Calculation of time

The speed of Sun never stays same for a whole year. It keeps on changing every time. That is why, time on earth is different at different places. The standard time for every country is different. The local time for every country is also not same. The time of Greenwich is taken as standard time for the whole world and the standard time of all the countries is calculated according to this time. The standard time of India is 82 degrees 30 minutes. Let us first understand that what is local time and then you will be able to make changes in local time. We should study the Indian Standard Time now.

Indian Standard Time (I.S.T)

The standard longitude of India is 82 degrees 30 minutes. Sun will be exactly above this standard longitude at 12:00 pm.

Local Mean Time (L.M.T)

The time decided for a particular place is known as Local Mean Time(L.M.T).

Greenwich Mean Time (G.M.T)

The Sun is above Greenwich Meridian line at 12 p.m in Greenwich. When Sun shifts from Greenwich Meridian towards west, the time changes by 1 degree. 1 degree is equal to 4 minutes.

The time calculated like this is known as Greenwich Mean Time.

• 15 degrees = 1 hour
• 1 degree = 4 minutes
• 1 minute = 4 second

Zonal Standard Time (Z.S.T)

The time determined according to the longitude and meridian of a particular place according to the Greenwich Mean Time is known as Zonal Standard Time (Z.S.T). Ephemeris is required to find out the time of a particular place. The standard time of all the countries is available in an ephemeris. The Local Standard Time of various cities of India are also available in an Ephemeris. You can also find out the Mean time of all the cities with the help of an ephemeris.

We are explaining you the method of finding out Mean Time of a city with the help of an example. We hope that this will help you to understand the method in a better way.

The local Time of Dispur is 11 degrees 50 minutes at night. You should first note down the Longitude and Latitude of Dispur from an ephemeris. Now, note down the change of the Local Time from the ephemeris. The longitude of Dispur is 93.48 east and the latitude of Dispur is 25.05 north. The change in local time is +45.12.

We will add the Local Standard time of Dispur which is 11:50 pm and the change in local time. The Indian Standard Time of this place is 11:50 p.m. The Local Meridian Time (L.M.T) will be now added in this. We will obtain a new time now. We have 00:35:12 hours of night. If this time is of 30 July 2006, then it will now become the time of 30/31 July 2006. You can also write it as 00:35:12 hours of 31 July.

Let us understand this difference of time in detail.

• Longitude of Dispur                            :          93 degrees 48 minutes
• Standard Longitude of India                :          82 degrees 30 minutes
• Difference between these two            :          11 degrees 18 minutes
• You should now understand that 1 degree = 4 minutes (according to Greenwich Mean Time), then 11 degrees*4 = 44 minutes.
• If 1 minute = 4 seconds, then 18 seconds*4 = 72 seconds. Hence, 60 seconds = 1 minute.

1 minute when added to 44 minutes gives 45 minutes 12 seconds. This was the Local Meridian Time change which was done in the time of Dispur. The results obtained from Local Meridian Time change done according to an ephemeris or the change done by finding out the difference of longitudes would remain same.