Dante and the early astronomers

by M. A. Orr

The distances of all other heavenly bodies are too great to be determined by naked eye methods, for the displacement (technically called “parallax”) is so small that it is quite invisible without a telescope. Hipparchus had tried repeatedly to measure the distance of the sun, but recognized that neither the method of Aristarchus nor any other was really conclusive. It was, however, the best attempt that had been made, and Ptolemy assumed that he knew the distance to be about twenty times that of the moon, so he gave the sizes and distances of these two bodies as follows, taking Earth’s semi-diameter as unit:—

	According to Ptolemy.		Modern Values.
	Semi-diameter.	Distance.	Semi-diameter.	Distance.
Moon	⁵/₁₇ (=0·290)	59	0·273	60·3
Sun	5½	1210	109·4	23,439

In Books VII and VIII of the Almagest Ptolemy describes the 48 ancient constellations and the course of the Milky Way among them. The position of each star is noted as it appears in its constellation-figure, but the celestial latitude and longitude is also given, and this great catalogue is evidently taken from Hipparchus.

PTOLEMY’S FORTY-EIGHT CONSTELLATIONS.[54]

Ursa Minor	Pegasus
Ursa Major	Andromeda
Draco	Triangulum
Cepheus	Cetus
Auriga	Orion
Corona Borealis	The River (Eridanus)
The Kneeler (Hercules)	Lepus
Lyra	The Dog (Canis Major)
The Bird (Cygnus)	Canis Minor (Procyon)
Cassiopeia	Argo
Perseus	Hydra
Böotes	Crater
Ophiuchus	Corvus
Serpens	Centaur
Sagitta	The Wild Beast (Lupus)
Aquila	Ara
Delphinus	Corona Australis
Equuleus	Piscis Australis,
Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra,
Scorpio, Sagittarius, Capricornus, Aquarius, Pisces.

Ptolemy sometimes remarks on the colour of the brighter stars, and always mentions the brightness, or “magnitude” as we call it now, for the classification of Ptolemy (or Hipparchus?) was found convenient and accurate enough to be retained by modern astronomers, and the same system is now continued with the faint telescopic stars.

The stars ranked as “first-magnitude,” or brightest of all, are fifteen in number, and as these are evidently the “quindici stelle”[55] alluded to by Dante in Par. xiii. 4, it will be interesting to give a list of them here.

PTOLEMY’S FIRST-MAGNITUDE STARS.

	Name and Description	Modern Name and Meaning. in the Catalogue.
1.	Arctouros, fire-coloured	Arcturus (the Bear-Watcher).
2.	The brightest star in the Lyre	Vega, in Lyra. (Falling Eagle, Arabic).
3.	Aichs (the Goat)	Capella (Little Goat, Latin).
4.	The brightest star of the Hyades, fire-coloured	Aldebaran. (The follower, because following the Pleiades, Arabic).
5.	Basiliskos (the royal), the heart (of Leo).	Regulus (Latin equivalent).
6.	The tip of the tail (of Leo)	Denebola (Lion’s Tail, Arabic).
7.	Stachys (Ear-of-Corn)	Spica (Latin equivalent).
8.	The last of the Water and Mouth of the Southern Fish.	Fomalhaut. (Mouth of the Fish, Arabic).
9.	The fire-coloured bright star on the shoulder of Orion.	Betelgueux. (Shoulder of Giant, Arabic).
10.	On the left foot of Orion, common to the Water.	Rigel. (Foot of the Giant, Arabic).
11.	The last of the River	Achernar (Arabic equivalent).
12.	The very brilliant star, fire-coloured, at the mouth of the Dog, called the Dog.	Sirius (Greek Seirios, or Sothis, from the Egyptian Sept), in Canis Major.[56]
13.	Prokuon (the preceding Dog), on the thigh of Prokuon	Procyon (in Canis Minor), which rises before Canis Major.
14.	Canopos, on the rudder (of Argo)	Canopus (an Egyptian god, and a town on the Nile delta).
15.	The tip of the right forefoot (of the Centaur).	Alpha Centauri.
Altair and Antares were counted as second magnitude, though we now class them among the first.

It will be seen from the above that only two of these fifteen brightest stars of Ptolemy’s still bear their Greek names, Arcturus, and Procyon; but most of the other modern names are direct translations, either into Latin or Arabic, of Ptolemy’s description or name for the star, while Vega and Aldebaran have preserved their original Arabic names (much corrupted), and Canopus and Sirius are derived from the Egyptian. Regulus-Basiliskos apparently comes from Babylon, for the name of this star on tablets of the second century b.c. was Shar-ru, which means “royal.”