On May 19, 1910, Earth had its closest encounter with Halley’s Comet. Most comets, sometimes called “dirty snowballs” or “icy dirtballs”, usually follow an orbit that keeps them far away from the sun. Those comets are invisible from Earth, even with the best telescopes. However, if a comet gets closer to the sun, heat from the sun may begin to evaporate some of its ice, which releases the comet’s dust and rocks. This material is the “tail” of a comet seen from Earth. 
 
It takes about 76 years for Halley’s Comet to make one orbit around the sun, and astronomers have been able to predict its return since the 1700s. Centuries earlier, ancient astronomers from China to Babylonia had accurately, consistently reported seeing the comet, but failed to recognize it as a single object or recurring event.
accurately
Adverb

exactly or perfectly.

ancient
Adjective

very old.

astronomer
Noun

person who studies space and the universe beyond Earth's atmosphere.

comet
Noun

celestial object made up of ice, gas, and dust that orbits the sun and leaves a tail of debris.

consistent
Adjective

maintaining a steady, reliable quality.

encounter
Verb

to meet, especially unexpectedly.

evaporate
Verb

to change from a liquid to a gas or vapor.

orbit
Noun

path of one object around a more massive object.

predict
Verb

to know the outcome of a situation in advance.

recognize
Verb

to identify or acknowledge.

sun
Noun

star at the center of our solar system.

tail
Noun

stream of gas or dust debris behind a comet.

telescope
Noun

scientific instrument that uses mirrors to view distant objects.

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