Aspect-based shading is calculated according to a modified cosine shading equation (Moellering and Kimerling, 1990):
gray value=cos(α)+12
Where α is the angle between aspect and the azimuth of the light direction
Slope can be expressed either in degrees or as a percentage. Aspect is the orientation of slope, measured clockwise in degrees from 0 to 360, where 0 is north-facing, 90 is east-facing, 180 is south-facing, and 270 is west-facing.
... the direction of a celestial object from the observer, expressed as the angular distance from the north or south point of the horizon to the point at which a vertical circle passing through the object intersects the horizon.
http://www.reliefshading.com/analytical/shading-methods/
Aspect-based shading is calculated according to a modified cosine shading equation (Moellering and Kimerling, 1990): gray value=cos(α)+12 Where α is the angle between aspect and the azimuth of the light direction
What is the difference between slope and aspect? https://grass.osgeo.org/grass82/manuals/i.topo.corr.html
Slope can be expressed either in degrees or as a percentage. Aspect is the orientation of slope, measured clockwise in degrees from 0 to 360, where 0 is north-facing, 90 is east-facing, 180 is south-facing, and 270 is west-facing.
What is the azimuth? https://en.wikipedia.org/wiki/Azimuth
... the direction of a celestial object from the observer, expressed as the angular distance from the north or south point of the horizon to the point at which a vertical circle passing through the object intersects the horizon.