Do now! We will have Wednesday in the computer suite to finish the presentations and you will do the presentations on FRIDAY

Development of models of the Solar system/universe You are here

Astronomical observations  The pattern of the stars remains the same from night to night (over the time of a life-time)  These patterns have been labelled constellations Orion’s belt

Astronomical observations  The constellations do not appear in the same place  Over the period of one night they appear to rotate around the pole star  Some stars rise above the horizon during the night, and some set behind it.

Astronomical Observations  This same movement is continued during the day. The sun rises in the east and sets in the west. Maximum height at midday (in the south in the Northern hemisphere.

Place to place  The constellation you can see also depends on where you are I can’t see the pole star! I’m on top of the world!

Constellation position  The location of the pole star varies from night to night as we go through the year, so different constellations may be visible at different times of the year. View toward horizon from 59°54'N 11°E, azimuth 0° (N) View toward horizon from 59°54'N 11°E, azimuth 0° (N)

Planets  Some objects that we can see a night behave differently!  Their position DOES change relative to the background of stars.  These objects are called planets (derived from the Greek word for “wanderer”).

Planets The planets can sometimes exhibit retrograde motion over the course of several nights.

The moon  The moon’s observed motion across the night sky is different again.  It takes part in the nightly rotation around the pole star, but it also wanders.

Phases  The moon’s appearance changes over the course of a month. We say it has phases. The Moon's Phases The Moon's Phases

Models of the Universe  Aristotle ( BC) Wisdom is the reward you get for a lifetime of listening when you'd rather have been talking.

Aristotle’s Universe  Earth at centre (geocentric)  Other objects attached to concentric perfect crystal spheres turning at slightly different speeds  All “fixed” stars were attached to the final sphere

Aristotle’s Universe

Ptolemy’s (AD 85 – 165) adaption of Aristotle’s ideas  To explain the retrograde motion of some planets, he allowed each planet to move in a small circle (an epicycle)  The centre of the epicycle moved on a bigger circle called a deferent.  Earth is still at the centre (geocentric)

Ptolemy’s adaption of Aristotle’s ideas The Universe of Aristotle and Ptolemy The Universe of Aristotle and Ptolemy The Universe of Aristotle and Ptolemy

Heliocentric models  Sun-centred (heliocentric)  First proposed by Aristarchus (2 nd century BC), but it has come to be associated with Copernicus ( )  Orbits are still taken to be circular  Explains retrograde motion of planets

Heliocentric models The Copernican Model: A Sun-Centered Solar System The Copernican Model: A Sun-Centered Solar System The Copernican Model: A Sun-Centered Solar System This model also explains why Mercury and Venus always appear close to the sun in the sky To account for slight differences between the model and actual observations, Copernicus was forced to add epicycles too!

Galileo "All truths are easy to understand once they are discovered; the point is to discover them."

Galileo  His observations with one of the first telescopes added further support to the Copernican view

Galileo’s observations  There are mountains on the moon  Venus has phases (like the moon) and appears to change size  Jupiter has objects orbiting it (moons)  There are dark spots on the sun  The sun rotates and the spots on the surface move

Galileo in trouble  In 1633 he was tried by the catholic Church and forced to recant his ideas. He remained under house arrest for the rest of his life

Kepler  At the same time as Galileo, Johannes Kepler ( ) was adapting Copernicus’s model  His new model gave very good agreement with observations

Kepler’s model  Each planet orbits the sun in an ellipse, with the sun at one focus

Universal Gravitation If I have ever made any valuable discoveries, it has been owing more to patient attention, than to any other talent. Isaac Newton (1642 – 1727)

Universal Gravitation  Newton came up with a physical explanation for Kepler’s model  Newton proposed that every object in the universe attracts every other object with a force that is proportional to the masses of the objects and inversely proportional to the distance between the objects squared

Universal Gravitation  This law applies everywhere in the universe  It can explain why an apple falls to earth, but also why the moon stays in orbit around the earth

That’s it!