When the sun sets, most of the land and part gets dark still some light at outlines of objects can be seen because of a phenomena known as Refraction.
In this, when the Sun sets, the light given out by it keeps refracting with in our atmosphere which help to see the outlines even after its dark on other parts.
<em>Important thing is that all unitless quantity is dimensionless quantity. .</em><em>A</em><em> dimensionless physical quantity may have an unit</em>
