The Geocentric Universe Cosmology: the study of the workings of the universe on the largest scale To the ancient Greeks, the universe was essentially only the solar system The ancient Greeks could see Mercury, Venus, Mars, Jupiter, Saturn and our moon The Greeks did not know these objects as we do today – they called them “planets”, which meant “wanderers” – they did not realize that Earth was a planet, too!
Geocentric Universe The Greeks noticed that planets sometimes underwent “retrograde” or backwards motion – the stars did not! Conjunction – when two or more planets or a planet and the sun are in the same direction in the sky Aristotle (384 – 322 BC) believed in a geocentric (Earth-centered) universe
Not all Greeks agreed with Aristotle Aristarchus: proposed that all the planets, including Earth, revolve around the Sun AND that Earth orbits on its axis once each day
Ptolemaic System Ptolemy, at about 140 AD, wrote Almagest (“the greatest”) gave a reason for retrograde motion, using epicycles and deferents Using today’s standards, this would be considered wrong because it is too complex
The Heliocentric Model – The Copernican Revolution Nicolaus Copernicus: (16 century Polish cleric) –Supported Aristarchus’ model –Said only the moon orbited Earth –Explained retrograde motion –Said that the planets orbited the sun in circles –Had little effect during his time –73 years after his death, his book was placed on the church’s banned book list, where it remained for until the end of the 18 th century
The Birth of Modern Astronomy – Galileo (1564-1642) Italian mathematician and philosopher After the telescope was invented, Galileo built one and used it for astronomical purposes
The Birth of Modern Astronomy – Galileo discovered the varied terrain of the moon sunspots – did he go blind? http://mintaka.sdsu.edu/GF/vision/Galileo. html http://mintaka.sdsu.edu/GF/vision/Galileo. html Jupiter’s four largest moons that Venus had phases like the moon
The Birth of Modern Astronomy – Galileo wrote Sidereus Nuncius (The Starry Messenger); discussed his observations and his support of the Copernican ideas (1610) In 1616 his ideas are judged heretical by the Catholic church Dialogue Concerning the Two Chief World Systems (1632) – compares the Ptolemaic and Copernican models – pokes fun of the current Pope
The Birth of Modern Astronomy – Galileo’s final years Galileo writes Dialogue in Italian instead of Latin, which makes it accessible to the public In 1633, during the Inquisition, he is forced to retract his ideas He is placed under house arrest for the rest of his life He is finally pardoned by the church … in 1992
Tycho Brahe Born in Denmark Studied astrology, alchemy and medicine Had his own observatory, Uraniborg, in Denmark Kept great records on the stars and planets Noted a comet and supernova in his lifetime
Tycho Brahe 1597 – moves to Prague Becomes Imperial Mathematician of the Holy Roman Empire 1600 - Johannes Kepler becomes Brahe’s assistant Brahe dies a year later Kepler inherits his position and all of his accumulated data and observations
Johannes Kepler Believed in the heliocentric theory Develops “Three Laws of Planetary Motion”
Kepler’s First Law States “The orbital path of the planets are elliptical (not circular), with the sun at one focus”. Eccentricity: a measure of how flattened the ellipse is This broke the Greek idea of the “perfect circle” that even Galileo and Copernicus supported
Kepler’s Second Law States “an imaginary line connecting the Sun to any planet sweeps out equal areas of the ellipse in equal time intervals” Means that planets move quickest when closest to the sun Kepler published his first two laws in 1609. Had proved his laws with Mars only. 1619 – proved his laws with the other known planets.
Kepler’s Third Law States “the square of a planet’s orbital period is proportional to the cube of its semimajor axis See page 48 Kepler’s laws apply to all known planets, moons and now, artificial satellites
Isaac Newton “If I have seen farther, it is because I have stood on the shoulders of giants.”
What do Newton’s Law of Motion have to do with astronomy? Aristotle believed that the natural state of an object was at rest. This was due to his observations with friction. But he didn’t know what friction was. When we talk about the planets, friction is not a factor. Therefore “on object in motion (a planet) remains in motion in a straight line unless acted upon by an outside force”.
What do Newton’s Law of Motion have to do with astronomy? But the planets do not move in a straight line – why? Two factors result in the elliptical orbit: –Gravity from the sun –Inertia from the mass of the planet
What do Newton’s Law of Motion have to do with astronomy? Newton’s 2 nd Law –Force = mass x acceleration –This applies to your weight, which is a result of the Earth’s gravity. –Weight = mass x acceleration due to gravity
What do Newton’s Law of Motion have to do with astronomy? Newton’s 3 rd Law “To every action there is an equal and opposite reaction” For example, the Earth exerts a force on the moon (to keep it in orbit) and the moon exerts an equal but opposite force on the Earth? So why does the Earth not orbit the moon?