1. Biographical notes
2. The "De Admirandis Coeli Characteribus"
3. Classification of the nebulae
4. Luminosae
5. Nebulosae
6. Occultae
7. Other Nebulae
8. The unsolved problems
9. Conclusions

Biographical notes

Giovan Battista Hodierna born in Ragusa (Sicily) Apr. 16th, 1597. His real name was Dierna (a name still quite common in Ragusa); Hodierna himself added the first syllable to his name, to signify his enthusiasm in the renewal that his century promised. In fact, Hodiernus, in Latin, means "of today", "modern".

His interest in astronomy was precocious. When he was twenty years old, he obtained the authorization to use, for his astronomical observations, the bell tower of the church of S. Nicola in Ragusa. From there he observed, between 1618 and 1619, the three comets which spurred the famous polemic that culminated in Galileo's Saggiatore.
In 1622 he was ordained in Siracusa. Beginning in 1637, he served the barons of Montechiaro (who were also the Dukes of Palma) as chaplain and parish priest of the town of Palma di Montechiaro (well to the west of Ragusa near the southern shore of Sicily). They gave him an apartment on the top floor of their palace for his astronomical observations and later (1645), after endowing him with benefice, thus named him archpriest and, in 1655, court mathematician. He died in Palma di Montechiaro, April, 6th, 1660.

Hodierna was interesting in every branch of human knowledge. He was attracted by optics, astronomy, astrology, corpuscular philosophy, natural history, meteorology, microscopy. After studying the passage of light through prisms he offered a vague explanation of the rainbow and of the spectrum. His "Thaumantia junonis nuntia praeconium pulchritudinis" (1647), was followed by "Traumantiae miraculum" (1652). In natural history his explanation of the structure and function of the retractile poison fangs of vipers (Dentis in Vipera Virulenti - Anothomia (sic!), 1644) anticipated the work of Redi. Hodierna developed an early microscope and studied the eyes of flies and other insects with it (L'occhio della mosca, 1644). He studied the human eye (Il Sole del Microcosmo, 1644) and pursued also meteorological studies: cyclones, thunder, and springs.

But his greatness consists most of all in his astronomical observations. His studies on the satellites of Jupiter (the first published ephemeris on them) were published in "Medicaeorum ephemerides" (1656), dedicated to the Grand Duke Ferdinand II. He studied also Saturn, and came close to the true nature of its ring.

But there is a work, particularly, whose weight his contemporary never understood and whose features make Hodierna an unequalled figure in the scientific outline of the XVII century.

The "De Admirandis Coeli Characteribus"

The extraordinary historic weight of this work was shown by an important article in the Journal for the History of Astronomy, by Giorgia Foderà (Osservatorio Astronomico di Palermo), Lucio Indorato (Istituto di Matematica, Università di Palermo) and Pietro Nastasi (Istituto di Geodesia, Università di Palermo). This article, entitled G. B. Hodierna's Observations of Nebulae and his Cosmology explained Hodierna's booklet De Systemate Orbis cometici; Deque Admirandis Coeli Characteribus..., printed in 1654 in Palermo.

The first section of the book is devoted to the theory of comets. Hodierna follows Galileo's theories on the comets, but we are not interested in this: the second section is the object of our exposition. In the second part of the booklet, indeed, we can find:

1. the earliest attempt at a classification of the nebulae;
2. the earliest attempt at a theory on the nebulae;
3. and, most of all, the earliest true catalogue of nebulae.

To begin, Hodierna suggests a classification of nebulae, based on the assumption that, ultimately, all the nebulae are made up of stars. This assumption was common at that time, and Herschel too was to follow this theory.

Classification of the nebulae

Hodierna divides the nebulae into three classes (see on the left): Luminosae, Nebulosae and Occultae . The Luminosae, according to Hodierna, was the nebulae where a few stars were visible to the naked eye; the Nebulosae didn't show stars to the naked eye, but a telescope was able to make them out as stars. The definition of the Occultae is somewhat contradictory and includes two possibilities: the very little and faint star clusters and the unsolvable nebulae, the biggest as well.

Then, Hodierna starts to list the nebulae. Although he observed more nebulae than anyone before him, the Sicilian astronomer refers also to several other objects of which he was informed: for example, the Magellanic Clouds. But, obviously, we are more interested in his observations.

Luminosae The first listed are the Luminosae: the Pleiads begin the series.

1. Hodierna treats the Pleiads on pages 11-15 of the "De Admirandis..."; he made also two woodcuts, the first (pag.12) showing the appearance of the cluster to the naked eye; the second (pag. 14) is a "telescopic" drawing. Moreover, he counts 33 stars in the cluster, and gives (pag. 13) a table with the latitude and longitude of the seven brighter stars (the seven sisters).
2. The second place among the Luminosae is for the Hyades (pag. 15-17). The stars counted in the nebula through the telescope are "numero inexplicabiles" (innumerable).
3. The third nebula is the Coma cluster (pag. 12-13).
4. And here is the first original discovery of Hodierna: the fourth Luminosa, according to his words, "shines to the right side of Perseus, near the Milky Way, where, besides a star of great magnitude that shines on the same side, there is also some nebulosity". This is almost certainly the early description of this cluster, today called a Persei cluster or Melotte 20.
5. The fifth Luminosa is that "in Orions' Sword. Hodierna counted 22 stars surrounding an unresolvable nebulosity. Moreover, he resolves three stars of the "Trapezium" and gives a plate (pag. 19) that is the first telescopic drawing of M42 (on the right).
6. The next item is merely an asterism ("in Orion's Head").
7. Another original discovery: the "Luminosa Scorpionis (sic!)", of which Hodierna made a careful drawing (pag. 21), corresponds undoubtedly with NGC6231 (with z1 and z2 Scorpii).
8. The eighth item ("in the water of Aquarius", pag. 21) is somewhat indecipherable.

