Astronomical Images : Problem 12: Measuring a comet's parallax

Johannes Regiomontanus

Astronomical Images

<p style='text-align: justify;'>Johannes Regiomontanus died in 1475, leaving behind a printing press, instruments and a library containing printed books and manuscripts. Most of the library was bought by Bernhard Walther (1430-1504), the merchant-astronomer at Nuremberg and then ended up in the possession of Willibald Pirckheimer (1470-1530), the patrician friend of Albrecht Duerer. Pirckheimer sold on several of Regiomontanus's works to Johannes Schoener (1477-1547), who taught mathematics at the gymnasium in Nuremberg. Regiomontanus's work on comets, which was listed in his own printing advertisement, was first edited and published by Schoener in 1531 as <i>Sixteen Problems on the Magnitude, Longitude and True Position of Comets</i>. It was printed again, with several other works of Regiomontanus in 1544. In the twelfth problem, Regiomontanus introduces an instrument in the form of a cross-staff, with which the apparent diameter of a comet may be measured. The instrument should be a fine ruler, five or six cubits long, and divided into many equal parts along line AB ' the more parts, the better. To AB is joined another ruler CD with its arms at equal length. CD is mobile and always at right angles to AB and has the same gradations as AB. Affix three fine keys at A, C, and D. Place point A next to the right eye, with the left eye closed, and point the ruler to the centre of the comet. This is done easily if the ruler is set on some kind of stand. Move the little ruler CD until it covers the entire diameter of the comet. Then, take the number of divisions between point A and CD, and take from the table prepared for this purpose (and constructed elsewhere) the apparent diameter of the comet. This instrument may be used to measure the diameter of the Moon and even of the Sun, so long as the Sun's brightness does not deter the eye. This is the instrument that Regiomontanus used for his own observations. J. Jervis points out, however, that this instrument was rather crude compared to Levi ben Gerson's (1288-1344) similar instrument, which adjusted for the centre of the eye; this is particularly puzzling since Gerson's work (translated into Latin) was available to Regiomontanus.</p>