8/10/2019 Solar Time

1/3

8/10/2019 Solar Time

2/3

-

So we cannot use a clock according to the solar time.-

To overcome this difficulty, a fictitious sun is called mean sun is defined,

which transits the observer meridian at a fixed interval of time every 24hours.

-

Two Successive transits of this mean sun across the same meridian define

one mean solar day.- Solar time is converted to standard time by the equation:

Solar time = Standard Time 4 (Lstandard-LLocal) +EOT

-

There are two correction terms are introduced to convert the watch timeinto solar time.

- First term, due to difference in the observers longitude and the longitude

on which the local standard time is based.

-

The sun traverses each degree of longitude in 4 minutes.-

The (- ve) sign is used if the observer is in the east of reference meridian-

The (+ ve) sign is used if the observer is in the west of reference meridian.-

Second term, due to arising out of the variation in the length of the solar

day throughout the year and is called Equation of Time.-

This in minutes may be determined either by fig or using the followingequations:

E = 9.87 Sin 2B7.53 Cos B1.5 Sin B

-

Where, the Value of B is measured from,

364

)81n(*360B

- where, n = Day of the year, 1< n < 365.

Local Solar Noon:-

The solar noon at a place is determined form the following equations:

Local Solar noon = 12Equation of time in hours onthat day

8/10/2019 Solar Time

3/3

(Longitude of reference meridian -Observers longitude)/15

Numericals:

Calculate solar time on Feb. 2, 10.30 a.m. at Vellore(79.1833E, 12.9833N)

Solar time = Watch Time - 4 (Lst - Llo) + EoTn = 33 (on Feb. 2)EoT = -13.5 min. (on February)

Solar time = 10.304*(82.5-79.1833) - 13.5 (minutes)= 10 hours 03 minutes

Angular Measurements:

Hour angle():

- Hour angle ():- The angular displacement of the sun from east to west of the local

meridian due to rotation of the earth on its own axis.- It gives how close the observer to the solar noon. (solar radiation

intensity is maximum).- It is considered as - in the morning and + in the evening and at

the solar noon 0.- It is the angle through which the earth must turn to bring the

meridian directly in line with the sun.- It is determined by the following expression.

Hour angle ()= 15*(Solar time12) (Degrees)