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Introduction to Astronomy and Astrophysics
Introduction to Astronomy and Astrophysics
o 
Lectures
Planets
1. 
Astronomy – and Observational Science
2. 
The Sun
3. 
Planets of the Solar System
4. 
Extra-solar Planets
5. 
Observing the Universe
6. 
Properties of Stars
7. 
Life and Death of Stars
8. 
Galaxies and Large Scale Structure of the Universe
9. 
Cosmology – Origin and Evolution of the Universe
Cluster of Stars
Stars and Planets
Galaxies (Whirlpool Galaxy)
Cosmic Microwave Background
Introduction to Astronomy and Astrophysics
Introduction to Astronomy and Astrophysics
o  Recommended text: Introduction to Astronomy and Cosmology (Morison; Wiley)
o  Lecturer:
o  Prof. Peter Gallagher
o  Head of Solar Physics and Space Weather Research Group
o  Director of Astrophysics Degree
o  Email: [email protected]
o  Assessment:
o  Examination – written paper: 70%
o  Online tutorials (3):
Cluster of Galaxies
30%
Lecture 1: Astronomy – An Observational Science
o  Overview:
Early Models of the Solar System
o  Ptolomy’s (AD 100-170) Geocentric Model
o  Early astronomy – motion of the planets
o  Ptolomy, Copernicus, Galileo
o  Laws of Planetary Motion and Gravity
o  Kepler, Newton
o  Earth at centre
o  Planets move in circular ‘epicycles’, whose
centres move around Earth in circular
‘deferents’
o  Note: Mercury nearer to Earth than Venus
Retrograde motion
o  The Solar System Today
o  Chapter 1 of Introduction to Astronomy and Cosmology
o  Explained ‘retrograde’ motion of planets
like Mars and Jupiter
Early Models of the Solar System
o  Retrograde motion of Mars
Early Models of the Solar System
o 
Copernicus’s (1473-1543) Helcentric Model
o 
Centre of Universe is near Sun
o 
Distance from Earth to Sun is imperceptible compared with distance to stars.
o 
Rotation of Earth accounts for the apparent daily rotation of the stars.
o 
Apparent annual cycle of movements of Sun is caused
by the Earth revolving round it.
o 
Apparent retrograde motion of planets caused by motion
of Earth from which one observes.
o 
Explains retrograde motion – Earth overtakes Mars on
“inside track”
Retrograde motion
Early Models of the Solar System
Orbits of the planets
o  Ptolemaic model:
o  Venus between Earth and Sun
o  Could only show crescent phases
o  Little variation in angular size
o  Copernican model:
o  Venus orbits Sun
o  Phases and almost full phase
o  Large chance in angular size
o 
Laws governing planetary motion formulated
by Johannes Kepler (1571-1630) based on
Tycho Brahe’s observations
o 
Kepler’s Laws:
1.  Planets have elliptical orbits with the Sun at
one focus
Galileo’s
drawings
of Venus’
phases
o  Galileo (1564-1642) proved Sun not Earth at
centre of solar system by observing
Venus with telescope => Copernicus correct!
Modern
images
Kepler s 1st Law: Law of Orbits
2.  As a planet orbits, a line connecting the
planet to the Sun sweeps out equal areas in
equal times
3.  The square of the orbital period is
proportional to the cube of the semi-major
axis of the orbit
Kepler s 2nd Law: Law of areas
o  Planets move in elliptical orbits with the Sun at one focus.
o  The radius vector (line joining planet to Sun) sweeps out equal areas in equal
times:
dA
= const
dt
=> Planet movies faster at perihelion.
Semi-minor axis
€
Semi-major axis
Aphelion
Perihelion
Kepler s 2nd Law: Law of areas
Kepler s 3rd Law: Law of Periods
o  The square of a planet’s period (T) is proportional to the cube of the semimajor axis of the orbit (a):
o  Consequence of conservation of energy:
Kinetic Energy + Potential Energy = const
r ⎯⎯
→ max
GM s m p
PE = −
⎯⎯
→ max
r
2
KE = 1 / 2m p v p ⎯⎯
→ min
GM s m p
= const
r
mp
r
Ms
v p ⎯⎯
→ min
where k is a constant.
r ⎯⎯
→ min
GM s m p
PE = −
⎯⎯
→ min
r
2
KE = 1 / 2m p v p ⎯⎯
→ max
o  Note: If a is in Astronomical Units (AU),
then k = 1 and T is in years
o  1 AU = Earth-Sun semi-major axis
= 149 million km
Semi-major Axis (AU)
1 / 2m p v 2p −
T 2 = k a3
T 2 = k a3
v p ⎯⎯
→ max
Period (T) in Years
In Class Problem
Consequences of Kepler’s Laws
o  Calculate the semi-major axis of Mars in AU and km given that the period
of its orbit is 1.88 years.
Gave superb map of the Solar System
o  BUT, could not give a scale. No idea of distances.
o  Answer:
o  Know:
o 
T2
=k
a3=>
a=
o  Cassini in 1672 using observations of Mars from Paris and French Guiana measured
Earth-Mars distance. Using Kepler’s 3rd Law, he then calculated Earth-Sun distance (140
million km).
T2/3
1 AU
o  Therefore, for Mars
a = (1.88)2/3
= 1.523 AU
o  As 1 AU = 149 million km =>
Mars’ semi-major axis = 227.9 million km
1.523 AU
Consequences of Kepler’s Laws
o 
Led Newton (1642-1726) to the Law of Gravity.
o 
Used Newton’s Laws of Motion (F = ma) and Kepler’s 3rd Law to derive Law of Gravitation.
The Solar System Today
Oort Cloud
Edgeworth-Kuiper Belt
Asteroid Belt
Lecture 1 Practical Task
o 
Find Venus, Mars and Jupiter just before sunrise in East. What can you see after sunrise?
Moon on Oct 8
Moon on Oct 9
Moon on Oct 10
o 
Find out more at www.jb.man.ac.uk/astronomy/nightsky/