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
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Find out more at www.jb.man.ac.uk/astronomy/nightsky/