The Local Mean Time

The time elapsed from the midnight of the place is  known as local mean time. Thus L M T at midnight is  zero hours. This is different from the hour-angle of the mean Sun.
The Earth is rotating from the west to east and it completes one rotation with respect to the Sun in one civil day. However, it completes one complete rotation with reference to any distant star in one sidereal day.  Its  spin in a sidereal day is 360°, while the same is about 361° for a civil day. A mean solar day is of 24 hours 3 minutes and 56.56 seconds in sidereal time. Mean sidereal day is equal to 24 hours, in sidereal hours, and is 23 hours 56 minutes 4.09 seconds in mean solar time. For simplicity, both solar day and  sidereal day are taken as 24 hours in terms of their own hours. Thus, the Earth rotates 360° in 24 hours or, say, 1° in 4 minute. The places which are in the east  will see the rising of the Sun early and those in the  west will see it later. If the difference is 10° in terrestrial longitude, the difference in sunrise will be  of 10 x 4 = 40 minutes provided the terrestrial latitude is the same. This way, we can see that the local mean time of places at different terrestrial longitudes will be different in a country or a zone and the day-to-day work of the society will face a lot of trouble and practically be disrupted in the present era. The terrestrial latitudes and longitudes of Mumbai are 18°58’ (N) and 72°50′ (E), and of Calcutta 22°35′ (N) an 88°23’ (E).
The difference between latitudes is only 3°37′ while in the longitudes it is 15°33′, i.e. the difference between their local time will be of 15°33′ x 4 = 62 minutes 12 seconds, i.e. 1 hour 2 minutes 12 seconds. The person at Mumbai will at his noon say that the time is 12 hours while at Calcutta he will say no, it is 1 hour 2 minutes 12 seconds p.m. and the schedule of railway timing, plane timing, radio, television, etc. will not be possible. So, a way was devised that within a country or a zone (in large countries), one standard meridian is fixed and the time of that meridian is taken as standard time for that country or the zone. It is called the standard time of a country or zonal standard time of that zone.
In India the standard meridian is having a longitude 82°30′ (E) and this meridian passes through a place near Varanasi. The local mean time of this place is the Indian standard time and is followed throughout India. Similarly, other countries or zones have also fixed their standard meridians. When going through the last few pages of the table of ascendents , it will be seen that the standard meridians of a country or a zone are fixed in such a way that normally the time  difference between that of Greenwich is a multiple of half an hour. The difference between the 1ST l Indian Standard Time) and that of the Greenwhich mean time is 5‘/2 hours.

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