the mean sun is 195° (13 hours), the mean time is 12 + 13 =25 hours, which is the same as 1
o’clock mean time after the midnight (i.e., next. Day).
The Equation of Time
The difference between the mean and the apparent solar time at any instant is known
as the equation of time. Since the mean sun is entirely a fictitious body, there is no means to
directly observe its progress. Formerly, the apparent time was determined by solar
observations and was reduced to mean time by equation of time. Now-a-days, however, mean
time is obtained more easily by first determining the sidereal time by steller observations and
then converting it to mean time through the medium of wireless signals. Due to this reason it
is more convenient to regard the equation of time as the correction that must be applied to
mean time to obtain apparent time. The nautical almanac tabulates the value of the equation of
time for every day in the year, in this sense (i.e. apparent – mean). Thus, we have
Equation of time = Apparent solar time – Mean solar time
The equation of time is positive when the apparent solar time is more than the mean
solar time ; to get the apparent solar time, the equation of time should then be added to mean
solar time. For example, at 0h G.M.T. on 15 October 1949, the equation of the time is + 13m
12s. This means that the apparent time at 0h mean time is 0h 13m 12s. In other words, the true

sun is 13m 12s ahead of the mean sun. Similarly, the equation of time is negative when the
apparent time is less than the mean time. For example, at 0h G.M.T. on 18 Jan., 1949, the
equation of time is – 10m 47s. This means that the apparent time at 0h mean time will be 23h
49m 13s on January 17. In other words, the true sun is behind the mean sun at that time.
The value of the equation of time varies in magnitude throughout the year and its
value is given in the Nautical Almanac at the instant of apparent midnight for the places on
the meridian of Greenwich for each day of the year. For any other time it must be found by
adding or subtracting the amount by which the equation has increased or diminished since
midnight.
It is obvious that the equation of time is the value expressed in time, of the difference
at any instant between the respective hour angles or right ascensions of the true and mean
suns.
The amount of equation of the time and its variations are due to two reasons :
(1) obliquity of the ecliptic, and (2) elasticity of the orbit. We shall discuss both the effects
separately and then combine them to get the equation of time.
11. Explain the conversion of local time to standard time and vice versa.
The difference between the standard time and the local mean time at a place is equal to
the difference of longitudes between the place and the standard meridian.
If the meridian of the place is situated east of the standard meridian, the sun, while
moving apparently from east to west, will transit the meridian of the place earlier than the
standard meridian. Hence the local time will be greater than the standard time. Similarly, if

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