introduction to astronomy and astrophysics · o head of solar physics and space weather research...
TRANSCRIPT
Introduction to Astronomy and Astrophysics
Cosmic Microwave Background
Stars and Planets
Galaxies (Whirlpool Galaxy)
Introduction to Astronomy and Astrophysics o Lectures
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
Planets
Cluster of Stars
Cluster of Galaxies
Introduction to Astronomy and Astrophysics
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): 30%
Introduction to Astronomy and Astrophysics
o Recommended text: Introduction to Astronomy and Cosmology (Morison; Wiley)
Lecture 1: Astronomy – An Observational Science
o Overview:
o Early astronomy – motion of the planets o Ptolomy, Copernicus, Galileo
o Laws of Planetary Motion and Gravity o Kepler, Newton
o The Solar System Today
o Chapter 1 of Introduction to Astronomy and Cosmology
Early Models of the Solar System
o Ptolomy’s (AD 100-170) Geocentric Model
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
o Explained ‘retrograde’ motion of planets like Mars and Jupiter
Retrograde motion
Early Models of the Solar System
o Retrograde motion of Mars
Early Models of the Solar System
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”
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.
Retrograde motion
Early Models of the Solar System
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 Galileo (1564-1642) proved Sun not Earth at centre of solar system by observing Venus with telescope => Copernicus correct!
Galileo’s drawings of Venus’ phases Modern images
Orbits of the planets
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
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 1st Law: Law of Orbits
o Planets move in elliptical orbits with the Sun at one focus.
Semi-minor axis
Semi-major axis Perihelion Aphelion
Kepler�s 2nd Law: Law of areas
o The radius vector (line joining planet to Sun) sweeps out equal areas in equal times:
=> Planet movies faster at perihelion.
€
dAdt
= const
Kepler�s 2nd Law: Law of areas o Consequence of conservation of energy:
Kinetic Energy + Potential Energy = const
1/ 2mpvp2 −
GMsmp
r= const
r⎯→⎯ min
PE = −GMsmp
r⎯→⎯ min
KE = 1/ 2mpvp2 ⎯→⎯ max
vp ⎯→⎯ max
r⎯→⎯ max
PE = −GMsmp
r⎯→⎯ max
KE = 1/ 2mpvp2 ⎯→⎯ min
vp ⎯→⎯ min
mp
Ms
r
Kepler�s 3rd Law: Law of Periods
o The square of a planet’s period (T) is proportional to the cube of the semi-major axis of the orbit (a):
T2 = k a3
where k is a constant. 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 T2 = k a3
Period (T) in Years
Sem
i-maj
or A
xis (
AU
)
In Class Problem
o Calculate the semi-major axis of Mars in AU and km given that the period of its orbit is 1.88 years.
o Answer:
o Know: T2 = k a3 => a = T2/3
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 AU
1.523 AU
Consequences of Kepler’s Laws
o Gave superb map of the Solar System
o BUT, could not give a scale. No idea of distances.
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).
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
Asteroid Belt
Edgeworth-Kuiper Belt
Oort Cloud
Lecture 1 Practical Task
o Find Venus, Mars and Jupiter just before sunrise in East. What can you see after sunrise?
o Find out more at www.jb.man.ac.uk/astronomy/nightsky/
Moon on Oct 8
Moon on Oct 9
Moon on Oct 10