the mean sun is 195 13 hours the mean time is 12 13 25 hours which is the same

# The mean sun is 195 13 hours the mean time is 12 13

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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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