Visual Angle
The angle subtended at the eye's nodal point by an object, determining the size of the image formed on the retina.
Governing formula
θ ≈ h / d (in radians for small angles)
SI unit
rad (or arcminutes ')
In depth
An object appears larger when it subtends a larger visual angle. Magnifying instruments increase visual angle subtended at the eye.
Examples in the real world
The Moon and Sun subtending nearly identical visual angles ($≈ 0.5^°$ or $30'$) from Earth, allowing total solar eclipses.