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Measures of Astronomical Distances
•  Let s view the Universe in terms of kilometers
–  The size of the observable universe is roughly
130,000,000,000,000,000,000,000 km !
- Obviously measuring astronomical distances in km is
inconvenient.
- We need much BIGGER units of distance:
  Some commonly used distance units:
1 ly (light year) = Distance traveled by light in 1 year
= 9.46 trillion km
1 au (astronomical unit) = average distance between the sun &
earth = 150 million km
Reading: Chapter 1
Measures of Astronomical Distances
The light year (ly)
1 ly is the distance light travels in 1 year
Speed of light = 300,000 km/s
In 1 second light travels 300,000 km
In 1 minute light travels 300,000 km x 60 = 18 million km
In 1 hour light travels 18 million km x 60 = 1.08 billion km
In 1 day light travels 1.08 billion km x 24 = 25.92 billion km
In 1 year light travels 25.92 billion km x 365 = 9.46 trillion km
1 ly = 9.46 trillion km = 9,460,000,000,000 km
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Measures of Astronomical Distances
The astronomical unit (au)
The earth s orbit around the sun is elliptical. Hence the
distance between the earth and sun varies at different points
in its orbit.
Minimum distance between earth & sun
(Perihelion) = 148,000,000 km
Maximum distance between earth & sun
(Aphelion) = 152,000,000 km
1 au = Average distance between earth &
sun
= (Min. distance + Max. distance) /2
= (148000000km+152000000km)/2
= 150,000,000 km
= 150 million km
Measures of Astronomical Distances
– The size of the observable universe is roughly
130,000,000,000,000,000,000,000 km !
What is the size of the observable universe in astronomical units ?
150,000,000 km = 1 au
130,000,000,000,000,000,000,000 km
= (1au/150,000,000 km) x 130,000,000,000,000,000,000,000 km
= 8,666,666,666,667 au
= roughly 8.67 trillion au
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Measures of Astronomical Distances
– The size of the observable universe is roughly
130,000,000,000,000,000,000,000 km !
What is the size of the observable universe in light years?
9,460,000,000,000 km = 1 ly
130,000,000,000,000,000,000,000 km
= (1ly/ 9460,000,000,000km) x 130,000,000,000,000,000,000,000 km
= 13,742,071,882 ly
= roughly 14 billion ly
Scientific Notation
Scientists use a convenient shorthand for expressing very large or
very small numbers:
Every number can be expressed as a multiple of powers of 10:
Example: 4812 = 4.812 x 1000 = 4.812 x 103
Example: 0.0000312 = 3.12 x 1/100000 = 3.12 x 10-5
Rules for expressing any number in scientific notation:
1.  Place the decimal after the first nonzero digit
2.  Count the number of places that the decimal point moved.
This gives the power of 10.
3.  If the decimal moves to the left, the power is positive. If the
decimal moves to the right the power is negative.
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How old is the Universe?
Let s build a scale model: The Cosmic Calendar
Say we speed up our clocks by a factor of 13.8 billion.
- Then the entire age of the Universe would be one calendar year.
- one month would be approximately 1 billion real years
What date on the cosmic calendar would these events occur?
Big Bang : 13.8 billion years ago
Milky Way Galaxy disk forms: 8.8 billion years ago
Sun and planets form: 4.6 billion years ago
First life forms: 3.8 billion years ago
First dinosaurs: 300 million years ago
First mammals: 200 million years ago
Extinction of dinosaurs: 65 million years ago
First apes: 15 million years ago
How old is the Universe?
Let s build a scale model
The Cosmic Calendar
Say we speed up our clocks
by a factor of 13.8 billion.
- Then the entire age of the
Universe would be one
calendar year.
- one month would be
approximately 1.15 billion
real years
http://school.discoveryeducation.com/schooladventures/universe/itsawesome/cosmiccalendar/
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How old is the Universe?
•  On the cosmic calendar, Earth was formed in mid
September. How old is the earth ?
On the Cosmic Calender the Earth is 3.5 months old.
1 yr on the cosmic calendar = 13.8 billion actual years
Thus 3.5 months on the cosmic calendar
= (1.38 x 1010 yrs / 1 year) x (3.5 mths x 1 yr/12mths)
= 4.02 x 109 actual yrs
The earth is roughly 4 billion years old
Fundamental Questions of Modern
Astronomy
•  When did we come to be ?
