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 Incidence angle (θ): 

- 

It is the angle between the sun’s direct rays and the normal to anysurface.

- 

 The general expression for calculating the indirect angle of the beam

radiation is,

- 

It can reduce for the horizontal surface, ie when (β = 00), then aboveequation can be reduces to,

- 

It can reduces to vertical surfaces, ie when ((β = 900), then aboveequation can be reduces to,

Cosθ = (-sinδ cosø cosγ )+ (cosδ sinø cosγ cosω) + (cosδ sinγ sinω) 

- 

if a surface is inclined surface facing due to south, the surface azimuthangle (γ) is Zero, then above equation reduces to, 

- 

Cosθ = sinδ sin(ø –  β) + cosδ cosω cos(ø –  β) 

- 

If a vertical surface facing south, the β =900 , and γ = 00, then the

incidence angle is measured from the following expressions,

- 

Cosθ = sinø cosδ cosω - cosø sinδ 

Measurements of Day length, sun rise and Sun set:

a. 

Day length:- 

It is the time each day from the moment the upper limb of the sunappears above the horizon during the sun rise to the moment when theupper limb of the sun disappears below the horizon during sun set.

- 

It is also termed as length of a day or length of the day time.- 

It is expressed in terms of (Smax)- 

It is measured from the following expression.

- 

)tan*tan(cos15

2S

  1

max  

 

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b. 

Solar hour angle (ωs):- 

It is the hour angle corresponding to sunrise or sunset (ωs) on ahorizontal surface can be found from the following expressions.

- 

 This hour angle is used to find the day length of any location on earth

surface and any particular day.

-  ωs  = cos-1 (-tan δ* tan ø)

c.  Sun Rise Time (SRT): - Although the Sun appears to "rise" from the horizon, it is actually

the Earth’ s  motion that causes the Sun to appear. The illusion of amoving Sun results from Earth observers being in a rotating referenceframe. 

- It is time at the sun rise in the east direction of the local observer everyday.

- It is measured by the given,

SRT = 12-(0.5*Smax)

c.  Sun Set Time (SST): - Although the Sun disappears to "set" from the horizon, it is actually

the Earth’ s  motion that causes the Sun to disappear. The illusion of amoving Sun results from Earth observers being in a rotating referenceframe. 

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  - It is time at the sun is set at the West direction of the local observerevery day.

- It is measured by the given,

SRT = (0.5*Smax)