Astronomy (from Greek: ἀστρονομία) is a natural science that studies celestial objects and phenomena. It applies mathematics, physics, and chemistry, in an effort to explain the origin of those objects and phenomena and their evolution. Objects of interest include planets, moons, stars, galaxies, and comets; the phenomena include supernova explosions, gamma ray bursts, and cosmic microwave background radiation. More generally, all phenomena that originate outside Earth's atmosphere are within the purview of astronomy. A related but distinct subject, physical cosmology, is concerned with the study of the Universe as a whole.
My lord, they say five moons were seen tonight:
Four fixed, and the fifth did whirl about
The other four in wondrous motion.
William Shakespeare, King John (1598), Act IV, scene 2, line 182.
It's unique because it doesn't look like a comet with the typical tail; it looks like a cloud. It's not what you would normally see at all … With the naked eye, it looks like a star or planet, but with binoculars it's really weird looking; it doesn't happen every day.
George Masterson, physics and astronomy teacher on Comet 17P/Holmes - quoted in "Mysteriously bright comet provides sky-high teaching moment at NHS" in the Daily News of Newburyport (9 November 2007).