1. Our Place in the Cosmos and Introduction to Astrophysics
Lecture 2
Historical Milestones in Astronomy

2. 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.

3. Thales (624 - 545 BC) Pythagoras (582 - 500 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 ( BC)
Produced the first geocentric model of the Universe, with everything making perfectly circular orbits around the Earth.

4. Geocentric Model

5. Problem: Retrograde Orbits

6. Plato ( BC)
Invented the idea of epicycles, later `perfected’ by Ptolemy.

7. 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.

8. Eratosthenes (276 - 195 BC) Hipparchus (190 - 120 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 ( 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 (~ AD)
Perfected the geocentric model with epicycles. It will go unchallenged for 1300 years.

9. Ptolomey’s revised epicycle mode See also http://astro. unl

10. 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.

11. Finally we shall place the Sun himself at the centre of the Universe
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

13. Tycho Brahe ( )
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.

14. Johannes Kepler ( )
Painstaking studies of Tycho’s observations led him to the now-accepted 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.

15. Galileo Galilei ( )
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.

16. 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.

17. Isaac Newton ( )
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.

18. 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.

19. 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.

20. Albert Einstein ( )
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

21. GR in words
Gravity is caused by the curvature of space-time; 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

22. Edwin Hubble ( )
Discovered in 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

23. The expanding Universe

24. Expansion causes redshift

25. Hubble Law

26. 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

27. 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

28. 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"

30. 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

32. “Precision Cosmology” Era
COBE (1989)
WMAP (2003)

33. 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?

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36. 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?

37. Discussion contd.
What two basic kinds of models have been 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?