Solar time

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Apparent solar time (Local solar time) is determined by a sundial (The sun’s position in the sky).

15° longitude —» I hour 1° longitude —» 4 min

Mean time (Standard time): GCT = Greenwich Civil Time (0° longitude)

Local Civil Time (LCT): Determined by longitude.

Local Stand Time: Through the center of a time zone

EST = Eastern Standard Time (75° W longitude)

CST = Central Standard Time (90° W longitude)

MST = Mountain Standard Time (105° W longitude)

PST = Pacific Standard Time (120° W longitude)

Equation of time is a factor of earth orbital velocity. Due to non-symmetry of earth’s orbit, earth’s rotational speed is irregular (T Table 6-1 or A Table 8).

Local solar time = Local standard time

+ Equation of time

+ 4 (Local standard time meridian — local longitude) [Minutes]

Local Solar Time (LST): Time measured by position of sun Daylight Saving Time (1 hr advance in summer)

Example 7-1

Longitude of a PSU classroom is 78° W. Determine the local solar time at EST 12:00 noon on February 21.

Solution:

EST is for longitude of 75° W. From T Table 6-1 or A Table 8, equation of time is about -14 minutes. Then

Local solar time = Local standard time + Equation of time

+ 4 (Local standard time meridian — local longitude)

= 12:00-0:14 + 0:4 (75-78)

= 11:34 am.

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