It is presumed that he published a star catalogue (now lost) and that this would have been an extension or refinement of the work of Timocharis. Six of his stellar declinations are preserved by Ptolemy in the Almagest (7.3; H2.20‒22) interspersed with twelve of Timocharis, where he appears to work to a precision of ¼°:
| Star | Reference | Reported Dec | Actual Dec (258 BCE) |
|---|---|---|---|
| α Aur (Capella) | H2.20 | +40.00° | +39.898° |
| α Gem (Castor) | H2.20 | +33.00° | +33.142° |
| β Gem (Pollux) | H2.21 | +30.00° | +29.979° |
| η UMa (Alkaid) | H2.22 | +61.50° | +61.413° |
| ζ UMa (Mizar) | H2.22 | +67.25° | +67.317° |
| ε UMa (Alioth) | H2.22 | +68.50° | +68.454° |
A least squares analysis of these declinations using the VCW model reveals that the observational errors are minimised for the year 258 BCE with an average error of 0.087 These observations are extraordinarily accurate for the C3rd BCE. However, it may be that Ptolemy chose just these few observations for this reason, since he reports (7.1; H2.3) that Hipparchos himself was dismissive about the accuracy of observations made by astronomers who had preceded him, excepting those of Timocharis and Aristyllos, but that even those were not always without fault.
This result, of course, is not a proof that the observations were made in that year or even in a single year, but it gives a reasonable working date for Aristyllos' activities. It also agrees well with a previous study which found a date of 261 BCE (Isis 73: 259-265 [1982] p. 263).
Lunar Crater: Aristillus (diameter 54 km)
Last updated 27/04/20
