Transcript Our Place in the Cosmos

Our Place in the
Cosmos
and
Introduction to
Astrophysics
Lecture 2
Historical Milestones in Astronomy
The Ancient Greeks
By far the most famous early astronomers are the ancient
Greeks. Between about 500BC and 100BC, they built a
picture of the Universe which dominated for over 1000 years.
Thales (624 - 545 BC)
Realised that celestial objects were at
different distances, that the Earth was
spherical, and that the light of the moon
was reflected sunlight.
Pythagoras (582 - 500 BC)
Produced the first geocentric
model of the Universe, with
everything making perfectly
circular orbits around the Earth.
Geocentric Model
Problem: Retrograde Orbits
Plato (428 - 347 BC)
Invented the idea of epicycles, later
`perfected’ by Ptolemy.
Aristarchus of Samos (310-230 BC)
 He created the first heliocentric cosmology;
that is, he was the first to propose that the Earth,
and the other planets, went around the Sun.
1,750 years later Copernicus will claim the credit.
 Aristarchus is also famous for devising a way of
measuring the size of the moon in terms of the size of the Earth.
 Hipparchus later used this to conclude that the moon had about
one quarter the radius of the Earth (modern value 0.27) and that its
distance was about 60 Earth radii away (modern value 60.4).
However, his calculations did not give absolute values; only those
relative to the unknown radius of the Earth.
 Aristarchus also measured the relative distances of the Sun and
Moon. He underestimated that badly, but even so realised that the
Sun was bigger than the Earth.
Eratosthenes (276 - 195 BC)
Measured the circumference of the Earth with
amazing accuracy. He did so with a particularly
powerful piece of observational technology,
namely a long stick.
Hipparchus (190 - 120 BC)
Powerful insights into many aspects of
astronomy. Invented the magnitude scale,
worked out the size of the moon, developed star
and eclipse catalogues.
Ptolemy (~85 - 165 AD)
Perfected the geocentric model with epicycles.
It will go unchallenged for 1300 years.
Ptolomey’s revised epicycle mode
See also
http://astro.unl.edu/naap/ssm/animations
/ptolemaic.swf
Nicolaus Copernicus (1473-1543)
Credited with the heliocentric
model of the Solar System.
He divided the planets into the
`inferior’ ones closer to the Sun
than the Earth, and the `superior’
ones outside the Earth’s orbit.
Only planets out to Saturn were
known at that time.
The orbits were all taken as
circular.
Finally we shall place the Sun himself at the centre of the Universe. All this
is suggested by the systematic procession of events and the harmony of the
whole Universe, if only we face the facts, as they say, `with both eyes open'.
Nicolaus Copernicus
Tycho Brahe (1546 - 1601)
Almost as famous for his silver nose
(he lost the original in a duel) as for
his observations. With the support of
the King of Denmark, he developed
instruments of unprecedented quality,
capable of positional accuracies of
one arcminute.
He is less remembered for his
cosmological model, an attempted
compromise in which the Sun goes
round the Earth but the planets round
the Sun.
Johannes Kepler (1571-1630)
Painstaking studies of Tycho’s
observations led him to the nowaccepted conclusion that planets
moved not on circles but on
ellipses.
He went on to formulate his
three laws of planetary motion,
which we will be studying in a
later lecture.
Galileo Galilei (1564 -1642)
• Didn’t invent the telescope (and
might well not have dropped
cannonballs off the leaning
tower of Pisa either).
• But he is considered the
inventor of the modern scientific
method, with its emphasis on
experimental verification of
theoretical models.
Galileo was the first to properly exploit the telescope for
astronomical purposes.
Considering that his original telescope had a magnification power
of only three, he made amazing new discoveries, including the four
large moons of Jupiter, sunspots, and the rings of Saturn.
GALILEO
NOW
His belief in Copernicus’s heliocentric Universe nearly led
him to a nasty end.
