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.