Solar time

Table 3.9 summarizes the solar zenith angles at latitudes of 20, 30, 40, and 50°N as a function of month and true solar time. True solar time, also known as apparent solar time or apparent local solar time, is defined as the time scale referenced to the sun crossing the meridian at noon. For example, at a latitude of 50°N at the beginning of January, two hours before the sun crosses the meridian corresponds to a true solar time of 10 a.m. from Table 3.9, the solar zenith angle at this time is 77.7°. 

Figure 3.22 shows the solar angle 0 as a function of true solar time for several latitudes and different times of the year. As expected, only for the lower latitudes at the summer solstice does the solar zenith angle approach 0° at noon. For a latitude of 50°N even at the summer solstice, 0 is 27°. 

TABLE 3.9 Tabulation of Solar Zenith Angles (deg) as a Function of True Solar Time and Month... 

FIGURE 3.22 Effect of latitude on solar zenith angle. On the scale of true solar time, also called apparent solar time and apparent local solar time, the sun crosses the meridian at noon. The latitudes and seasons represented are as follows I, 20°N latitude, summer solstice II, 35°N latitude, summer solstice III, 50°N latitude, summer solstice IV, 20°N latitude, winter solstice V, 35°N latitude, winter solstice VI, 50°N latitude, winter solstice (from Leighton, 1961). 

DAY (d). A unit of time, the exact definition of which is dependent upon which system of time measurement Is referred to, i.e., apparent solar time, mean solar time, universal time, apparent sidereal time, ephemera time, or atomic time. See Time. With exception of atomic time, the time base is referenced to rotation of the Earth. For general purposes, a day is considered tire period taken for 1 revolution of the Earth about its axis... 

Solar time may deviate from Ihe local lime. Solar noon at a location is the time when the sun is at ihe highest location (end thus when the shadows are shortesi) Sciat radialron data are sym neltic about the sc ar noon the value on a west vrall belote the solar noon is equal lo Ihe value on an east wall two hours alter Ihe solar noon. 

The declination <5 of the sun, for 1st September, that appears in this equation, can be taken from tables presented by M. Iqbal [5.34] (5 = 8.51°. The hour angle ui is calculated from the local solar time ts, according to... 

I8 This equation yields the mean local solar time. In order to obtain the actual (date dependent) local solar time, a small correction, the so-called equation of time has to be used. This correction amounts to only a few minutes. 

Figure 3.15a,b. Zonally averaged wind velocities for the equinox period (February 12 to May 3, 1993) at 12 00 h local solar time derived from the combined observations of HRDI and WINDII on board the UARS satellite. Zonal (a) and meridional (b) wind velocities (m/s) are shown between 65 and 140 km altitude. Dashed lines correspond to westward winds (easterlies) for the zonal component and to southward winds for the meridional component. The signature of solar tides is visible. Adapted from McLandress et al., 1996. 

Figure 5.29. Measured concentration of OH and H02 on 30 April 1997 at 69°N, 149°W 9 15 local solar time using a balloon-borne thermal emission far-infrared transform spectrometer. From Jucks et al. (1998).

Several estimates of the per cent of diffuse radiation were found (1, 7,10,12). Typical values for three solar times in July are ... 

Values used in this study for each month by hourly solar time can be derived from data presented in Table II. 

With the total insolation determined and the per cent of diffuse radiation established, it was possible to calculate the quantity of direct and diffuse solar radiation available on a monthly average basis for each hour of solar time (Table II). The next step was to determine how much of this available energy could be captured by a solar heat collector. 

Universal time (t j, UT) - Mean solar time counted from midnight at the Greenwich meridian. Also called Greenwich mean time (GMT). The interval of mean solar time is based on the average, over one year, of the time between successive transits of the sun across the observer s meridian. 

The period of the Earth s rotation is constant and is measured by the interval of time between two successive passages of a star in the observer s meridian. This is called the sidereal day. The interval of time between two successive pas.sages of the Sun in the observer s meridian, the solar day, is slightly longer (by about 4 minutes) than the sidereal day. Also, because of the ellipticity of the orbit and the inclination of the axis, the solar day is not constant throughout the year. For practical purposes, we use a mean solar day divided into 24 hours. Local mean time (LMT) noon is defined on this basis. The real Sun passes in the meridian either earlier or later than the average Sun. The passage of the real Sun in the observer s meridian defines the local true solar time (TST) noon. The difference between the true solar time and the local mean time is ET = TST — LMT, which varies between 15... 

Local Solar Time - A system of astronomical time in which the sun crosses the true north-south meridian at 12 noon, and which differs from local time according to longitude, time zone, and equation of time. 

Solar Time - The period marked by successive crossing of the earth s meridian by the sun the hour angle of the sun at a point of observance (apparent time) is corrected to true (solar) time by taking into account the variation in the earth s orbit and rate of rotation. 

Solar time and local standard time are usually different for any specific location. 

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