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NOTICED A NEW AND UNUSUAL STAR

BEFORE

1503 The most accurate star positions to date are recorded by Bernhard Walther at Nuremberg. 1543 Copernicus introduces the idea of a sun-centered cosmos, improving the prediction of planetary positions. These, however, are still inaccurate

AFTER

1610 Galileo’s use of the telescope starts a revolution that eventually supersedes naked-eye astronomy. 1620 Johannes Kepler completes his laws of planetary motion. 1670s Major observatories are established in all the capitals of Europe.

The astronomy of Tycho’s time still followed the teachings that Aristotle had laid down nearly 1,900 years earlier. Aristotle had stated that the stars in the heavenly firmament were fixed, permanent, and unchanging. In 1572, when Tycho was 26, a bright new star was seen in the sky. It was in the constellation of Cassiopeia and stayed visible for 18 months before fading from view. Influenced by the prevailing Aristotelian dogma, most observers assumed that this was an object high in the atmosphere, but below the moon. Tycho’s careful measurements of the new object convinced him that it did not move in relation to nearby stars, so he concluded that it was not an atmospheric phenomenon but a real star. The star was later discovered to be a supernova, and the remnant of this stellar explosion is still visible in the sky as Cassiopeia B. The observation of a new star was an extremely rare event. Only eight naked-eye observations of supernovae have ever been recorded. This sighting showed that the star catalogs in use at the time did not tell the whole story. Greater precision was needed, and Tycho led the way.

Precision instruments

To accomplish his task, Tycho set about constructing a collection of reliable instruments (quadrants and sextants, and armillary spheres) that could measure the position of a planet in the sky to an accuracy of about 0.5 arcminute (± 1⁄120º). He personally measured planetary positions over a period of around 20 years, and for this purpose in 1576 he oversaw the building of a large complex on the small island of Hven in the Øresund Strait, between what is now Denmark and Sweden. This was one of the first research institutes of its kind.

Tycho carefully measured the positions of the stars and recorded them on brass plates on a spherical wooden globe about 5 ft 3 in (1.6 m) in diameter at his observatory on Hven. By 1595, his globe had around 1,000 stars recorded on it. It could spin around a polar axis, and a horizontal ring was used so that stars positioned above the horizon at any given time could be distinguished from those below the horizon. Tycho carried the globe with him on his travels, but it was destroyed in a fire in Copenhagen in 1728. Further evidence of a changing cosmos came from Tycho’s observation of the Great Comet in 1577. Aristotle had claimed that comets were atmospheric phenomena, and this was still generally believed to be the case in the 16th century. Tycho compared measurements of the comet’s position that he had taken on Hven with those that had been taken at the same time by Bohemian astronomer Thaddaeus Hagecius in Prague. In both instances, the comet was observed in roughly the same place, but the moon was not, suggesting that the comet was much farther away. Tycho’s observations of the way the comet moved across the sky over the months also convinced him that it was traveling through the solar system. This overturned another theory that had been believed for the previous 1,500 years. The great Graeco-Egyptian astronomer Ptolemy had been convinced that the planets were embedded in real, solid, ethereal, transparent crystalline spheres, and that the spinning of these spheres moved the planets across the sky. However, Tycho observed that the comet seemed to move unhindered, and he concluded that the spheres could not exist. He therefore proposed that the planets moved unsupported through space, a daring concept at the time.

Tycho used his immense wealth to design and build fine instruments, such as this armillary sphere, which was used to model the night sky as seen from Earth

No parallax

Tycho was also very interested in Copernicus’s proposition that the sun, rather than Earth, was at the center of the cosmos. If Copernicus was right, the nearby stars should appear to swing from side to side as Earth traveled annually on its orbit around the sun—a phenomenon known as parallax. Tycho searched hard, but could not find any stellar parallax. There were two possible conclusions. The first was that the stars were too far away, meaning that the change in their position was too small for Tycho to measure with the instruments of the day. (It is now known that the parallax of even the closest star is about 100 times smaller than the typical accuracy of Tycho’s observations.) The second possibility was that Copernicus was wrong and that Earth did not move. This was Tycho’s conclusion.

Tycho Brahe’s observatory complex on the island of Hven attracted scholars and students from all over Europe between its founding in 1576 and its closure in 1597.

The Tychonic model

In reaching this conclusion, Tycho trusted his own direct experience. He did not feel Earth moving. In fact, nothing that he observed convinced him that the planet was moving. Earth appeared to be stationary and the external universe was the only thing that appeared to be in motion. This led Tycho to discard the Copernican cosmos and introduce his own. In his model of the cosmos, all the planets except Earth orbited the sun, but the sun and the moon orbited a stationary Earth.

For many decades after his death in 1601, Tycho’s model was popular among astronomers who were dissatisfied with Ptolemy’s Earth-centric system but who did not wish to anger the Catholic Church by adopting the proscribed Copernican model. However, Tycho’s own insistence on observational accuracy provided the data that would lead to his idea being discredited shortly after his death. His accurate observations helped Johannes Kepler to demonstrate that the planets’ orbits are ellipses and to create a model that would displace both the Tychonic and Copernican models.

Tycho’s improved measurements would also allow English astronomer Edmond Halley to discover the proper motion of stars (the change in position due to the stars’ motion through space) in 1718. Halley realized that the bright stars Sirius, Arcturus, and Aldebaran had, by Tycho’s time, moved over half a degree away from the positions recorded by Hipparchus 1,850 years earlier. Not only were the stars not fixed in the sky, but the changing positions of the closer stars could also be measured. Stellar parallax was not detected until 1838.

The Tychonic model kept Earth at the center of the cosmos as in the Ptolemaic model, but the five known planets were now orbiting the sun. Although he was impressed by the Copernican model, Tycho believed that Earth did not move.

TYCHO BRAHE

1588 Astronomiæ Instauratæ Progymnasmata (Introduction to the New Astronomy)

Born a nobleman in 1546 in Scania (then Denmark, but now Sweden), Tyge Ottesen Brahe (Tycho is the Latinized version of his first name) became an astronomer after sighting a predicted solar eclipse in 1560. In 1575, King Frederick II gave Tycho the island of Hven in the Øresund Strait, where he built an observatory. Tycho later fell out with Frederick’s successor, Christian IV, over the potential transfer of the island to his children and closed the observatory. In 1599, he was appointed Imperial Mathematician to Emperor Rudolph II in Prague. There, Tycho appointed Johannes Kepler as his assistant. Tycho was famed for his distinctive metal nose, the legacy of a duel he fought as a student. He died in 1601, allegedly of a burst bladder, having refused out of politeness to take a toilet break during a long royal banquet.

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