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• panda.unm.edu/courses/sanfratello/sp09/astro101
• Remember - Read syllabus and schedule
• (contains textbook reading assignments and
• suggested problems from the text.)?

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Astronomy Picture of the Day
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Powers of Ten Video
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What properties of physical objects would you
expect to be important in Astronomy?

• Mass
• Size
• Distance from Earth
• Temperature
• Speed and Velocity
• Brightness (Luminosity)?
• Age
• Chemical composition

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Question
Which is longer a Solar Day or a Sidreal Day?
Why? A. Solar, Earth both rotates on it's axis
and orbits the sun B. Sidreal,Earth both rotates
on it's axis and orbits the sun C. Solar, the
Earth slows down as it rotates D. Sidreal, the
Earth speeds up as it rotates
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One solar day later, the Earth has rotated
slightly more than 360o . A solar day is longer
than a sidereal day by 3.9 minutes (24 hours
vs. 23 hours 56 minutes 4.091
seconds). Difference due to rotation and
revolution of Earth!
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Question
The wobble of the Earth on its axis is called
_______. It is caused by _______. A.
precession, the Earths rotation on it's axis B.
rotation, conservation of angular momentum C.
precession, the gravitational pull of the moon D.
Earth's orbit around the sun, the seasons
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Precession
What causes precession? How long does one cycle
take?
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Precession
The Earth has a bulge. The Moon "pulls down" on
the side of the bulge closest to it, causing the
Earth to wobble on its axis.
Earth
Moon

Vega

Polaris
Spin axis
Precession Period 26,000 years!
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Question
Why is looking out into space like looking back
in time? A. ancient civilizations named the
constellations B. the speed of light is
infinite C. the speed of light is finite
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How are the Earth, Moon, Sun, and stars laid out
in space?

• What lies at the center?
• Motion(s) of the Earth?
• Motion(s) of the Moon?
• What about the stars?

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The Moon has a cycle of "phases", which lasts
about 29 days.
The Motion of the Moon
Reflected Light
Which way is the Sun here?
Moon moves in synchronous motion it rotates on
its axis in exactly the same amount of time it
takes to orbit earth.
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(DEMO)?
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Why doesn't the earth prevent the sunlight from
reaching the moon during its Full phase?
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Moon's orbit tilted compared to Earth-Sun orbital
plane
Moon
Sun
Earth
5.2o
Side view
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Eclipses

• What sort of alignment of the Earth, Moon, and
  Sun is required for a lunar eclipse?
• How about for a solar eclipse?

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Eclipses
Lunar Eclipse (around 100 minutes)?
When the Earth passes directly between the Sun
and the Moon.
Sun
Earth
Moon
Solar Eclipse (no more than about 8 minutes)?
When the Moon passes directly between the Sun and
the Earth.
Earth
Moon
Sun
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(No Transcript)
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Lunar Eclipse
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Total Solar Eclipse
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Varieties of Solar Eclipses
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During which phase(s) can a lunar eclipse
occur? What about a solar eclipse? Why don't
eclipses occur every month?
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Regions of Earth that saw or will see total (red)
or annular (blue) solar eclipses between the
years 2000 and 2020. Each track represents the
path of the Moons umbra across Earths surface
during an eclipse. High-latitude tracks are
broader because sunlight strikes Earths surface
at an oblique angle near the poles and because of
the projection of the map.
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Triangulation - Using Geometry to Measure
Distances

• Measure
• Angle at A
• Angle at B
• Length of Baseline
• Calculate
• Distance to object
• distance baselinetan(B)?

(SOH CAH TOA )?
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Parallax

• Triangulation - Measure angles at points A and B
• Parallax - Know Baseline. Measure third angle in
  triangle made by A, B, and object in space
• Baseline problem

The apparent displacement (shift) of a foreground
object relative to the background as the
observers location changes is known as parallax.
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Parallax Geometry

• Parallax angle is inversely proportional to
  distance of object
• Close object causes large parallax
• Distant object causes small parallax

The closer an object is to the observer, the
larger the parallax.