What are We? How Do We Know? Chapter 1

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? Guidepost

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. Guidepost (continued)

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.

A Campus Scene 16 x 16 m

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

The Landscape of Pennsylvania 100 miles x 100 miles

The Earth Diameter of the Earth: 12,756 km

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

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

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

The Solar System Approx. 100 AU

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

The Solar Neighborhood Approx. 17 light years

The Solar Neighborhood (2) Approx. 17 light years New distance scale: 1 light year (ly) = Distance traveled by light in 1 year = 63,000 AU = 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

The Extended Solar Neighborhood Approx. 1,700 light years

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

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

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

Scales of Size and Time

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