In ancient times phenomena in the sky were not understood! Chapter 2 The Science of Life in the Universe The Ancient Greeks The Scientific Method Our ideas must always be consistent with our observations! In science observations are made with the five senses! Laid the foundations of modern science Observations Hypotheses Ideas Predictions Experiments Hypotheses must be modified or even thrown out if they are found not to consistent with observations! 1
At the turn of the 20 th century the astronomer Percival Lowell claimed to see canals on Mars! Reproducibility Valid experiments are those that are repeatable by other scientists working independently Observations could not be reproduced by other scientists! Hypotheses that have been verified experimentally and shown to be reliable are then collected together to form theories and laws They are also subject to further testing Laws Summaries of observed behavior without explanation as to why the behavior occurs Laws predict WHAT will happen! Example: Kepler s Laws of Planetary Motion Theories (Models) Explanations of observed behavior Theories explain WHY something happens (and will also allow a prediction of what will happen!) Hypothesis-driven Science Example: Einstein s Theories of Relativity 2
Thomas Edison (1847-1931) Discovery Science Sir Alexander Flemming (1881-1955) Non-science Anything that cannot be measured with the 5 senses Example: religious belief Discoverer of Penicillin An Alien Autopsy? Pseudoscience 3
The Constellations star patterns Some well known constellations Which constellation is this? Ursa Major the great bear Orion the Hunter Which constellation is this? 4
What about this one? Leo the Lion Canis Major the Hunting Dog Cygnus the Swan Sagittarius the archer Scorpius the scorpion 5
State Boundaries Constellations Today 88 constellations cover the sky, each with its own boundary 12 Constellations are special! Astronomy is NOT Astrology! Constellations of the Zodiac Direct and Retrograde Motion The five naked eye planets 1. Mercury 2. Venus 3. Mars 4. Jupiter 5. Saturn Planets are brightest during retrograde motion 6
The Geocentric Universe Ptolemy (100-170 AD) Epicycles and Deferents The Ptolemaic explanation for retrograde motion Occam s Razor The simplest ideas are usually the best The simplest ideas are those with the fewest assumptions Assumptions are things which are assumed to be true but which have not been confirmed experimentally Nicolai Copernicus (1473-1543) 7
The Heliocentric Cosmology The Copernican explanation for apparent retrograde motion Inferior and Superior planets Johannes Kepler (1571-1630) Tycho Brahe (1546-1601) Made detailed observations of planetary motion The Empirical Method The Empirical Method: An Example Planetary Orbits are Ellipses! 8
Eccentricity measuring the shape of ellipses 1 st Law: the orbit of a planet is an ellipse with the Sun at one focus Zero Low High 2 nd Law: an imaginary line joining the Sun and a planet sweeps out equal areas in equal times What does this mean? 9
Planets move faster when they are closer to the Sun! 3 rd Law: the Harmonic Law P 2 = a 3 where: P = orbital period = time taken for planet to move around the Sun a = average distance from the Sun What does this mean? The further a planet is from the Sun, the longer it takes to orbit! a P Galileo Galilei (1564-1642) Galileo was one of the first people to study the sky with a telescope 10
The Galilean Moons of Jupiter The Phases of Venus Can only be explained if Venus orbits the Sun! If Venus moved about the Earth, we would not see the observed phases! The Inquisition of Galileo (1616) Isaac Newton (1642-1727) The Analytical Method 11
Newton s 1 st Law of Motion An object will remain at rest or move in a straight line at constant speed unless a force acts In their elliptical orbits around the Sun, are the planets at rest or moving in straight lines? No! So what does this mean? A force must be acting! This force is called gravity! Newton s Universal Law of Gravitation Applications G is a constant (does not change) 12
Proof of Kepler s 3 rd Law Newton s modified form: P 2 = a 3 /(M 1 +M 2 ) where: P = orbital period (Earth years) a = average distance of planet from the Sun (compared to average Sun- Earth distance) M 1 +M 2 = combined mass (compared to the mass of the Sun) Application to Solar System M Sun = 1 M planet = 0.001 M Sun + M planet = 1.001 1 P 2 = a 3 /1 P 2 = a 3! Kepler only deduced an approximation! For any two masses in orbit about each other, if we know their average separation, a and their orbital period, P it is possible to use this formula to calculate their combined masses: Sir Edmund Halley (1656-1742) M 1 + M 2 = a 3 / P 2 Halley s Comet Adoration of the Magi: Giotto (1304-6) Inspired by Halley return in 1302? 13
The Bayeux Tapestry The Planet Uranus discovered telescopically in 1781 Depicts the Norman conquest at the Battle of Hastings (1066) The planet Neptune discovered by gravitational calculations in 1846 14