1. A laser is used at the Very Large Telescope (VLT) site in Chile to measure the distortions of Earth's ever changing atmosphere. Constant imaging of high-altitude atoms excited by the laser - which appear like an artificial star - allow astronomers to instantly measure atmospheric blurring. This information is fed back to a VLT telescope mirror which is then slightly deformed to minimize this blurring.

2. The VLT consists of four 8.2 meter (27.3 ft) wide telescope units:
Yepun (Sirius), Antu (Sun), Kueyen (Moon), & Melipal (Southern Cross).
The light from each of these is combined, achieving an equivalent aperture of 16.4 meters (54.7 ft)

3. Homework #1 will be posted soon
Homework #1 will be posted soon. An announcement and instructions will be sent to the class once it is posted.

4. Where do we fit in the universe?

5. Science begins with observation:

6. The origins of astronomy and the physical sciences stem from:
observing the sky and nature
finding repeatable patterns
seeking natural explanations for these patterns

7. Understanding and explaining motions observed in sky led to geometric models for the geometry of the universe.
These models, in turn, motivated the development of the physical sciences: required to explain this geometry.
Ultimately, this led to an understanding of how stars and planets form.

8. The development of Astronomy as a Science
Prehistory - development of myths to explain natural phenomena - no physical model
Later, many threads of physical models were developed, most were lost
We owe the origins of the physical sciences to the Greeks, later threads came from many other cultures.
Physical models are developed to explain observations (everything starts with observations!)
These models then make further predictions that can be tested.

9. Motions in the sky:

10. Stellarium: www. stellarium
Stellarium: -- A free open source planetarium for your computer. It shows a realistic sky in 3D, just like what you see with the naked eye, binoculars or a telescope.

11. Screenshots from Stellarium

12. Planets Known in Ancient Times
Mercury: difficult to see; always close to Sun in sky
Venus: very bright when visible; always close to Sun in sky, but can be farther than Mercury
Mars: noticeably red
Jupiter: very bright
Saturn: moderately bright

13. Q: Why are there seven days in the week
Q: Why are there seven days in the week? A: There were seven objects in the sky that moved independently of the stars.
Roman gods associated with planets
1. Saturn: Saturday
2. Sun: Sunday
3. Moon: Monday
Anglo-Saxon or Nordic analogues to Roman gods
4. Mars: Tuesday (Tiw’s day)
5. Mercury: Wednesday (Woden’s day)
6. Jupiter: Thursday (Thor’s day)
7. Venus: Friday (Freys’ day)

14. The origins of astronomy and the physical sciences stem from:
observing the sky and nature
finding repeatable patterns
seeking natural explanations for these patterns

15. Patterns Observed in the sky:
(observed facts)

16. Celestial Sphere
Large imaginary spherical surface centered on the Earth.
Stars and other celestial objects “fixed” on its surface.
Conceptual Model, not a physical model

17. This path is called the “Ecliptic”
The sun follows the same path around the sky (celestial sphere), repeating this journey once every year.
This path is called the “Ecliptic”

18. Why does the sun appear to move like this?

19. Locations of planets in the sky
Mercury: always close to Sun in sky
Venus: always close to Sun in sky
Mars: no restrictions on distance from Sun in sky
Jupiter: no restrictions on distance from Sun in sky
Saturn: no restrictions on distance from Sun in sky
What causes these differences?

20. Motions of the planets
On short term (diurnal motion), planets appear to move with the stars, east to west, making a full circuit around the sky (meridian to meridian) in approximately one day
Most of the time, planets move slowly eastward each day relative to the stars: different planets at different rates
What causes these motions?

21. Planets are always close to the “ecliptic”, the apparent annual path of the sun through the sky.

22. Why are the planets restricted to these locations?
Close grouping of five planets in April 2002.
This is a pattern that was well known to the “ancients”
Why are the planets restricted to these locations?

23. apparent retrograde motion
Some planets occasionally reverse their motion relative to the stars, moving slowly westward relative to the stars, for a few days
apparent retrograde motion
What causes this?

24. What causes this?

25. It did a reasonably good job explaining these motions.
What causes the observed motions of the stars, sun, moon, and planets in the sky?
The Greeks developed a model for the Universe that lasted for nearly 15 centuries.
It did a reasonably good job explaining these motions.

26. Claudius Ptolemy (100-170 CE)
Developed a model of the universe designed to fit the observational data.

27. “all natural motion is circular”
Ptolemy and later scientists were strongly influenced by the belief of Plato that …
“all natural motion is circular”

28. Ptolemy’s Geocentric Model
Earth is at center (Geocentric)
Sun orbits Earth
Planets orbit on small circles (epicycles) whose centers orbit the Earth on larger circles (this explains retrograde motion)

29. Apparent retrograde motion in geocentric model

30. Planet orbits lie in approximately the same plane (this explains why the planets are always near the ecliptic)
Inferior planet epicycles were fixed to the Earth-Sun line (this explained why Mercury & Venus never stray far from the Sun).
Geocentric Model

31. Ptolemy’s model fit the data and made accurate predictions, but was horribly contrived!

32. Although the geocentric model of Ptolemy gained dominance, Aristarchus of Samos actually proposed that the earth rotated daily and revolved around the sun

33. Ptolemy’s Geocentric Model
Relied upon circles upon circles (epicycles & defferents) to explain the motions of planets and the sun.
Tied to Plato & Aristotle’s belief that “all natural motion is circular”
With modifications (e.g., additions of epicycles upon epicycles), remained the standard through the middle-ages.

34. Plato proposed that the orbits of the planets have what shape?
conical
circular
elliptical
equal-angular
epicycles

35. Plato proposed that the orbits of the planets have what shape?
conical
circular
elliptical
equal-angular
epicycles

36. The diurnal (daily) motion of stars is due to
the motion of the earth around the sun
the rotation of the earth
the epicyclic nature of the celestial sphere
the rotation of the celestial sphere

37. The diurnal (daily) motion of stars is due to
the motion of the earth around the sun
the rotation of the earth
the epicyclic nature of the celestial sphere
the rotation of the celestial sphere

38. What is the ecliptic? when the Moon passes in front of the Sun
the constellations commonly used in astrology to predict the future
the Sun's daily path across the sky
the Sun's apparent path across the celestial sphere

39. What is the ecliptic? when the Moon passes in front of the Sun
the constellations commonly used in astrology to predict the future
the Sun's daily path across the sky
the Sun's apparent path across the celestial sphere

40. About how long does it take the Sun to complete one “trip” around along the ecliptic around the entire sky?
One day
One month
One year
The time varies from one trip to the next
This never happens

41. About how long does it take the Sun to complete one “trip” around along the ecliptic around the entire sky?
One day
One month
One year
The time varies from one trip to the next
This never happens