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I'm confused by the phrase, "it was obsolete as an astronomical instrument, having been replaced by the armillary sphere". IIUC, the armillary sphere was a teaching/study tool, while the dioptra was a measuring tool, something like a theodolite without optics. If I thought I knew what I was talking about, I'd rip it out. Wcoole (talk) 22:20, 5 November 2008 (UTC)
I wrote that line. I got it from James Evans: The History and Practice of Ancient Astronomy, page 35: "However, by the time of Hipparchus (second century BC), the dioptra was replaced by the armillary sphere as the instrument of choice for measuring the positions of the stars. From then on, the dioptra's chief role in astronomy was that of a demonstration device or teaching tool." Concerning the armillary sphere, he says on page 80: "if an armillary sphere is made well enough, and large enough, and equipped with sights, it can also function as an instrument of observation. It can be used, for example, to measure the celestial coordinates of stars or planets in the night sky. By Ptolemy's time, the armillary sphere had become the preferred instrument of the Greek astronomers." Singinglemon (talk) 20:14, 9 November 2008 (UTC)