Isaac Newton (1643 - 1727)
Widely acknowledged as one of the
greatest scientific geniuses of all
time. Co-inventor of calculus and
founder of the modern theories of
dynamics and optics, he also
developed a theory of gravity.
His theory explains Kepler’s Laws,
and shows that the gravity we feel
on Earth is the same as the gravity
that governs planetary orbits.
Despite the development of the telescope, it would be many years
before the remaining planets of the solar system were discovered.
Uranus was discovered in
1781 by William Herschel.
Neptune was first observed in
1846, after its position was
predicted by Adams and
Leverrier by analysing
perturbations to the motion of
Uranus...
… while Pluto was not
discovered until 1930,
and its moon Charon
only in 1978.
Albert Einstein (1879 - 1955)
 Made numerous vital
contributions to physics, the
most relevant for astronomy
of which is his general
theory of relativity (GR) , a
new law of gravity which
supplanted that of Newton
 GR unifies space and time
into a single entity:
space-time
GR in words
 Gravity is caused by the curvature of spacetime; the curvature is induced by the presence of
matter
 “Matter tells space how to curve, space tells
matter how to move” (John Wheeler)
 Light rays are bent when they pass near a large
mass, a
prediction confirmed
by Arthur Eddington
in 1919
Edwin Hubble (1889-1953)
 Discovered in 1929-30
that some nebulae lie
outside our Galaxy and
that these objects are
receding from us at a
speed proportional to
their distance, the
Hubble expansion
The expanding Universe
Expansion causes redshift
Hubble Law
Discovery of Cosmic
Microwave Background
 Once Hubble had discovered the expansion of
the Universe, cosmological models predicted a
Universe of infinite density in the past: the Big
Bang cosmology
 This theory widely accepted once Arno Penzias
& Robert Wilson accidentally discovered the
Cosmic Microwave Background (CMB) radiation
in 1965, a low-temperature (3K) relic of the hot
big bang fireball
Dark Matter
 Swiss astronomer Fritz
Zwicky was first to suggest
in the 1930s that much of
the matter in the Universe
is in some dark, unseen
form
 Not taken seriously until the
1970s, when evidence from
rotation curves of spiral
galaxies became
compelling
Detection of CMB
anisotropies
 Cosmic Background Explorer (COBE) satellite,
launched in 1989, made the first detection of
anisotropies in the CMB radiation
 2006 Nobel Prize in Physics awarded to COBE
principal investigators John Mather and George
Smoot “for their discovery of the blackbody form
and anisotropy of the cosmic microwave
background radiation"
Accelerating Universe
 Ever since Hubble’s discovery of the expansion
of the Universe, it was widely assumed that
expansion rate would slow down due to
gravitational pull of matter in the Universe
 In the late 1990s, surveys for distant supernovae
showed the surprising result that they were
fainter than expected in a matter-dominated
Universe
 Observations indicated that the expansion of the
Universe is in fact accelerating, due to presence
of a cosmological constant or dark energy
“Precision Cosmology” Era
COBE (1989)
WMAP (2003)
Outstanding Questions
 What are the dark matter and dark
energy?
 Is Einstein’s model of gravity correct?
 How do galaxies form?
 Is there other life
in the Universe?
Seminar 1
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Topics for Discussion
 What do you hope to get from this course?
 What is a scientific model and what must the
model be able to do to be useful?
 How can an incorrect scientific theory still be
considered a good scientific theory?
 What distinguishes a scientific truth from a
religious truth?
 How is astrology different from astronomy?
Discussion contd.
 What two basic kinds of models have been
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
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proposed to explain the motions of the planets?
What is the Ptolemaic model? What new things
did Ptolemy add to his model?
In what ways was the Ptolemaic model a good
scientific model and in what ways was it not?
What is the Copernican model and how did it
explain retrograde motion?
What important contributions did Tycho Brahe
make to astronomy?