Unit 2 : Activity 2 : Ptolemaic Model : Copernican Model : Brahe : Kepler : Galileo : Newton : Unit Exam


King Fredrick II of Denmark built Tycho Brahe an observatory to measure the position of planets with high accuracy. However, the telescope had not yet been invented, so the observatory was a naked eye observatory with oversized instruments.

Tycho Brahe (1546 – 1601) was born three years after Copernicus’ death

A painting of Brahe’s naked eye observatory. The quarter circle measurement device is drawn to the scale of the two people in the foreground of the painting.

These instruments allowed Brahe to not only measure the positions of planets with high accuracy, but he also calculated the measurement error in his instruments. Ascertaining the error in scientific instruments is expected today, but Brahe pioneered the practice.

Brahe also noted that previous measurements of the planets, as recorded in the texts of the time, were incorrect. With his new measuring devices, he was able to record the position of the planets with the best possible accuracy for naked eye observations - ten times more accurately than people had been getting until then.

Even with his high quality observations, Brahe was not able to detect any changes in the positions of the stars over the year, which should occur if the Earth revolves around the Sun. If he had, this would have constituted direct proof of the Earth's motion. This parallax effect was not detected by astronomers until the 19th century. You can see this effect by holding a finger about six inches from your face and alternately closing one eye and then the other. You should see your finger "jump" relative to background objects.

Brahe proposed a model of the solar system to explain Galileo's observation that Venus has phases without making it necessary for Earth to be moving. His model had all the planets (except Earth) orbiting around the Sun, but then the Sun orbited around the Earth. This model satisfies ALL the observations because it corresponds to reality except that is is viewed from the point of view of someone on Earth.