Dawn
occurs before sunrise, before the top of the Sun reaches the
horizon.
Twilight is the
name given to the period between dawn and sunrise, or between
sunset and dusk, when light is still visible in the sky due to
sunlight scattering off the atmosphere.
Sunrise and
sunset are the points at which the top edge of the Sun
reaches the horizon; the only difference between them is the
direction in which the Sun is moving at the time. It actually
occurs when the top of the Sun is 0.57° below the horizon due to
refraction of the Sun’s light by the atmosphere.
Dusk occurs
after sunset, once the top of the Sun has passed the horizon.
Dawn, dusk, and
twilight can also be separated in
astronomical, nautical, and civil sections by how far below
the horizon the Sun is (18°, 12° and 6° below
the horizon respectively).