Comets at dusk and dawn, illustration. Comets are best viewed when they are relatively close to the Earth, as they approach the Sun and as they depart from the Sun. When approaching the Sun, a comet is visible at dusk (top) as is sets behind the Sun. When moving away from the Sun, a comet is visible at dawn (bottom) as it rises ahead of the Sun. In both cases, the tail is streaming away from the Sun. At other times, comets are usually not visible in the glare of the Sun during the day. Comets are icy bodies from the outer solar system that boil and form a bright tail of gas and dust as they approach the Sun.