The phenomenon you are describing is known as "atmospheric refraction."
Atmospheric refraction is the bending of light waves as they pass through Earth's atmosphere, caused by the variation in air density at different altitudes. This bending of light allows us to see celestial objects like the sun for a short period of time after they have actually set below the horizon.
The specific angle you mentioned, 89 degrees perpendicular to the surface, refers to the concept of the "astronomical horizon." The astronomical horizon is the theoretical horizon where an observer's line of sight is tangent to the Earth's surface. When the sun is just below the astronomical horizon, its light is refracted enough by the atmosphere to become visible even though it's technically already below the geometric horizon.
In simpler terms, because of the way the Earth's atmosphere bends light, we can see the sun for a little while after it has set. This is responsible for the phenomenon often referred to as the "green flash" – a brief burst of green light that can sometimes be seen just as the sun disappears below the horizon.