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] ------------------------------------------------------------------------ [54] Taken from the _Almagest_ catalogue, as given in Delambre’s History. ------------------------------------------------------------------------ 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.