Un cometa se percibe como un disco luminoso de niebla con una cola, semejante a una cabellera que ondea al viento. Peludo o cabelludo se dice en griego kométes. De ahí apareció el nombre cometa. Ahora bien, estos objetos aparecían en los momentos más inesperados. Y claro, cuando se veía alguno, ¿qué podía significar? Pues que se avecinaba un desastre...

REVISAR

Workmanlike book about comets, written and published just as Halley's comet re-entered the solar system in 1986. Numerous black and white pictures. Explains the importance of Halley's comet to astronomers and also how exceptional Halley's comet is, having a period short enough that it can pass through the planets of the solar system many times within recorded history, yet long enough that it is not rapidly reduced to nothing by frequently passing near the sun.

Chapter 1: The Fear of Comets
Comets can be visible to the naked eye, in fact some dominate the sky for a few months. They were generally considered to be portents of some kind of doom or other.

Chapter 2: The Paths of Comets
Scientific observation of comets begins in the 1400s. Orbits are observed and also the fact that the comet's tail always points away from the sun. Tycho Brahe can discover no parallax in observations of comets, so concludes that they are pretty distant, much farther than, e.g., the moon, at any rate. Kepler and then Newton explain the motion of the planets. The same laws are applied to comets, yielding to the conjecture of highly eccentric cometary orbits. Halley predicts the return of Halley's comet.

Chapter 3: The Return of Halley's Comet
It comes back! Astronomers tot up its various recorded appearances in the past, including that in 1066 and going back as far as 400 BC.

Chapter 4: Dim Comets
Numerous short period comets are discovered during the latter half of the 18th century. Astronomers realize that comets are actually quite numerous and develop the mathematical techniques to calculate their orbits. It was in this time that Herschel discovered a new planet and thought at first that it was a comet. Some comets are tricky, because their orbits are disturbed by the planets in the solar system.

Chapter 5: The Death of Comets
Fred Laurence Whipple's "dirty snowball" theory of comets is generally accepted. Each time a comet approaches the sun its frozen bits turn to vapor and dust, the comet becomes visible and simultaneously loses a lot of its mass. As it recedes from the sun it freezes and becomes less visible.

Chapter 6: Nineteenth Century Comets
This century saw the appearance of a bunch of very brilliant long period comets as well as the discovery of many smaller comets and the uses of photography. At the time, it was not possible to determine the orbit of the long period comets.

Chapter 7: Comet Tails and Meteors
Comet tails are blown out by the solar wind. The debris left by the comet continues to follow the comet's orbit yielding meteors and the odd meteorite.

Chapter 8: The Distant Comets
Millions of comets reside in the Oort cloud, a spherical shell with a radius far greater than that of the orbit of Neptune. We see the comets that get their orbits wildly altered by some chance event, which is why they come in from all directions and moving in directions which are unrelated to the planetary orbits.

Chapter 9: The Birth of Comets and the Solar System
The comet that are stuck out in the Oort cloud are the most unaffected by the evolution of the solar system and therefore the most alike in composition to the solar system originally. In Asimov's time, there were no scheduled trips to the Oort cloud to pick up a comet or two for study. So, Halley's comet actually seemed like a good way to study the early solar system.

Chapter 10: Comets and Catastrophes
Meteor craters and the Tunguska explosion of 1908. Neil de Grasse Tyson has described earth as being at the center of a shooting gallery, I guess he was referring to comets. There is good evidence for the impact of a giant meteorite around the time of the extinction of the designers. Speculative: Big extinctions on earth seem to occur with a period of 65 million years. Perhaps they are caused by some event that causes a whole lot of comets to leave the Oort crowd and adopt orbits which enter the solar system, thereby increasing the chances of a great meteorite strike. ( )

Less about Halley's Comet than comets in general, Asimov turns his prodigious knowledge and lucid writing style to our hairy visitors from Kupier and Oort Land. Written, like most of his non-fiction, for a general, very general, audience, it gets the job done. Especially because it is chock-full of history. That and the numerous black and white illustrations, many of them old prints of comets add to the allure of the book. Also, I've always had an interest in astronomy and have glorious visions of becoming an amateur astronomer when I have wealth and time in great abundance, or perhaps in a past life I was gentleman antiquarian who fancied himself a dabbler in astronomy. Either way, quite entertaining if you can get it. A quick, fun read. ( )

A short, yet interesting book about the history of comets and comets in general. There are --ALOT-- of pictures in this book, but that's not necessarily a bad thing. It does make the 118 pages fly by pretty quickly, but they add so much to the narrative. And besides, comets are pretty cool to look at. ( )