AnnouncementsFive planets were known in ancient times! Venus, Jupiter, Saturn, Mars, and Mercury! All five are visible to the naked eye 19 The Changing Appearance of the Planets!

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Announcements

!  Tonight’s observing session is cancelled (due to clouds)!  the next one will be one week from now, weather

permitting!  The 2nd LearningCurve activity was due earlier today!  Assignment 2 and Quiz 2 are due on Tuesday at 2 p.m.,

and will cover Chapter 2 from the textbook!  If you do not yet have access to LaunchPad, you need

to do this ASAP1

Topics To Be Covered in this Lecture

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!  I am assuming that you have read Chapter 2 in advance of this lecture

!  I will focus on the following in this lecture:!  ancient astronomy!  the geocentric and heliocentric models!  the Copernican revolution and Galileo’s observations!  planetary orbits and Kepler’s laws!  Newton’s laws and gravity

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GRAVITY AND THE SOLAR SYSTEM

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Overview

!  We will review some of the history of astronomy, including the development of the concept of gravity

!  We will begin with ancient civilizations!  We will see how scientists like Galileo and Newton

changed our view of the Solar System and our place in it

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Ancient Astronomy

Many ancient civilizations studied the sky, for various reasons:!  Navigation (using the sky to determine directions and

location)!  Agriculture (predicting the changing seasons)!  Religious/philosophical/scientific meaning

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Early Discoveries

!  Egyptian & Chinese astronomers knew the length of the year at least 3000 years ago

!  Numerous cultures independently built elaborate structures to keep track of the Sun, moon and planets!  Egyptian pyramids (~2500 B.C.)!  Stonehenge, in England (~2000 B.C.)!  Mayan structures (~1000 A.D.)!  etc.!

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The Egyptian Pyramids

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Alignment of the Pyramids

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Stonehenge, England

Simulated view of Stonehenge, as it would have looked 4000 years ago 9

Mayan Observatory�(Chichen Itza, Mexico)

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Early Concepts of the Universe

!  For thousands of years, people believed that the Earth was at the centre of the universe

!  Most people thought that the Sun, moon, planets, and stars all revolved around the Earth

!  All celestial bodies were thought to be perfect and unchanging (unlike the Earth itself)

!  This was a very natural (and comforting) view of the universe

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The Early Days of Science

!  Early Egyptians, Mesopotamians, and Greeks developed geometry and mathematics about 2500 years ago

!  At around 350 B.C., Aristotle and others determined that the Earth is spherical

Aristotle (384-322 B.C.) 12

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Measuring the Size of the Earth

Eratosthenes made a fairly accurate estimate of the size of the Earth in about 200 B.C.

His answer was accurate to 1%!

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Early Observations of the Sky

!  The stars appeared to revolve around the Earth !  The Sun also appeared to revolve around the Earth, but

at a slightly different rate !  once every solar (not sidereal) day

!  The Moon appeared to revolve around the Earth, at a different rate than the stars or the Sun

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The Unchanging Appearance of the Sun

Further observations of the Sun showed that as it circled the celestial sphere every year, !  its speed didn’t change!  its brightness didn’t change !  its size didn’t change

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A Model to Describe Early Observations of the Sky

!  Similarly, the Moon’s motion and size appeared to be quite constant

!  These observations implied that the Sun and Moon each revolved around the Earth at a fixed distance and speed

!  These observations led to the first widely accepted model of the universe …

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The Geocentric Model

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The Geocentric Model

This so-called geocentric (Earth-centred) model postulated that:!  the Earth is at the centre of the universe!  the Earth is stationary and does not rotate!  the stars are fixed onto a sphere that rotates east-to-

west around the Earth once per day!  the Sun and Moon also go around the Earth

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The Planets

!  Some objects seemed to wander in the night sky!  These strange objects were called planets!  Five planets were known in ancient times

!  Venus, Jupiter, Saturn, Mars, and Mercury!  All five are visible to the naked eye

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The Changing Appearance of the Planets

!  The planets were seen to change in apparent brightness!  They also appeared to move through the sky at

changing speeds!  Sometimes they even appeared to change direction and

go backwards! !  this is called retrograde motion

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The Retrograde Motion of Mars

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Ptolemy’s Geocentric Model

!  The Greek astronomer Ptolemy (140 A.D.) explained the changing appearance of the planets using epicycles

!  He claimed that planets travel on small circles (epicycles) as they orbit the Earth

!  This explains how they can change in brightness, and also explains their wandering motions

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Epicycles

See LaunchPad Animation 2.2 23

The Heliocentric Model

A Greek astronomer named Aristarchus (310-250 B.C.) proposed instead that:

!  all the planets (including Earth) orbit the Sun

!  the Earth rotates on its axis!  this model explains retrograde

motion as being due to the Earth’s motion relative to the other planets

This is called the heliocentric (sun-centred) model

!  this model went against strongly held beliefs (both scientific and religious)

!  the geocentric model was widely accepted as correct until the 16th century!

