# Why is SCNNode.rotation property a four dimensional vector?

I don't understand why SCNNode's rotation property is a SCNVector4 instead of a SCNVector3. The documentation states that:

The four-component rotation vector specifies the direction of the rotation axis in the first three components and the angle of rotation (in radians) in the fourth. The default rotation is the zero vector, specifying no rotation. Rotation is applied relative to the node’s pivot property.

What's the fourth axis? I tried adding a light (SCNLight) and if I don't specify the value of the 4th float in the vector the light has no effect (the scene is dark). Why? shouldn't a 3-dimensional vector (x,y,z) be enough?

• I'm confused, you seem to ask about 2 un-related topics. How to you set-up the light? – Alexandre Vaillancourt Feb 15 '16 at 15:45

For instance, specifying (pseudo-code) (0, 0, 1, degToRad(45)) will have the effect of a light that will be rotated 45 degrees around the vertical axis (assuming that z is up), coming from the pivot location.