Tycho Brahe and his Instruments

Tycho Brahe (1546 – 1601) was the greatest observer of the early part of the scientific revolution. He built some of the most accurate instrument prior to the telescope.

Tycho’s main interest in astronomy was that of astrological predictions. He became interested in astronomy when he was at university. In 1563, there was a conjunction of Jupiter and Saturn. The planetary table references of the time were both inaccurate in their prediction of when this event would occur. The Alphonsine Tables produced a date that was out by a month, while the Prutenic Tables were off by a 2-3 days.

Tycho sets outs to make the most accurate observations possible, to create an accurate model of planetary motion, with the idea that this will make possible the most accurate astrological predictions. Tycho is a scientific astrologer.

During his life then, Tycho sets out to make these observations and in the process develops some of the most accurate instruments to measure the heavens that had ever been seen. He designs and builds many instruments himself and the way he does many of his observations were also unusual for his time.

Tycho was one of the first scientists to build redundancy into his observations. He tries to confirm his own observational accuracy by make repeat observations. He also uses constructions techniques, like building the observatories underground in order to protect the instruments from wind.

In his observations there were two key phenomenon that he observed and measured, that would upset the Aristotelian view.

The new star of 1572 - ( we now call this Tycho’s nova). In 1572, a ‘new star’ or stella nova appeared in the constellation of Cassiopeia. This phenomenon lasted for a few weeks, then faded away. The appearance and disappearance of a heavenly object is in clear contradiction to Aristotle’s notion of an unchanging heavens.

The comet of 1577 - tycho observes this object and he is able to measure the distance of this object from Earth. The number he obtains puts the comet out among the planets, and moving through the solar system.

This contradicts Aristotle’s theory that comets are part of the realm of fire, and should be closer than the moon. It also raises the question of how an object could traverse the crystalline spheres that supposed make up the orbits of the planets.

These early observations establish Tycho’s reputation, and the Danish king becomes his patron, and gives him the island of Hven to support his research. He builds two observatories - the first was Uraniborg - from 1576-1580, but as his research progressed this proved insufficient and a second observatory Stjerneborg was built around 1584.

Near the end of the century however, his patron dies, and he ends up moving to the court in Prague to become the Imperial mathematician to Rudolph II.

Tycho produces accurate measurements of the heavens particular in the ring of the zodiac - along the ecliptic, and many measurements of planetary motion. He intended to produce a complete sky map, but never completed it. His measurements were accurate to within 1 minute of arc.

He does however have sufficient data on planetary motion to try to create a model.

Tycho is familiar with Copernicus’ work, but being a devote Lutheran, can not accept the sun centred model (luther having basically called Copernicus an idiot), however he does recognize some of the scientific value of the Copernican model. The model he than supports, which we now refer to as the Tychonic model has Earth at the centre with the moon and sun orbiting, while the other planets orbit the sun. This model has the inherent problem that the orbit of the sun and mars cross (page 103 cambridge )

Tycho realizes that he is unable to produce an accurate model and goes looking for help. He invites Kepler to come and join him to work on this model.

The relationship between Kepler and Tycho was not an easy one, as they were distinctly different personalities, and Tycho was unwilling to part with data for which Kepler might get credit in building a successful model from.

Tycho dies in 1601, and his data and position within the court of Rudolph II are inherited by Kepler.

Instruments

Prior to the telescope instruments were used mostly to measure positions of objects in the sky.

http://www.sil.si.edu/DigitalCollections/HST/Brahe/thumbs.htm

1. Gnomon - this is stick used to cast shadows that are measured to tell time. This is the simplest and earliest of the astronomical instruments. It can give an accurate length of the year, and be used to find dates such as solstices, and equinoxes.
2. Cross staff – (page 79 Cambridge) this instrument was used to measure the angular separation between two objects, like stars in the sky. The staff would have angular markings calibrated on it. The cross piece would slide along the staff so that the two objects you were interested in would be at either end. You could then read the angle between objects from the markings on the staff.
3. Sextant – this was also used for angular measurement. In this case one arm was fixed, while another could slide along a calibrated arc. The observer at the end of the arms would line up to two objects, and reads the angle off the calibrated arc. This instrument was used also in navigation. To find latitude, particularly at sea, you just needed to find the angular height of the North Star above the horizon.

4. Quadrant - this instrument was always on a north- south alignment and was mostly used to measure the angular height above the horizon of an object as it crossed the meridian. (page 98-99 Cambridge)

5. Armillary Sphere - this is a 3-D instrument. It is essential a globe of the sky. These could either be a solid globe with the stars marked on it, or it could be a series of rings representing the zodiac (ecliptic), the tropic lines, and the orbits of the planets in the sky. This sphere is always centered on Earth. This was used as either a 3D model, but could also be used as an angular measuring device.

6. Astrolabe – ( Cambridge text 63-67) http://www.astrolabes.org

1. there are a variety of different instruments called the astrolabe, but the astronomical version is a circular disk used to mimic the motions in the sky in a 2D representation. This instrument was a series of disks or plates. The bottom most one was a 2D representation of the coordinate system of the sky. There would be a zenith point and a horizon, as well as lines for angular height above the horizon (altitude), and compass direction (azimuth).This plate would have to change with latitude as the projection would be different. Different latitude plates were sometimes referred to as climates. On top of the climate would be a rete. This plate had markings for the various objects in the sky, and the outer ring would have markings for date and time. This could be used in two ways. If you knew the date and time, you could spin the rete to see where in the sky stars should be. If you could observe the stars, by spinning the rete you could tell time. This was both an astronomical device and used for time keeping. The earliest examples date back to the Greeks, but it is uncertain as to when exactly this device was invented.