Astronomy 101

The Reflection of the Atmosphere does not seem to be the only Cause of the Twilight; but there is an Aetherial Air, or Atmosphere, likewise round the Sun, which shines after the Body of the Sun is set: This Orb of the Sun's Atmosphere rising sooner, and setting later than the Sun itself, shines out at Mornings and Nights in a circular Figure, it being a Segment of the Sun's Atmosphere cut by the Horizon; and its Light is quite of another Sort, than that which is made by the Reflection of our Atmosphere.

— John Keill, An Introduction to the True Astronomy: or, Astronomical Lectures Read in the Astronomical School of the University of Oxford (1769).