An introduction to Comets from a 20th century stereograph:

This is a wonderful view of a comet sweeping across the sky. Formerly these were looked upon as evil omens. They were thought to forecast the coming of famine, pestilence, or war. But scientists, even in ancient days, did not accept these statements. In Western Asia, and Northern Europe the old star-gazers believed them to be what they are, “wanderers” across the sky following regular paths. 

Generally a comet is made up of three parts. You will notice in the view that the head of the comet or the bright part is at the extreme lower end. This is called the nucleus. About this is a thin halo of light called coma. Streaming off from the head of the comet, upward in the case of this view, is a thin, airylike tail. It is so thin that you can easily see the stars beyond it. Usually a comet has one tail, but some of them have as many as five or six. Sometimes these are millions of miles long. 

Comets move around the sun as planets do, but not always in the same direction. They do no, as planets do, revolve in the same plane. That is, if you were to draw in space the orbit of the earth about the sun, you could lay a piece of flat paper across your drawing so that it would touch all points of your circle. But comets wander, or wobble up and down in their circuits. 

Comets are much larger than we sometimes think. The head of Donati’s in 1858 was measured as 250,000 miles in diameter. Tails of the short ones are come 10,000,000 miles long; and the comet of 1882 had a tail 100,000,000 miles long. Some of them appear regularly in view of the earth so that we know when to expect them. Halley’s famous comet moves about the sun once every 76 years and can thus be seen at regular intervals from earth. Comets are usually named for the one who discovers them.

Morehouse’s Comet, Yerkes Observatory, about 1900, Keystone View Co. J. Paul Getty Museum, 84.XC.873.7631.