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Chapter 1
Here and Now
The Scale of the Cosmos
Guidepost
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 (continued)
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.
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)
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
(= 1 + 13 zeros)
= 10 trillion km
Nearest star to the Sun:
Approx. 17 light years
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.