Longitude and Time Summary

Sun is the best time-keeper throughout the world. It Sun regularly rises and sets every day. Local time can be measured by the shadow cast by the sun. All the places on a meridian have midday at the same moment. If the earth rotates from West to East, places East of Greenwich are ahead of Greenwich time and those in the West behind it. Since the earth rotates 360° in 24 hours, every 15° there is a difference of 1 hour.

      -: Standard Time :- 
Local time of places which are on different meridians differ. In India there will be a time difference of about 1 hour and 45 minutes in the local time of Dwarka in Gujarat and Dibrugarh in Assam. It will be difficult to prepare a time-table of trains which move from one corner to another. It is therefore necessary to adopt the local time of some central meridian of a country as the standard time for the country. In India 82°30'E is treated as the standard meridian. The local time at this meridian is taken as the standard time for the whole country. Some countries have a great longitudinal extent and so they adopt more than one standard time. For example USA has as many as 5 standard times. The earth has been divided into 24 time zones of one hour each. A few countries like India adopt a time zone in between the two, like 5½ hour zone.

     -: Time Correction :- 
In countries with high latitudes, day duration changes drastically from say 6 hours in winters to 18 hours in summers. At poles this difference becomes so large that there is day for six months and night for six months. In Northern latitude May, June are longer, whereas in Southern latitude December January are longer and they have summer at this time. In summer the sun rises very early. To take the advantage of sun light, the clocks are advanced by one hour during summer for about six months and it is set back to original position during winter. This advanced time is called "summer time" or "day light saving time". At some places the correction is done for 2 hours and it is called as "double summer time". Similarly sometimes it is only half an hour correction. Since this adjustment is only to save light, it is subtracted before we do any astronomical calculation.

      -: Summary :-
For astrological purposes we need date, time and place of birth. City of birth is normally sufficient as place of birth. Along with this one should know the standard time zone and the day light saving time correction to know exactly and correct the time of birth. With the knowledge of coordinates we can even determine the distance between two points. Longitudinal distance play vital role in time gap compared to latitude.