The parallactic instrument of Ptolemy

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The parallactic instrument of Ptolemy


It was an astronomical instrument suitable for measuring vertical angles and especially for the parallax of the moon and its distance from the Earth. It belongs to the instruments which were designed without calibrated discs to overcome the problem of the exact division into degrees. It consisted of a stable, four-cubit (two- metre), calibrated rod with a vertical disposition, a sighting rod of equal length and a thin rod with a longitudinal slit. The last two, at one end were pivoted to the ends of a calibrated rule while at the other end they were connected to a movable pivot so as to form an equilateral triangle of variable (in length) base. The sighting disposition consisted of a plate with a small hole (eyepiece) and a plate with a larger hole such that when the eye is placed at the eyepiece the whole celestial body could be seen. The operator of the instrument aimed the object by rotating the sighting rod and sliding it on the slit of the thin rod. Then he noted the value on the thin rod and by its rotation it corresponded to the value with the calibrated indication of the vertical rod. Knowing the value of the base of the isosceles triangle, with the help of chord-angle tables, the operator calculated the required vertical angle sighting.

SOURCES: "Ptolemy, The Great Syntaxis of Astronomy (Almagest)"