1. The list starts with the famous "Praesepe" (M44, after Messier); Hodierna counted 38 stars in the cluster, while Galileo stopped at 36.
2. The second item is M7, the fine cluster in Scorpius, already listed by Ptolemy in the Almagest.
3. Then, Hodierna mentions the double cluster, h and c Persei, another nebula already listed by Ptolemy.
4. The fourth nebula is another original discovery: M6, the fine cluster close to M7, also called "Butterfly Cluster". Hodierna described it as "The second and smallest of the two in the Scorpion's sting, westwards" (see on the right).
5. The fifth item is an asterism, n1 and n2 Sagittarii.
6. Here is another discovery of Hodierna: "Beyond the arc of Sagittarius, westwards, near the Galaxy" there is NGC6530, the galactic cluster associated with M8. The position of the nebula is determined by the coordinates given in the book and by the drawings found in the manuscripts of the Archivio Capitolare di Vigevano. In a map from the manuscript "Delineatio signorum Zodiaci", we can see another nebula close to the first one. It could be hazardous to suggest it represents M20.
7. The seventh item shows the Hodierna's discovery of three clusters in Auriga, today known as M36, M37, and M38.
8. Then, Hodierna lists an asterism, the so-called "Al Sufi's Nebula" or "Briocchi's Cluster" (Cr399).
9. Another asterism (88 Her.).
10. Other asterisms, in Capricornus.

Other Nebulae in the book, and more

Besides the nebulae systematically listed, the sicilian astronomer mentions some other objects in a way rather untidily. It must consider pp. 7, 11 and 48.

Some of these nebulae had been identified since 1985. They are:

1. "...quae in eadem recta ducenda ab eductione caudae Syrii ad Algomeisa in Caniculae" (in the line from Sirius to b Canis Minoris): the description may be too indefinite, but there is a manuscript map that removes any doubt: Hodierna discovered M47.
2. "...iuxta Syrium ad ortum" (near Sirius, to the East): in spite of the mistake (East rather than South), the Hodierna's observation of M41 is unquestionable: there are even two maps, in the manuscripts, to testify it.
3. Hodierna's next nebulae don't lie in the book, but only in the manuscript maps found in the Archivio Capitolare di Vigevano. In a map of Canis Major, from the Registrum omnium constellationum, we can easily see a nebula (labeled as "K: nebulosa in lumbis") in the region where NGC2362 and 2354 lie. Since NGC2362 is much more visible than NGC2354, the article proposes the identification of the nebula with NGC2362. But in the same dossier there is another map of this region, apparently unfinished, showing two nebulae in that position. Shall we exclude Hodierna could have seen them both?
4. Another map of the "Registrum omnium constellationum" proves the discovery of NGC2451.

The Unsolved Problems.

The article of G. Foderà Serio, P. Nastasi and L. Indorato leaves some unsolved problems. They are:

1. The nebula described (pp. 7, 11 and 48) as "close to Triangle"; the article proposes M33, but Kenneth Glyn Jones fail this identification beacuse the Hodierna's telescope could not be able to show him that too faint nebula; according to me, Hodierna discovered not only M33, but even NGC752. See the pages of M33 and NGC752.
2. Another nebula mentioned at pp. 7, 11 and 48 (over the head of the Medusa and similar expressions). Even in this case, the authors propose an identification: M34; but, they add, "...as in the case of M33...the evidence...being far from conclusive"; according to me, there isn't any doubt, Hodierna discovered this cluster: see the page of M34.
3. "...In Bracchio dextro Orionis, & in baculo" (in Orion's right arm, and in club): they could be NGC2169 and 2175. Unfortunately, the only map of the whole constellation found among the manuscripts shows no nebulae apart from the asterism "Luminosa Capitis Orionis" o M42 ("Luminosa Ensis Orionis"). Therefore, the identification is rather doubtful.
4. "...inter Leporem, & Columbam..." (between Lepus and Columba), p. 48: is the region of M79. This is a globular cluster, probably too faint for the Hodierna's telescope.
5. "...in corpore Caeli post Caudam..." (in the body of the sky, after the tail), p. 48: I think I had understood what it means: in "La Composizione del Mondo" by Restoro d'Arezzo, the early work on astronomy and cosmology published in Tuscan vernacular (more precisely: in dialect of Arezzo), the expression "corpo del cielo" (body of the sky) recurs frequently, and the author explains the sense of it. There is no need to dwell, here, on this line of reasoning. Anyway, I think this nebula is the star clouds in Scutum.
6. "Nebulosa intercanicularis..." (nebula between two dogs), p. 48: the article links this expression to the next (quae in eadem recta...), relating it, therefore, to M47. I think the two phrases must be kept apart. "Nebulosa intercanicularis" does not differ much from the one used by Cassini II to list in his Elements d'Astronomie the discovery of his father: M50.

Hodierna, in "The Admirandis Coeli Characteribus", besides the treatment and the cataloguing of the nebulae, derives a cosmological theory of great historical importance: it comes before, indeed, the de Mairan's theory (1731). This space, however, doesn't allow to expound it here.

However powerful is Internet, it isn't a suitable means to study a subject in depth. I had intention, in a simple way, of pointing out to this Messier's forerunner, this underestimated self-made astronomer able to discover, by oneself, more nebulae than all the astronomers of his century.

If you have any suggestions, or find any errors, please e-mail me. Any constructive feedback is highly appreciated!