Paper 2 MA (Astrology) year I

Time Variation due to different geographical locations:

 

Difference of time due to difference of places:

 

The duration of a mean solar day is 24 hours.  i..e the Earth rotates on its axis once in a day in 24 hours or 24x 60 = 1440 minutes.  This rotation is of 360 degrees.  Therefore to ratate 1 degree, the Earth will take 1440÷360=4 minutes.  Due to this rotation on its axis, the Sun appears to rise in the east and set in the west.  The eastern parts of the Earth get sunrise earlier and each 1 degree longitude towards the west gradually get the sunrise later, at about 4 minutes interval. For every 15 degrees of difference of longitude there is a time difference of one hour.

 

This time difference of 4 minutes per 1 degree longitude is not uniform due to various factors like latitude, tilt of Earth’s axis etc.  The time of each place therefore will be different and is called the Local Mean Time of that particular place.

 

Longitudes:

We have already discussed under “Fundamentals of Astronomy” that Longitudes are the imaginary circles drawn joining the North and South poles and perpendicular to Equator. For mutual understanding in International affairs, the longitude passing through the Greenwich place in England is treated as standard zero degree longitude. The part 180 degrees east of it is treated as east half of the Earth  while the part 180 degrees west of it is treated as west half of the earth.  These imaginary longitudes are drawn at one degree intervals. 

 

Standard Time:

The change of time for each degree of longitude causes inconvenience in management. So each country adopts a specific time as its’ standard time. Greenwich time for England, Paris time for France, Berlin time for Germany etc.  have been adopted as respective Standard times.  The time of the middle place between west located Bombay and east located Calcutta  which is at 82½ degrees longitude east of Greenwich is taken as a standard time in India. This passes through TUNI town of East Godavari District of A.P..   Indian Standard Time is 5½ hours advance to the time of Greenwich time. (82.5 x 4/60 = 5½ hours)  As United States of America is spread from east to west, four standard times are adopted considering four different places. These are called “Zonal Times”.

 

Standard Zones:

The surface of the earth is divided into 24 standard zones each at 15 degrees interval of longitudes east to Greenwich and also at west to Greenwich. The watches show the same time between the places located at 7½ degrees on either side of the respective longitude of the standard zone.

 

 

Meridian:

In many places, the time is measured with reference to mid-noon. The mid-noon is known from the meridian.This is an imaginary line technically.  The meridian can be defined  as a great circle passing through the zenith (the part of the sky that is directly above an observer) and the celestial poles. It corresponds to the grographical longitude of a place.

 

International Date Line:

The longitude at 180 degrees (East or west) to Greenwich is called “International Date Line”.  Ships and Planes crossing the line in Westernly direction from west to east should increase one day in the date column.  Similarly ships and planes crossing the line in Easternly direction from east to west should decrease one day in the date column.Also when crossing the Standard Zones, 1 hour per Zone should be either increased or decreased basing  upon the journey to  east or west side respectively.

 

War time:

During the second world war from 1-9-1942 to 14-10-1945, the Indian standard time was advanced by one hour all over India (including  present Bangladesh and Pakistan) for the purpose of day light saving.  Any recorded time of this period should therefore be reduced by 1 hour to get I.S.T.

 

LOCAL MEAN TIME:

 

Indian Standard Time and Local Mean Time are different.   IST is common for any place in India where as the LMT is the mean solar time arrived based on the longitude of the particulalr place for which the LMT is calculated.  This is essentially required to prepare a horoscope. Any place before the standard longitude (82˚-30’) falls west of it and the time by every 4 minutes per degree should be reduced from IST.  Likewise, any place after Standard longitude  falls east of it and  and the time be added by every 4 minutes per degree to IST.