The development of ideas regarding the Solar System

Nicolaus Copernicus (1473-1543)

Nicolaus Copernicus (1473-1543) First to argue for Heliocentric model Better at predicting movements of planets – in particular retrograde motion Copernicus was a canon in the church and worried about how his ideas would be received. His models became more and more complicated, and when they were published, the preface stated that they were just a means of calculating the planets’ positions and not what was actually happening.

Tycho Brahe (1546 – 1601)

Tycho Brahe (1546 – 1601) Tycho had instruments which allowed him to calculate distance to objects, using a technique called ‘parallax’ Found that stars show no parallax Observed supernova – heavens not eternal and unchanging Observed comet – passed through ‘crystalline spheres Compromise model with Earth at the centre, the Moon and Sun going round it, and the other planets going round the Sun.

Johannes Kepler (1571-1630)

Johannes Kepler (1571-1630) Convinced by heliocentric model, because he believed it not only predicted but explained the movement of the planets Proposed that planets moved in ellipses, rather than circles – better fitted with observations Came up with simple mathematical models

Galileo 1564-1642

Moons of Jupiter – Io, Europa, Callisto, Ganymede

Phases of Venus

Moon

Dialogue Concerning the Two Chief World Systems Written in Italian, rather than Latin Sagredo – heliocentric explanation Simplicio – not very clever, stuck with the geocentric explanation Salviati – open-minded and willing to listen to the arguments

Conflict with Church

Newton and the force of gravity

Questions How do you measure mass? How do you measure the mass of the Earth? Do all objects fall at the same rate? Why doesn’t the Moon crash into the Earth? How is Newton linked to apples?

Isaac Newton (1643 – 1727)

Force and Acceleration force = mass x acceleration (2nd Law) N = kg x ms-2 A force of 1 newton will accelerate a 1kg mass by 1ms-2

The Apple Supposedly, a question occurred to Newton when he saw an apple falling from a tree. John Conduitt, Newton's assistant at the royal mint and husband of Newton's niece, had this to say about the event when he wrote about Newton's life: In the year 1666 he retired again from Cambridge ... to his mother in Lincolnshire & while he was musing in a garden it came into his thought that the power of gravity (which brought an apple from a tree to the ground) was not limited to a certain distance from earth, but that this power must extend much further than was usually thought. Why not as high as the Moon thought he to himself & that if so, that must influence her motion & perhaps retain her in her orbit, whereupon he fell a-calculating what would be the effect of that superposition... ( Keesing, R.G., The History of Newton's apple tree, Contemporary Physics, 39, 377-91, 1998)

Gravitational Field Force felt by an object in a region of space No strings attached!

Gravity and Orbits

Weight and Mass weight and mass are not the same mass(kg) is a measure of amount of matter weight(N) is the force pulling down on a mass in a gravitational field At sea level weight = mass x 9.81. At the top of Everest weight = mass x 9.78. At GCSE we use weight = mass x 10

Acceleration due to gravity What acceleration will be produced by the gravitational field? use F=ma and weight = m x 10 For 1kg mass: a = F/m = weight/m = (1kg x 10)/1 = 10 m/s2 For 0.1kg mass: a = F/m = weight/m = (0.1kg x 10)/0.1 = 10 ms2 1N 10N

Law of Gravitation Force is proportional to the mass of each object, but decreases as they are moved apart. F = G (m1 x m2)/r2 (G = 6.67 x 10-11 Nm2kg-2)

Problems F = G(m1 x m2)/r2 G = 6.67 x 10-11 Nm2kg-2 Estimate the gravitational force between 2 people, masses 55kg and 70kg, if they stand with their centres of mass 0.75 m apart The radius of the Earth is 6400km. Use this information to calculate the mass of the Earth

Measuring mass How do we go about measuring mass

William Herschel (1738 – 1822)

William Herschel (1738 – 1822) Discovery of Uranus 1781

John Couch Adams and Urbain Leverrier Observations showed that Uranus’ orbit was strange, appearing to speed up Was Newton’s Law of gravitation wrong? Adams explained this in terms of planet further out and predicted its position Leverrier was first to observe Neptune Discovery of Neptune 1846