–  if the entire age of the Universe were one calendar year
–  Human civilization occurred in the last minute of December 31st.
–  We are newcomers to the universe !
•  The Egyptian pyramids were built about 10 seconds ago
•  Copernicus and others convinced humanity that the Earth orbits the Sun
about one second ago
•  Elizabeth II became Queen of England about 0.14 seconds ago
•  You were born about 0.04 seconds ago (assuming your age is 18)
•  Canada won the Olympic Gold Medal in Women s and Men s hockey in
2002 about 0.023 seconds ago;
•  You started reading this about 2 millionths of a second ago;
•  And it is now exactly midnight on December 31 - Happy New Year!!!
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Looking back in time
•  Light, although fast, travels at a finite speed.
•  Light takes:
–  8 minutes to reach us from the Sun
–  8 years to reach us from the star Sirius (8 light-years
away)
–  1,500 years to reach us from the Orion Nebula
•  Thus we see the sun as it was 8 minutes ago, Sirius as it
was 8 years ago, & Orion as it was 1500 years ago
•  The farther out we look into the Universe, the farther back
in time we see!
Looking back in time
Ultra Deep Field image of about 10,000
galaxies taken by the Hubble Telescope
Lookback time: 13 billion years!
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Fundamental Questions of Modern
Astronomy
•  How did we come to be ?
•  The Big Bang theory of an initial cosmic
explosion is the most widely accepted theory of
the origin of the universe
-  Matter and energy was hurled in all directions
-  The simplest elements hydrogen and helium initially
dominated
-  Gravity caused matter to be drawn together to create stars,
galaxies and clusters
Fundamental Questions of Modern
Astronomy
•  How did we come to be ?
-  Stars manufacture more complex elements such as
nitrogen and oxygen.
-  When the star dies, they are expelled into space….
to form new stars and planets.
Most of the atoms in our
bodies were created in
the core of a star!
http://chandra.harvard.edu/photo/2007/g292/
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Fundamental Questions of Modern
Astronomy
•  What is our motion in the Universe ?
The Earth rotates around
it s axis once every day
from west to east
We do not feel this motion
because we are moving with
the earth.
http://www.physicalgeography.net/
Fundamental Questions of Modern
Astronomy
•  What is our motion in the Universe ?
If you are located at
the equator, you will
travel 40,070 km in
24 hours at a speed
of 1670 km/hr!
http://www.physicalgeography.net/
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Fundamental Questions of Modern
Astronomy
•  What is our motion in the Universe ?
The Earth orbits around
the Sun once every year!
The Earth s axis is tilted by
23.5º relative to the plane of its
orbit
Fundamental Questions of Modern
Astronomy
•  What is our motion in the Universe ?
Our Sun and the stars of the local Solar neighborhood
orbit around the center of the Milky Way Galaxy every
230 million years!
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The Expansion of the Universe
•  Galaxies outside our Local Group appear to be moving
away from us.
•  The farther away they are, the faster they are moving.
•  This means that the universe is expanding
Summary
•  What makes up the Universe?
•  The universe is made up of various astronomical objects,
gasses, dust and energy within and in between galaxies.
•  Where are we in the Universe ?
•  Earth is a planet in a solar system, which is one of some
100 billion star systems in the Milky Way Galaxy, which is
one of about 40 galaxies in the Local Group, which is part
of the Local Supercluster, which is part of the Universe.
•  What is the scale of the Universe ?
•  The observable Universe is about 14 billion ly in size and
hence 14 billion years old. It is vastly bigger and older than
human scales.
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Summary
•  When did we come to be ?
•  On a cosmic calendar that compresses the history of the
Universe into one year, human civilization is just a few
seconds old.
•  How did we come to be ?
•  The Universe began in the Big Bang, which produced
Hydrogen & Helium. Gravity caused formation of stars and
galaxies. The more complex elements of life were produced
by stars.
•  What is our motion in the Universe ?
•  Earth rotates on its axis once each day and orbits around the
Sun once each year. The Solar System orbits the center of
the Milky Way Galaxy every 230 million years. Galaxies in
the Local Group move relative to each other. Other galaxies
are moving away from us with expansion of the Universe.
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