The standard clock time is reckoned from a selected meridian near the center of a time zone or from the standard meridian, the Greenwich, which is at longitude of 0°. Since the sun takes 4 min to transverse 1° of longitude, a longitude correction term of 4 X (Standard longitude - Local longitude) should be either added or subtracted to the standard clock time of the locality. This correction is constant for a particular longitude, and the following rule must be followed with respect to sign convention. If the location is east of the standard meridian, the correction is added to the clock time. If the location is west, it is subtracted. The general equation for calculating the apparent solar time (AST) is
LST = local standard time. ET = equation of time. SL = standard longitude. LL = local longitude.
DS = daylight saving (it is either 0 or 60 min).
If a location is east of Greenwich, the sign of Eq. (2.3) is minus (—), and if it is west, the sign is plus (+). If a daylight saving time is used, this must be subtracted from the local standard time. The term DS depends on whether daylight saving time is in operation (usually from end of March to end of October)
or not. This term is usually ignored from this equation and considered only if the estimation is within the DS period.
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