Since prehistoric times, comets have caused fear and even panic because they were considered to carry bad omens. It is attributed to the fact that, unlike the Sun, the Moon, the planets, and the stars, which follow regular patterns of movement in the celestial canopy, comets appear unexpectedly in any part of the sky showing variability in their brightness and changes in shape and size, as the tails they develop can vary and extend across much of the sky; some can even be seen in broad daylight. Since for millennia nothing was known about them, it is understandable that they were seen as messengers sent by the gods to foretell destruction, sickness, and death.

In western culture, Aristotle was the first to try to give a rational explanation about their origin, but he did not consider them as astronomical objects but phenomena that occurred in our atmosphere, which is why he did not include this study in his astronomical text About the Sky, where he discussed the physical structure of the Universe considering it geocentric. He analyzed comets in Meteorology, a work that he dedicated to the discussion of phenomena such as rain, snow, northern lights, winds, sea, rainbows, etc.; and even the Milky Way.

Aristotle affirmed that the immobile Earth was the universal center surrounded by spheres of water, air, and fire; that beyond it really began the sky formed by the solid and crystalline sphere that transported the Moon and then followed those that moved Mercury, Venus, and the Sun, which in turn were contained by the spheres that dragged Mars, Jupiter and Saturn. Finally, there was the sphere of the fixed stars that enclosed all the others. This geocentric model was in place for over 2,000 years.

Based on this paradigm, Aristotle explained the presence of comets and affirmed that terrestrial exhalations produced them when looking for their natural place; they went up to the high part of the atmosphere where the friction produced by the friction between the sphere of the air with the one of fire, heated those vapors until they were set on fire and as a final result they achieved the presence of a comet.

When in the sixteenth century the Spanish implanted the western culture in Mexico, they brought that vision of comets, in addition to the astrological burden that over time was added so that, in general, rather than the Aristotelian interpretation was accepted that they were messengers of the wrath of God, in a society immersed in the dogmas of the Church caused the New Spaniards to fear them.

Derived from the observations of the brilliant comet of 1577, prior to the telescopes, Tycho Brahe and the lesser-known Spanish Jerónimo Muñoz contributed information that contradicted the Aristotelian ideas about those stars, the origin of the processes of change in the paradigms of Astronomy and Physics that contributed to the emergence of modern science.

During the 17th century, the presence of several comets was recorded and their study contributed to this change. In New Spain they were also observed and, although they were mostly seen as evil omens, there were those who began to see them as natural objects.

In 1606, Enrico Martinez, in his Report of the Times published in Mexico City, spoke of a remarkable star in astronomical history: the 1604 supernova now known as Kepler's Supernova. Observed from the Mexican capital, Enrico wrote that: "last year of one thousand six hundred and four, in the first days of October, a comet appeared in this New Spain that was a little bigger than the biggest star of first grandeur and in the same very scintillating way, that lasted more than a year before it had just been consumed".

What is relevant in this note is the warning of that star that was twinkling like the stars, visible for more than a year, which comets do not do. So he reported an exceptional occurrence of something that did not behave like a comet and, even without knowing what it was, his observation was already heading in the direction of the change in astronomical paradigms.

Friar Diego Rodriguez, Professor of Astronomy and Mathematics at the Royal and Pontifical University of Mexico, published in 1652 the Etheorological Discourse on the new comet. Although this work has much of the ancient knowledge, it shows a rejection of Aristotelian ideas, in terms of data derived from observation. Referring to the planetary movement, Rodriguez wrote that "all this, then, does not admit of solid skies, but of fluid ones, because unless there is penetration in some parts and emptiness in others, it cannot be"; thus he denied Aristotle's geocentric model.

Referring specifically to comets, he wrote that "being to the observations and demonstrations of mathematicians, either there are no solid heavens or they are corruptible". According to these opinions, in Fray Diego Rodriguez there is another New Spaniard who was changing his ideas about the shape of the Universe.

One more case is that of Carlos de Sigüenza y Góngora, who was also a professor of that university chair. He observed the brilliant comet of 1680-1681 and wrote a couple of pamphlets where he explained that there was no need to fear, because that star was a natural object. This attitude caused other inhabitants of New Spain to defend the astrological interpretation of comets.

One of them was the Jesuit Eusebio Francisco Kino, trained in Europe, professor at the University of Ingolstadt, who published in the capital of New Spain an astronomical exhibition of the comet where, in addition to defending astrological ideas, he attacked the observations made by Sigüenza, who in turn wrote Libra Astronómica y Filosófica, a rational defense around those stars. By means of trigonometric methods, he established the distance at which that comet was, demonstrating that it moved beyond the lunar orbit which, as has already been said, contradicted the Aristotelian model.

Without a doubt, Sigüenza's observations, but above all the rational analysis he made of the movement followed by that comet in the sky, contributed to a more adequate perception of these spectacular stars among the people of New Spain, thus opening up a path that, in the following century, would lead other students of astronomy in our country to spread heliocentrism, the rational vision of comets and an expanded concept of the shape of the Universe in these lands.

What are comets and understanding some of their properties?

Comets are frozen bodies, about 50 kilometers (31 miles) in diameter, that are part of the Solar System. They spend most of their existence far from the Sun and therefore cannot be observed. When they approach the Sun, they heat up and evaporate, therefore, they are surrounded by a gaseous envelope.

The Sun is a sphere of incandescent gases that is in continuous evaporation, therefore it throws substances into space in the form of solar wind. The solar wind drags the gases that surround the solid and frozen part of the comet producing its tail. This is why the tail of comets always points away from the Sun.

In general, comets are brighter when their tails are longer; this happens when they are closer to the Sun since they suffer more evaporation because they receive more solar energy and the wind from the Sun interacts more vigorously with the cometary gases.

But the brightness of the Sun is so high that when it is in the daytime sky, other objects cannot be observed, so the stars cannot be seen when the Sun is in the sky even if they are there. So to observe a comet you generally have to do it before sunrise or at sunset, so that the horizon covers the Sun. Both the Earth and comets revolve around the Sun. Comets usually have much more elongated orbits than planets.

Sources: Marco Arturo Moreno Corral Institute of Astronomy, Ensenada Campus, Universidad Nacional Autónoma de México. The original text of this article was published by Conversus magazine.