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.

16 x 16 m16 x 16 m

A Campus Scene

3

4

5

= 0.1 nm

1 mile x 1 mile1 mile x 1 mile

A City View

16 x 16 m16 x 16 m

100 miles x 100 miles100 miles x 100 miles

Pennsylvania

1 mile x 1 mile1 mile x 1 mile

Earth’s Diameter = 12,756 kmEarth’s Diameter = 12,756 km

Earth

100 miles x 100 miles100 miles x 100 miles

Earth–Moon Distance = Earth–Moon Distance = 384,000 km384,000 km

Earth and Moon

Earth’s Diameter = 12,756 kmEarth’s Diameter = 12,756 km

Earth–Sun Distance Earth–Sun Distance = 150,000,000 km= 150,000,000 km

Earth orbiting around the Sun

Earth–Moon Distance = Earth–Moon Distance = 384,000 km384,000 km

Earth orbiting around the Sun

In order to avoid large numbers beyond our imagination, we

introduce new units:

1 Astronomical Unit (AU)

= Earth–Sun Distance

= 150 million km

1 AU

Approx. 100 AUApprox. 100 AU

The Solar System

Earth–Sun Distance = 1 AUEarth–Sun Distance = 1 AU

Approx. 10,000 AUApprox. 10,000 AU

(Almost) Empty Space near our Solar System

Approx. 100 AUApprox. 100 AU

Approx. 1,000,000 AUApprox. 1,000,000 AU

The Solar Neighborhood

Approx. 10,000 AUApprox. 10,000 AU

The Solar Neighborhood

New distance scale:

1 light year (ly) =

Distance light travels in 1 year

= 63,000 AU ~ 1013 km

= 10,000,000,000,000 km

(= 1 followed by 13 zeros)

= 10 trillion km

Nearest star to the Sun:

Proxima Centauri, at a distance of 4.2 light yearsApprox. 16 lyApprox. 16 ly

Powers-of-ten notation

106 =10×10×10×10×10×10 =1,000,000

one thousand 1000 = 103

one million 1,000,000 = 106

one billion 1,000,000,000 = 109

one trillion 1,000,000,000,000 = 1012

one thousandth 0.001 = 10–3

one millionth 0.000001 = 10–6

Powers-of-ten notation

6.3 × 104 AU = 6 3 0 0 0 AU

= 63,000 AU

1 2 3 4

5.43 × 10–4 m = 0.0 0 0 5 4 3

= 0.000543

1234

The speed of light

• c ≡ 299,792,458 m/s, exactly. ( ≈ 3 105 km/s )

• The cosmic speed limit —You cannot go faster than this, ever!

• 1 light-year (ly) = (speed of light) (one year)

= 3×105 km

1 s

⎛

⎝⎜⎞

⎠⎟ ×  1 yr( ) × 

365.25 d

1 yr

⎛⎝⎜

⎞⎠⎟

 × 24 hr

1 d⎛⎝⎜

⎞⎠⎟

 × 60 min

1 hr⎛⎝⎜

⎞⎠⎟

 × 60 sec

1 min⎛⎝⎜

⎞⎠⎟

=  9.46 ×1012 km( ) × 1 mi

1.609 km⎛⎝⎜

⎞⎠⎟

= 5.88 ×1012 mi ≈ 6 trillion miles

Table A-6, p. 447

0.1 nm

Approx. 1,600 lyApprox. 1,600 ly

Extended Solar Neighborhood

Approx. 16 lyApprox. 16 ly

Diameter of the GalaxyDiameter of the Galaxy ~ 75,000 ly~ 75,000 ly

The Milky Way Galaxy

Approx. 1,600 lyApprox. 1,600 ly

Several million light years to Several million light years to the nearest large galaxiesthe nearest large galaxies

The Local Group of Galaxies

Diameter of the GalaxyDiameter of the Galaxy ~ 75,000 ly~ 75,000 ly

Clusters of galaxies are grouped into superclusters.Clusters of galaxies are grouped into superclusters.Superclusters form filaments and walls around voids.Superclusters form filaments and walls around voids.

The Universe on Very Large Scales

• For the units of physical quantities scientists use the metric system.

• The basic units are the meter (length), kilogram (mass), and second (time).

• To conveniently express large (and small) numbers, astronomers often use the scientific notation, also called the powers-of-ten notation.

• The distance scale of the cosmos, from the smallest to the largest, involves a factor of 1040, or 1 followed by 40 zeros. To express large distances, astronomers use the astronomical unit (AU) and the light-year (ly).

Science is a process in which conceptual models interact with observations to produce new pictures of reality.

Science is about the cosmic order.

Religion is about the cosmic purpose.