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The Copernican Revolution

!  Polish astronomer Nicholas Copernicus (1500 A.D.) argued in favour of the heliocentric model

!  He emphasized that it could explain the observations just as well as the widely accepted geocentric model

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Galileo and the Telescope

!  New observations were needed to settle the geocentric vs. heliocentric debate

!  In 1609, an Italian astronomer named Galileo Galilei heard about a new invention called the telescope

Galileo Galilei (1564-1642)26

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Galileo and the Telescope

!  Telescopes were originally designed for military purposes

!  Galileo built a telescope of his own in 1609!  He was one of the first people to use a telescope to

study astronomy

Galileo’sTelescope

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Craters on the Moon

!  Galileo was the first to discover that the Moon is not a perfect sphere

!  Instead, it has craters, valleys & mountains, much like the Earth

!  This implied that at least one celestial object was not perfect

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Sunspots and the Sun’s Rotation

!  Galileo also found blemishes on the Sun!  these markings are called

sunspots!  Using these sunspots, he also

discovered that the Sun rotates

!  Both findings implied the Sun wasn’t quite so perfect after all

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The Predicted Phases of Venus

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Geocentric vs. Heliocentric

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The Observed Phases of Venus

!  Galileo discovered that the planet Venus undergoes a full series of phases

!  It also appears to change in size!  it looks largest when it is a

thin crescent!  This implies that Venus orbits

the Sun

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Moons Around Jupiter

!  Finally, Galileo discovered 4 moons in orbit around Jupiter!  these 4 moons are now called

the Galilean moons!  This made it clear that the

Earth could not be the centre of all orbits

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Acceptance of the Heliocentric Model

!  These new observations clearly favoured the heliocentric model over the geocentric one

!  The work of Galileo and others helped to bring about verification and eventual acceptance of the Copernican heliocentric model

!  The key to resolving this debate was the scientific testing of these competing theories using new observations

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Planetary Configurations

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Planetary Orbits

!  Danish observational astronomer Tycho Brahe kept careful and extensive records of the stars and planets

!  German theoretical astronomer Johannes Kepler studied these observations, and realized that circular orbits could not explain the precise motions of the planets

!  He eventually realized that ellipses provide a more natural explanation

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Ellipses

!  An ellipse looks like a flattened circle!  An ellipse has a precise geometrical shape!  A circle is one type of ellipse

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A Planet’s Orbit

!  Kepler correctly deduced that the shape of a planet’s orbit is an ellipse

!  An ellipse has two focal points (foci)!  The Sun lies at one focus

!  the Sun is not in the middle of the ellipse!  The place in the orbit which is closest to the Sun is

called perihelion!  The furthest point is called aphelion

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Planetary Orbits

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The Sun at Perihelion and Aphelion

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Kepler’s Laws

!  1) Planets have elliptical orbits, with the Sun at one focus!  2) An imaginary line between the Sun and a planet sweeps

out equal areas of the ellipse in equal time intervals!  planets move fastest at perihelion!  planets move slowest at aphelion

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Kepler’s Third Law

3) The square of a planet’s orbital period (P) is proportional to the cube of its semi-major axis (a)!  planets with larger orbits take longer to orbit the Sun!  P2 ~ a3

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The Laws of Motion

!  Kepler’s laws describe the way in which planets orbit the Sun

!  These laws are based on measurements, rather than any specific underlying theory

!  They do not explain why planets orbit the Sun

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Sir Isaac Newton (1643-1727)

!  British mathematician Sir Isaac Newton was the first to explain planetary motions

!  Newton’s laws of motion also apply to most objects here on Earth and beyond the Solar System

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Newton’s Laws of Motion

1) An object will either remain at rest or continue moving at the same speed and in the same direction unless it is acted upon by a force!  no force means no acceleration

2) If a force (F) is acting on an object, the resulting acceleration (a) of the object is: !  directly proportional to the size of the force!  inversely proportional to the object’s mass (m)!  mathematically, this can be written as F = ma

3) For every action, there is an equal and opposite reaction

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Gravity

!  Newton deduced that an unseen force acting between the Sun and the planets could explain the planet’s elliptical orbits!  he named this force gravity

!  Using known measurements, he figured out what determined the size of this force

!  This led to his Universal Law of Gravity: F = Gm1m2/r2

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Newton’s Law of Gravity

!  Every object in the universe is attracted to every other object by the force of gravity

!  The strength of the gravitational force (F) between any two objects is directly proportional to each of their masses (m1,m2)

!  The force of gravity is inversely proportional to the square of the distance (r) between them

!  F = Gm1m2/r2 (G is the gravitational constant)

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Some Examples of Gravity at Work

!  Earth’s gravity keeps us on the ground!  Earth’s gravity keeps the Moon and artificial satellites

in orbit around the Earth!  The Sun’s gravity keeps planets in orbit around the Sun!  Our Galaxy is held together by gravity acting between

its billions of stars

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An Astronaut in Orbit

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Satellite Motion

!  A satellite that is orbiting the Earth is able to stay in orbit for many years (forever, in principle)!  it doesn’t slow down and stop, because there is no air

resistance to slow it down!  it does not need fuel or rockets to keep it moving

!  In order to be in orbit, it must have started at an appropriate speed and direction!  if it moves too fast, it will escape!  if it moves too slow, it will fall to the Earth

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How a Satellite Stays in Orbit

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Before Next Class

!  Complete Assignment 2 and Quiz 2!  due on Tue. Sep. 27 at 2 p.m.

!  Read Chapter 3 (we will cover it in class next week)!  Complete LearningCurve: Light and Telescopes

!  due on Thu. Sep. 29 at noon

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