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The VRaySun and VRaySky are are special features which are provided by the V-Ray renderer. Developed to work together, the the VRaySun and  and VRaySky reproduce the real-life sun and sky environment of the Earth. Both are coded so that they change their appearance depending on the direction of the VRaySun.

The VRayGeoSun node allows you to set the position of the V-Ray Sun for a specific place in the world at a given time.

The VRaySun is frequently used with the environment texture map VRaySky.

The V-Ray Sun and Sky systems are based largely on the SIGGRAPH 1999 paper A Practical Analytic Model for Daylight [1]. For a complete list of references, please see the Links & References section.

Note that for the Clouds to work with VRaySun, a VRaySky is required.

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 ||V-Ray Shelf|| > VRaySun: Create a VRay Sun light button

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 ||Render Settings window|| > Overrides tab > Environment rollout > Create Sun button

The V-Ray Shelf includes right-click options from the Sun button that allows an easy way to Select or Delete the Sun node if needed. 

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After the sun is created, its parameters appear in the Attribute Editor.

Enabled – Turns the sun light on and off.

Intensity multiplier – The brightness of the sun. Since the sun is very bright by default, you can use this parameter to reduce its effect. See the Notes section for more information. For more information, see the Intensity Multiplier example below.

Turbidity – Determines the amount of dust in the air, which in turn affects the color of the sun and sky. Smaller values produce a clear, blue sky and sun similar to what is seen in rural areas, while larger values make the sun and sky more yellow and orange as seen in a big city. For more information, see the Turbidity example below.

Ozone – Affects the color of the sun light. Available in the range between 0.0 and 1.0. Smaller values make the sunlight more yellow, while larger values make it bluer. For more information, see the Ozone example below.

Size multiplier – Controls the visible size of the sun. This affects the appearance of the sun disc as seen by the camera and reflections, as well as the blurriness of the sun shadows. For more information, see the Size Multiplier example below.

Sky Model – Determines the procedural model used to generate the VRaySky texture. See the Sky model example below for more information.

Hosek et al. – The VRaySky procedural texture is generated based on the Hosek et al. method. 
Preetham et al. – The VRaySky procedural texture is generated based on the Preetham et al. method.

Photon radius – Determines the radius of the area in which photons will be shot. This area is represented by the cylinder around the Sun's ray vector. This parameter has effect when photons are used in the GI solutions or caustics. See the Photon Emission Radius example for more information.

Caustics subdivs – Used by V-Ray when calculating Caustics. Lower values mean more noisy results but will render faster. Higher values produce smoother results but take more time. This option is inactive when using the V-Ray CUDA engine.

Caustics Multiplier – Used by V-Ray when calculating Caustics. This multiplier controls the brightness of the caustics. This option is inactive when using the V-Ray CUDA engine.