True solar time TST and hour angle x--Algorithm 1.13

True solar time differs from clock time. The difference is due to (a) differences between the site longitude and the standard meridian (for example: 0, Greenwich meridian), (b) the Equation of Time, and (c) summer time or daylight-saving conventions.

Local clock time, LT hours from midnight
Longitude of site, l degrees (positive west of Greenwich)
Longitude of standard meridian, ls degrees
Equation of Time, ET hours (algorithm 1.12)
Summer time or daylight saving, TD hours (positive when clock time later than standard time)


IES (1)

  1. IES Calculation Procedures Committee Recommended practice for the calculation of daylight availability Journal of the Illuminating Engineering Society of North America 13 (4) 381-392 (1984)

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