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Here and Now The Scale of the Cosmos Chapter 1

As you study astronomy, you will learn about yourself. You are a planet walker, and this chapter will give you a preview of what that means. The planet you live on whirls around a star drifting through a universe filled with other stars and galaxies, results of billions of years of events and evolution. You owe it to yourself to know where you are in the universe because that is the first step to knowing what you are. In this chapter, you will meet three essential questions about astronomy: Where are you in the universe? How does your life span and human history fit into the age of the universe? Why should you study astronomy? Guidepost

As you study astronomy, you will see how science gives you a way to know how nature works. In this chapter, you can begin thinking about science in a general way. Later chapters will give you more specific insights into how scientists work and think and know about nature. This chapter is a jumping-off place for your exploration of deep space and deep time. The next chapter continues your journey by looking at the night sky as seen from Earth. Guidepost (continued)

Where are You? To find our place among the stars, we will zoom out from a familiar scene, to the largest scales in the universe. From each frame to the next, we zoom out by about a factor 100.

A Campus Scene 16 x 16 m

A City View 1 mile x 1 mile

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 Diameter of Neptune’s orbit: Approx. 60 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: ~ 80,000 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.