Unit 1 Physics Detailed Study 3.1 Chapter 10: Astronomy

Early models of the universe  Back in ancient Greece, it was a widely held view that the earth was a solid immovable sphere, around which the rest of the universe revolved in their own celestial spheres.  This worked well for the stars motion, however the motion of the planets were slightly problematic. Section 10.3 Understanding our world

Early models of the universe  The motion of the planets was not quite circular, and could be often be quite irregular.  The way this was explained by Hipparchus, was that each planet moved in a circle, known as an epicycle, the centre of which moved in a larger circle, known as the deferent.  Ptolemy improved on this by moving the centre of the deferent slightly off centre and varying planets speeds. This is a really complicated idea, but it worked. Section 10.3 Understanding our world

The Copernican revolution  The first mention of the Heliocentric or sun-cantered model, was actually back in ancient Greece, however, this didn’t sit very well with the masses so was disregarded.  It was not until the 16 th Century, when Nicolaus Copernicus that the heliocentric model seemed viable.  Copernicus was able to use the heliocentric to explain the retrograde motion of the planets such as mars more easily. Section 10.3 Understanding our world

The Copernican revolution  Copernicus’ heliocentric model also explains why we never see Mercury and Venus in the midnight sky.  He concluded that these planets were closer to the sun then the other planets in the midnight sky.  Planets closer then Earth are known as Inferior planets, while planets further than Earth are known as Superior planets. Section 10.3 Understanding our world

The Copernican revolution  Though the Copernican heliocentric model was great at explaining some phenomena, it was not perfect, so still require the addition of epicycles, though much smaller then Ptolemy’s.  This breakaway from tradition, is said to have started a ‘revolution’ that lead to future physicists to question everything.  While the heliocentric model seemed to work better the then Ptolemaic model, it was still not widely accepted. Section 10.3 Understanding our world

Observation and Theory  Tycho Brahe as an astronomer had left behind a legacy of meticulous observation and as a result a huge amount of data.  Some of his observations seemed to agree with the Copernican model, while some seemed to be in disagreement.  Some of his observations placed the Ptolemaic system under further scrutiny, as he observed a supernova, this proved that the celestial sphere was not ‘unchanging’. Section 10.3 Understanding our world

Observation and Theory  Tycho Brahe was more interesting in observing and gathering data then using that data in calculations.  This was left up to Johannes Kepler.  Kepler used this data to question further the Ptolemaic system by suggesting that the orbits of the planet were not circular, but in fact elliptical, with the sun at one focus. (This is known as Kepler’s First Law) Section 10.3 Understanding our world

Observation and Theory  Kepler also found that speed of the orbits increased as the planets came neared the sun and slowed as they went further away.  A radius from the planet to the sun would sweep out an area as it travels between two points on the ellipse at a constant rate.  Put simply, this means that a planet will sweep out equal areas in equal times. (This is Kepler’s Second Law)  Kepler’s third and final law makes reference to the time it takes a planet to orbit the sun, and its radius from the sun. A planet twice as far from the sun as the Earth, will take more then twice the time to orbit the sun.  After some time, around 9 years after his first 2 laws, he discovered the relationship he was looking for.  He concluded that the square of the period (T 2 ) of the planet is proportional to the cube of the radius (R 3 ) of the orbit. Section 10.3 Understanding our world

Galileo and the Telescope  Many credit Galileo with the invention of the telescope, this is not the case. He did however use telescopes a lot in his observation of the night sky and made improvements, this may be why he is often credited with the invention.  Galileo made four key discoveries that upset the balance of the Ptolemaic system and the church:  The fact that the moon had mountains, indicating the the moon is an earth- like object.  Sun-spots, (dark spots on the sun that would move around the surface and sometimes disappear) furthered the argument that the universe was not a perfect, godly realm. Section 10.3 Understanding our world

Galileo and the Telescope  These first two discoveries upset the Ptolemaic system, but the following discoveries all but confirmed the Heliocentric view of the universe.  Galileo observed faint, tiny stars near to Jupiter. He notice that they would revolve around Jupiter. This is not a Ptolemaic idea.  The final, and probably most damning discovery, was the phases of Venus.  Galileo observed that Venus had phases similar to our moon. He also observed that during these phases, the size of Venus would change.  Venus was at its smallest when in its full stage, this indicated that the sun was in between the Earth and Venus. The complete opposite of the Ptolemaic system! Section 10.3 Understanding our world

Galileo and the Telescope  So the Heliocentric model wins!!! … Well not quite  By that point in time (~1616) the church played a major role in society.  They had adopted the Aristotelian view of the universe, believing that the Earth was a motionless object, with objects revolving around it.  Galileo was eventually put under house arrest for continuing to teach the Copernican model. Section 10.3 Understanding our world