Chapter 1:

Here and Now

• 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.

The following slides will take

you on a journey through the

vast range of scales in the

Universe.

From each image to the next

we are zooming out by a

factor of 100.

16 x 16 m

A Campus Scene

1 mile x 1 mile

A City View

100 miles x 100 miles

The Landscape of Pennsylvania

Diameter of the Earth: 12,756 km

The Earth

Distance Earth – Moon: 384,000 km

Earth and Moon

Distance Sun – Earth = 150,000,000 km

Earth orbiting around the Sun

Earth orbiting around the Sun

In order to avoid large

numbers beyond our

imagination, we introduce

new units:

1 Astronomical Unit (AU)

= Distance Sun – Earth =

150 million km

Approx. 100 AU

The Solar System

The sizes of the planets and their distances from the sun

make models of our solar system difficult to make.

Approx. 10,000 AU

(Almost) Empty Space

Around our Solar System

Approx. 17 light years

The Solar Neighborhood

Lightspeed!

• Light is the fastest thing in the universe,

traveling 670,000,000 mph.

• That’s 186,000 miles every second!

• In order to understand astronomical

sizes, we compare to how far light

travels in a certain amount of time.

• The distance light travels in a year is

called a lightyear.

The Solar Neighborhood

New distance scale:

1 light year (ly)

Nearest star to the Sun:

Proxima Centauri, at a distance of

4.2 light years

Approx. 17 light years

Approx. 1,700 light years

The Extended Solar Neighborhood

Diameter of the Milky Way: ~ 75,000 ly

The Milky Way Galaxy

Our View of the Milky Way Galaxy

Distance to the nearest large galaxies:

several million light years

The Local Group of Galaxies

Clusters of galaxies are grouped into superclusters.

Superclusters form filaments and walls around voids.

The Universe on Very Large Scales

What’s the point? • The universe is HUGE.

• The Earth is just one planet in our solar system; our sun is just one star in our galaxy; our Milky Way galaxy is just one galaxy in the universe.

• There is structure everywhere, even when you look at how galaxies are distributed in the universe!

• In this class, we will be learning about planets, other objects in our solar system, stars, galaxies, and the universe.