determine by the mid-day sun. All places on the same meridian experience noon at the same time.
• As the Earth rotates 360º in 24 hours, in 1 hour it would rotate 360º/24º or 15º.
• Similarly, the earth would take 4 minutes to rotate 1º.
• Therefore, when it is midday at a particular place, the time would differ by 4 minutes for each degree of longitude east or west of that meridian.
• As the Earth rotates from west to east, places to the east see the sun first and so the time is ahead, while places to the west see the sun later and so are behind the time.
• Hence, if the time at Greenwich or 0º meridian is noon, it would be 12.04 pm at 1º E and 11.56 am at 1º W and similarly it would be11 am at 15º W and 1 pm at 15º E.
• Thus, each country has many local times and larger the E-W extent, the more the differences in local times.

Latitudes and Longitudes 

Latitudes

1. The Parallel of Latitudes extends from the Equator to 90 degrees North Pole and 90 degrees South Pole.
2. All the latitudes are parallel to the Equator.

Longitudes

1. Longitude is the east-west measurement of the prime meridian.
2. There are 180 vertical east longitudes of the Prime Meridian and 180 vertical west longitudes of the Prime Meridian

Standard Time and Time Zones

• If each town were to keep the time of its own meridian, there would be much difference in local time between one town and the other.
• Travelers going from one end of the country to the other would have to keep changing their watches if they wanted to keep their appoint­ments. This is impractical and very inconvenient.
• To avoid all these difficulties, a system of standard time is observed by all countries.
• Most countries adopt their standard time from the central meridian of their countries.
• In larger countries such as Canada, U.S.A., China, and U.S.S.R, it would be inconvenient to have single time zone. So these countries have multiple time zones.
• Both Canada and U.S.A. have five time zones—the Atlantic, Eastern, Central, Mountain and Pacific Time Zones. The difference between the local time of the Atlantic and Pacific coasts is nearly five hours.
• S.S.R had eleven time zones before its disintegration. Russia now has nine time zones.

Indian Standard Time

• Indian Standard Time calculates on the basis of 82.5° E longitude, just west of the town of Mirzapur, near Allahabad in the state of Uttar Pradesh.
• The longitude difference between Mirzapur and the United Kingdom’s Royal Observatory at Greenwich translates to an exact time difference of 5 hours 30 minutes.

The International Date Line 

• A traveler going eastwards gains time from Green­wich until he reaches the meridian 180°E when he will be 12 hours ahead of G.M.T. 
• Similarly, in going westwards, he loses 12 hours when he reaches 180°W. There is thus a total difference of 24 hours or a whole day between the two sides of the 180° meridian. 
• This is the International Date Line where the date changes by exactly one day when it is crossed. A traveler crossing the dateline from east to west loses a day (because of the loss in time he has made), and while crossing the dateline from west to east he gains a day (because of the gain in time he encoun­tered). 
• 
  The International Date Line in the mid-Pacific curves from the normal 180° meridian at the Bering Strait, Fiji, Tonga, and other islands in order to prevent confusion of day and date in some of the island groups intersected by the meridian.
• Some of them keep Asiatic or New Zealand standard time, others follow the American date and time.

Why international dateline drawn in a zigzag manner?

• The time difference on either side of this line is 24 hours. So, the date changes as soon as one crosses this line. To avoid any confusion of date, this line is drawn through where the sea lies and not land. Hence, the IDL is drawn in a zig-zag manner.

Local & Standard Time on Earth

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