Powers of Ten

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Powers of Ten
1 m LCD TV 10 m Bus 100 m Soccer field 1
km Skyscraper (2 on top of each other) 100 km My
commute 1,000 km Length of CA 10,000 km
Radius of Earth 100,000 km 1,000,000 km
Radius of Sun 10,000,000 km 100,000,000 km
AU 1,000,000,000 km 10,000,000,000 km Solar
System 100,000,000,000 km 1,000,000,000,000
km 1013 km Light Year
?
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Making Sense of Large Numbers

• People generally cant grasp the concept of large
  
• Numbers. So we make up ways to deal with them
• Comparison
• Subdivision
• Ignore any contact with reality and count the
  digits.

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Examples How Far Do We Walk?
Low estimate ? 3 miles per day walking 365 days
per year Long lifespan ? 100 years ?3 miles/day
x ()365 days/year x 100 years ?100,000 miles
?160,000 km (almost half way to moon!)
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How Far Does a Business Traveler Fly?

• London to SFO ? 8,700 km
• Double this (account for return trip) ? 17,000
  km
• Every week ? 52 times per year
• Years of service ? 30 years
• 30 x 52 x 17,000 km
• 26.5 million km
• gt1/6 of the way to the sun

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Typical Sizes of Astronomical Objects
How big is a comet? 10 km Asteroid? varies Moon
(ours) 1,700 km (1/100,000 of an
AU) Planet? 6,400 km Star (non-compact)? 700,000
km (1/200 of an AU) Solar system? 50 AU
(1/4,000 of a pc) Galaxy? tens of kpc Cluster
of Galaxies? several Mpc Observable
Universe? Depends on how old the universe
is Kilo (k) ? x 1,000, Mega (M) ? x
1,000,000 Giga (g) ? x 109
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Were Always Seeing the Past

• Light travels as a finite speed, so the further
  away an object is, the longer it takes the light
  to reach us.
• ? Due to this delay, we see the object as it
  appeared when the signal left it.
• This is true in everyday life the delay is
  just
• too short to notice (important for electronics
  though).

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Timescales in Astronomy
Age of the Universe ? 14 billion yrs Age of the
Earth/Solar System ?4.6 billion yrs Compare
with tens of thousands to millions of years
timescale of life on Earth. Millisecond
timescales in pulsars
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Calendar of the Universe
See image in your textbook.
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Calendar of the Universe
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Why Do the Stars Move?
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Motion About the Sun
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Relative Motions
How fast are we moving? Depends on your frame of
reference With respect to Speed Center of
Earth 1,500 km/hr Sun 100,000
km/hr Galactic center 800,000 km/hr Local
Group 300,000 km/hr Other distant
objects depends So, why does the sky appear
static?
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The Universe is Expanding
The underlying fabric of the entire universe is
expanding that is all points are moving away
from each other (unless theyre gravitationally
bound). Baking a blueberry/currant
cake/muffin/scone is the usual example.
Resizing a photo is the same thing (in 2-d)
its just a re-scaling of the universe. And
recently (1990s) evidence has been presented
that suggests its expanding faster and faster.
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Constellations
Originally, stars defined the constellations. Now
we use them to define regions of the sky,
similar to dividing the Earth into oceans and
continents.
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Angular Sizes
We dont automatically know how far away an
astronomical object is, so we cant assign a
physical size to it (length, diameter, etc.).
Instead we measure the angular size.
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Local Sky and a Basic Coordinate System
Alt/Az Coords.
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The Ecliptic
Defined by the path of the sun. The sun slowly
inches across the sky all the way around in 1
year. This is due to our orbit around the sun.
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The Milky Way
Similar to swimming at the beach
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Rising and Setting of Stars

• Sun rises in the East and sets in the West
• Standing on the N Pole, we would be rotating
  counterclockwise
• Stars/galaxies will appear to move similar to the
  sun.
• ? Facing South, stars will appear to move from
  left to right across the sky (in arcs)
• Due to Earths rotation (not actual motion of the
  stars)
• ? take a picture and the star trails will be
  longer at the equator than towards the poles.

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Seasons
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Solstices and Equinoxes
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Why Does What We See In the Sky Depend on the
Time of Year?
Wheres the sun shining? (see last slide) This
depends on latitude, but not on longitude
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arctic circle a line of latitude near but
south of the north pole it marks the
northernmost point at which the sun is visible on
the northern winter solstice and the southernmost
point at which the midnight sun can be seen on
the northern summer solstice (66 deg 33 N)
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The Zodiac
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Precession
Earth behaves similarly to a spinning top, with
its axis inclined at 23.5 degrees. The period
of precession is 26,000 years. This means that
the north star wont be the north star in
13,000 years but will be again in 26,000
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Phases of the Moon
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Phases of the Moon
All 3 objects are nearly all in the same plane
Credit Duffett-Smith
Time
Credit digitalastro.com
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Why Do We See Phases of the Moon?

• Well, its all about the geometry of these
  objects.
• Half of the moon is always illuminated (well,
  almost always, e.g. eclipses).
• Half of the Earth is always illuminated (day vs
  night)
• Were orbiting around the sun, and the moon is
  orbiting around us ? what part of the moon we see
  is time dependent. AND different phases will
  only be visible at certain times of day/night

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Motion in the Solar System

• The Moon has two relevant periods were
  interested in the former
• 29.5 days between new moons (moon directly
  between us and the sun)
• Only 27.3 days until it reappears in the same
  place relative to the stars.

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3 Days During the Lunar Cycle
To simplify things, lets first consider what
wed see during a single day.When wil l we see
the moon that day? (we can approximate the moon
as being in the same place while the Earth
rotates once.) And will look something like this
Motion of Moon
Credit Forrest Egan
Rotation of Earth
Credit Me
Dark side (Always pointing away from sun)
NOT to scale
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When Is the Moon Visible?

• This depends on what phase of the moon we see.
• We see the moon for 12 hours each day (except
  during a new moon)
• Two particular phases of the moon have obvious
  viewing times
• 1. Full moon ? apex at midnight (ignore DST)
• 2. New moon ? apex at noon
• To figure out how the other phases fit in, we
  need to think about the rotation of the Earth and
  the orbit of the moon

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Phases of the Moon
Remember the right hand rule for orbits.