So following up on yesterday’s entry about the armillary sphere on the Vatican News Services website. Today I read about the armillary spheres used by Tycho Brahe and was honestly quite stunned. He had 4 different armillary spheres! Above is my favorite, the Great Equatorial Armillary Sphere, which looks quite a bit like the hexagon quilt projection.
Check out the others:
The texts about the instruments are brought from Arne Wennberg’s “TÃ¤nk, om det Ã¤r sÃ¥ : om Tycho Brahes instrument och vad han kunde gÃ¶ra med dessa” (1996), copied without the permission from the author.
The zodiacal armillary sphere (1581)
Tycho had several armillary spheres. They were of two kinds, known as zodiacal or equatorial armillas, depending on which coordinate system they referred to. In the zodiacal system, the coordinates are celestial longitude and latitude (not to be confused with longitude and latitude on the earth). Longitude is measured from the first point of Aries along the ecliptic in an easterly direction. Latitude is the perpendicular distance from the ecliptic. When an observation was to be taken, the sphere first had to be orientated against true north and the elevation of the pole (the observer’s latitude) had to be set. The innermost ring representing the ecliptic, had to be set in the right position momentarily because of the earth’s daily rotation. This could be done against a star with a known longitude. Tycho complained of the weight of the rings. They were too heavy which affected the accuracy. The outermost ring had a diameter of 117 cm. The instrument was placed in the northwestern crypt of Stjerneborg.
The north equatorial armillary sphere (1584)
The two small observatories in the castle each gave room to only one instrument. There were two equatorial armillary spheres, of the same size, but somewhat different in construction. The dia-meter of the outermost ring was 155 cm. This instrument was placed in the smaller northern observatory of Uraniborg. The outer ring was made of steel but the inner rings were of brass. In the equatorial system the coordinates are right ascension and declination. Right ascension is measured along the equator from the first point of Aries to the east. Declination is the perpendicular distance from the equator. Before observations were taken, the instrument had to be adjusted in the northerly direction and the elevation of the pole above the horizon (the observer’s latitude) had to be set. It was used in the same way as the next instrument. The instrument carried the portraits of four well-known astronomers, Ptolemy, Al Battini, Copernicus and Tycho himself.
The south equatorial armillary sphere (1584)
This sphere was functionally similar to the previous one. Its was placed in the smaller southern observatory of Uraniborg. The reason Tycho had two similar armillas, one in each end of the castle, was that the towers of the castle were a hindrance to making observations in some directions. Tycho also often made simultaneous observations with different instruments and different observers. This instrument had three movable rings of steel. The outer ring had no other function than to make the instrument more stable. After adjustments were made (see previous instrument) observations could be taken by the sights on the rings and the little cylinder in the centre of the instrument. The declination could be read directly on its ring. As the first point of Aries is not to be seen on the sky, the right ascension could not be read as easily. However, the differences in right ascension between stars with known right ascension and unknown stars could be taken, and the right ascension of the unknown stars could be calculated.
The great equatorial armillary sphere (1585)
Tycho was a precursor with equatorial armillary spheres. This instrument was built later than the armillas of the castle and meant a great advance in construction. It was bigger and simpler to use. It consisted of one single declination circle, 272 cm in diameter, and a semicircle representing the equator, 350 cm in diameter. It was made of steel with scales and sights in brass. The instrument’s axis was very carefully adjusted to be parallel with the axis of the earth, as well as the semicircle being accurately adjusted to the equatorial plane. Two rulers, turnable around the centre of the instrument, carried sights in their outer ends and were used to determine declination. Two more sights slid along the equator. Between those, the difference of right ascension between two heavenly bodies could be found. Also the hour angle could be read on the equator. The angles could be read to an accuracy of a quarter of a minute; that is fifteen seconds. The instrument was placed in the southern and largest crypt of Stjerneborg, under the observatory’s big cupola.
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