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Model of the universe
Ptolemy placed the Earth at the centre of his geocentric model. Using the data he had, Ptolemy thought that the universe was a set of nested spheres surrounding the Earth. He believed that the Moon was orbiting on a sphere closest to the Earth, followed by Mercury, then Venus and then the Sun. Beyond the Sun were a further three spheres on which Mars, then Jupiter and then Saturn orbited the Earth. Finally, the outmost sphere was where all the stars were located in the 48 constellations that Ptolemy described in his text.
It wasn’t until 1543 that Polish astronomer Nicholas Copernicus (1473-1543) proposed a revised model putting the Sun at the centre – the heliocentric model of the universe.
About
Claudius Ptolemy was born in Egypt in 100AD when it was part of the Roman Empire. He was a mathematician, geographer and astronomer and produced several important, ancient manuscripts. Ptolemy died in Alexandria in 186AD.
Working scientifically
Ptolemy carefully studied the work of all the astronomers who had lived before him – particularly the Babylonian and Greek astronomers. He learnt about all the methods that were used to observe and measure objects in the night sky using the naked eye. Ptolemy took all of the observations and measurements collected over the previous 800 years and used his excellent mathematical skills to develop his own model of the universe.
Ptolemy shared his model in an important manuscript called ‘Almagest’ which is the only surviving ancient text on astronomy. He created sets of tables which could accurately predict the position of any planet in the night sky at any time in the past or future. There were also tables that could predict the position of the Sun and Moon as well as the rising and setting of the stars. In addition, he included tables that predicted solar and lunar eclipses.
The most important feature of Ptolemy’s model was that the Earth was at the centre – the geocentric model of the universe. Ptolemy’s tables were so effective at predicting positions in the night sky that they were used to prepare astronomical and astrological charts for over 1,500 years.
Claudius Ptolemy
Model of the universe
Brahe’s model of the universe put the Earth at the centre with both the Moon and Sun in circular orbits going around it. This feature of his model can be described as geocentric. Brahe’s model then becomes heliocentric with the Sun (further from the Earth than the Moon) being orbited by the rest of the planets. Mercury was in a circular orbit closest to the Sun, Venus a little further out and then Mars. Beyond that, Jupiter and Saturn also moved in circular orbits around the Sun. Around all of this was a sphere of fixed stars arranged in the shapes of the constellations.
Tycho Brahe’s assistant, Johannes Kepler, tried to convince him to use the Copernican model but Tycho could not be persuaded. After Tycho’s sudden death from a bladder infection, Kepler used Tycho’s data to develop his laws of planetary motion.
About
Tycho Brahe was born in Denmark in 1546. He was born to a wealthy family and was very well educated. In 1559 he began studying at the University of Copenhagen and became interested in astronomy. Brahe is well known for his accurate and detailed measurements of astronomical objects (the Sun, the Moon, stars and planets). He died in Prague in 1601.
Tycho Brahe
Working scientifically
Tycho Brahe observed the night sky with his naked eye, using tools to make careful measurements of the objects he observed. He was obsessed with making the most accurate measurements he could. He developed more accurate versions of the sextant and quadrant (tools for measuring angles) than there had ever been by making them much larger.
Tycho Brahe admired Copernicus greatly and he was the first person in Denmark to teach people about Copernicus’ heliocentric model of the universe. Nevertheless, based on his religious and scientific beliefs, Brahe was certain that the Earth was at the centre of the universe. He combined the ideas of Ptolemy and Copernicus with his own accurate measurements to create a new geo-heliocentric model.
King Fredrick II gave Tycho an estate on the island of Hven so that he could build an observatory and laboratories to make increasingly accurate measurements. Unfortunately, after King Fredrick’s death, Tycho Brahe fell out of favour with the Royal Court and went into exile. He became the official Imperial Astronomer in Prague where he was assisted by Johannes Kepler.
Model of the universe
Kepler’s conclusions about the universe were aligned with Copernicus’ model that had the six known planets orbiting the Sun (heliocentric). It was different in that Kepler proposed that the planets moved on elliptical (oval) paths rather than being objects fixed on nested spheres. Kepler summarised his learning with the laws of planetary motion that are still used today.
About
Johannes Kepler was born in 1571 in Germany. He was a mathematician, astronomer and astrologer. Starting his life as a mathematics teacher he later became an assistant to astronomer Tycho Brahe. Kepler is best known for his laws of planetary motion which formed the foundations of Isaac Newton’s theory of gravitation in 1687. Kepler died in 1630.
Working scientifically
Kepler was an amazing mathematician and started his career teaching in Graz, Austria. In 1600 he moved to Prague to assist exiled Danish astronomer Tycho Brahe in analysing the amazingly accurate astronomical data he was collecting from his new observatory. Brahe was so impressed with Kepler’s ideas that he soon shared all his data and the pair worked closely for over a year. However, they did not always agree: Kepler was certain that Copernicus’ heliocentric model with the Sun at the centre was correct; Brahe was convinced that the geocentric model with objects all orbiting the Earth was the true model.
When Brahe suddenly died in 1601, all of his data was given to Johannes Kepler and it became his responsibility to finish Tycho Brahe’s work. For the next 11 years Kepler investigated mathematical patterns in the data, making and testing hypotheses until he developed an even better understanding of the arrangement and movement of our solar system than anything that had gone before. His findings and conclusions were published in his book Astronomia Nova in
Johannes Kepler
The planets all move in elliptical orbits with the Sun at one focus.
An imaginary line drawn from the centre of the Sun to the centre of the planet will sweep out equal areas in equal intervals of time meaning that the planet’s speed changes during its orbit.
Model of the universe
Voyager 2 developed our understanding of the outer planets as well as numerous new discoveries, including:
- observing changes to Jupiter’s Great Red Spot, collecting high resolution images of the surfaces of many of Jupiter’s moons as well as discovering new ones
- discovering that Saturn was orbited by many ringlets and collecting hundreds of high quality images of Saturn and its moons
- obtaining the only close up image of Uranus we have, and finding 10 new moons and a magnetic field that is at 55° to the planet’s axis
- skimming about 3,000 miles from the top of Neptune’s atmosphere, and spotting five new moons and four rings around the planet never seen before.
About
Voyager 2 was one of two robotic NASA space probes launched in 1977 to study the outer solar system. They were sent to explore Jupiter, Saturn, Uranus and Neptune and to send useful data back to Earth for analysis. Voyager 2 is the only space probe to have ever visited Uranus or Neptune. The probes have been travelling at 37,000mph for 40 years, and are now in interstellar space, 12 billion miles away.
Working scientifically
Hundreds of people worked on the Voyager mission, designing the spacecraft and launch system, designing and constructing the instruments, planning the trajectory and analysing the data once it was sent back to Earth.
The Voyager probes have many instruments to make a range of accurate measurements and observations, including:
Imaging Science System – two camera system to collect high resolution images
Radio Science System – to analyse planets’ atmospheres, masses and gravitational fields
Infrared Spectrometer – to measure temperatures of planets and moons
Ultraviolet Spectrometer – to find out what gases are in the different atmospheres
Magnetometer – to measure magnetic fields.
The onboard computer was state of the art in 1977 but now the processor in an iPhone is 200,000 time faster with 250,000 times more memory.
Voyager 2
photo: NASA
Scientific ideas over time
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