1. What are We? How Do We Know?
Chapter 1
What are We? How Do We Know?

2. Guidepost
As you study astronomy, you will learn about yourself. You are a planet walker, and you should understand what it means to live on a planet that whirls around a star drifting through a universe of stars and galaxies. You owe it to yourself to know where you are. That is the first step to knowing what you are.
In this chapter, you will meet four essential questions about astronomy:
Why should we study astronomy?
How do scientists know about nature?
Where are you in the universe?
How does human history fit on the time scale of the universe?

3. Guidepost (continued)
Besides learning about astronomy, you will consider important questions about science:
How do we know? How does science work?
How do we know? What is the difference between a scientific argument and an advertisement?
In this chapter, a cosmic zoom takes you roaring outward through the universe checking out its major features. In the next chapter, you will return to Earth and begin your study by looking at the stars in the night sky.

4. Scales of Size and Time
Astronomy deals with objects on a vast range of size scales and time scales.
Most of these size and time scales are way beyond our every-day experience.
Humans, the Earth, and even the solar system are tiny and unimportant on cosmic scales.

5. A Campus Scene
16 x 16 m

6. A City View
1 mile x 1 mile　(1 mile = 1.6 km)　

7. The Landscape of Pennsylvania
100 miles x 100 miles

8. Diameter of the Earth: 12,756 km
Diameter of the Earth: 12,756 km

9. Distance Earth – Moon: 384,000 km
Earth and Moon
Distance Earth – Moon: 384,000 km

10. Earth Orbiting Around the Sun
Distance Sun – Earth = 150,000,000 km

11. Earth Orbiting Around the Sun (2)
In order to avoid large numbers beyond our imagination, we introduce new units:
1 Astronomical Unit (AU) = Distance Sun – Earth = 150 million km

12. The Solar System
Approx. 100 AU

13. (Almost) Empty Space Around Our Solar System
Approx. 10,000 AU

14. The Solar Neighborhood
Approx. 17 light years

15. The Solar Neighborhood (2)
New distance scale:
1 light year (ly) =
Distance traveled by light in 1 year
= 63,000 AU = 1013 km
= 10,000,000,000,000 km
(= zeros)
= 10 trillion km
Nearest star to the Sun:
Proxima Centauri, at a distance of 4.2 light years
Approx. 17 light years

16. The Extended Solar Neighborhood
Approx. 1,700 light years

17. Diameter of the Milky Way: ~ 75,000 ly
The Milky Way Galaxy
Diameter of the Milky Way: ~ 75,000 ly

18. The Local Group: Our Cluster of Galaxies
Distance to the nearest large galaxies: several million light years

19. The Universe on Very Large Scales
Clusters of galaxies are grouped into superclusters.
Superclusters form filaments and walls around voids.

20. Scales of Size and Time

21. Scales of Size and Time

22. How Do We Know? The Scientific Method Observed facts - phenomenology
The Scientific Method
Observed facts - phenomenology
Possible explanation – hypothesis / theory
Testable predictions:
What else should we see if the hypothesis / theory is true?
Further observations to test predictions
Observations contradict predictions
Observations confirm predictions
Support for hypothesis/theory – Develop further predictions
Modify or reject hypothesis