Charting the Heavens: Foundations of Astronomy

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Charting the Heavens Foundations of Astronomy

• Learning Goals
• Describe the Celestial Sphere and how astronomers
  use angular measurement to locate objects in the
  night sky.
• Account for the apparent motions of the Sun and
  the stars in terms of the actual motion of the
  Earth. Explain why our planet has seasons.
• Understand the changing appearance of the Moon
  and how the relative motions of the Earth, the
  Sun, and the Moon lead to eclipses.

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The Earth's rotation axis is tilted with respect
to its orbit around the Sun gt seasons.
Summer
Winter
Sun low in northern sky
Sun high in northern sky
Orion
Day
Night
Scorpius
Day
Night
Tilt is 23.5o
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The Motion of the Moon
The Moon has a cycle of "phases", which lasts
about 29 days. Half of the Moon's surface is lit
by the Sun. During this cycle, we see different
fractions of the sunlit side.
Which way is the Sun here?
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See Tutorial on books website for animation.
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The Motion of the Moon
DEMO - Phases of the Moon
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Cycle of phases slightly longer than time it
takes Moon to do a complete orbit around Earth.
Orbit time or "sidereal month"
Cycle of phases or "synodic month"
27.3 days
29.5 days
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Eclipses
Lunar Eclipse
When the Earth passes directly between the Sun
and the Moon.
Sun
Earth
Moon
Solar Eclipse
When the Moon passes directly between the Sun and
the Earth.
Earth
Moon
Sun
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Solar Eclipses
Diamond ring effect - just before or after total
Total
Partial
Annular - why do these occur?
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Lunar Eclipse
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Why don't we get eclipses every month?
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Moon's orbit tilted compared to Earth-Sun orbital
plane
Sun
Moon
Earth
5.2o
Side view
Moon's orbit slightly elliptical
Moon
Distance varies by 12
Earth
Top view, exaggerated ellipse
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Types of Solar Eclipses Explained
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Certain seasons are more likely to have eclipses.
Solar eclipse season lasts about 38 days.
Likely to get at least a partial eclipse
somewhere. Animation
It's worse than this! The plane of the Moon's
orbit precesses, so that the eclipse season
occurs about 19 days earlier each year.
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Recent and upcoming total and annular solar
eclipses
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From Aristotle to Newton
The history of the Solar System (and the universe
to some extent) from ancient Greek times through
to the beginnings of modern physics.
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Clicker Review
What time of day does the first quarter moon
set? A 6am B noon C 6pm D midnight E Never
sets
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Clicker Question
Who was the first person to use a telescope to
make astronomical discoveries? A Aristotle B
Brahe C Kepler D Gallileo E Newton
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Brainstorm What is a model and how is it useful?
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"Geocentric Model" of the Solar System
Ancient Greek astronomers knew of Sun, Moon,
Mercury, Venus, Mars, Jupiter and Saturn.
Aristotle vs. Aristarchus (3rd century B.C.)
Aristotle Sun, Moon, Planets and Stars
rotate around fixed Earth.
Aristarchus Used geometry of eclipses to show
Sun bigger than Earth
(and Moon smaller), so guessed that Earth
orbits the Sun. Also guessed Earth spins on its
axis once a day gt apparent motion of stars.
Aristotle But there's no wind or
parallax (apparent movement of stars).
Difficulty with Aristotle's "Geocentric" model
"Retrograde motion of the planets".
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Planets generally move in one direction relative
to the stars, but sometimes they appear to loop
back. This is "retrograde motion".
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But if you support geocentric model, you must
attribute retrograde motion to actual motions of
planets, leading to loops called epicycles.
Ptolemy's geocentric model (A.D. 140)
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"Heliocentric" Model

• Rediscovered by Copernicus in 16th century.
• Put Sun at the center of everything.
• Much simpler. Almost got rid of retrograde
  motion.
• But orbits circular in his model. In reality,
  theyre elliptical, so it didnt fit the data
  well.
• Not generally accepted then.

Copernicus 1473-1543
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Illustration from Copernicus' work showing
heliocentric model.
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Planets generally move in one direction relative
to the stars, but sometimes they appear to loop
back. This is "retrograde motion".
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Planets generally move in one direction relative
to the stars, but sometimes they appear to loop
back. This is "retrograde motion".
Apparent motion of Mars against "fixed" stars
Mars
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July

Earth

7
6
6
5

3

4
4

5
3
2
2

1
1
January
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Galileo (1564-1642)
Built his own telescope (1609). Discovered four
moons orbiting Jupiter gt Earth is not center of
all things! Co-discovered sunspots. Deduced Sun
rotated on its axis. Discovered phases of Venus,
inconsistent with geocentric model.
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Kepler (1571-1630)
Used Tycho Brahe's precise data on apparent
planet motions and relative distances. Deduced
three laws of planetary motion.
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Kepler's First Law
The orbits of the planets are elliptical (not
circular) with the Sun at one focus of the
ellipse.
Ellipses eccentricity (flatness of ellipse)
distance between foci
major axis length
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Kepler's Second Law
A line connecting the Sun and a planet sweeps out
equal areas in equal times.
faster
slower
Translation planets move faster when closer to
the Sun.
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Kepler's Third Law
The square of a planet's orbital period is
proportional to the cube of its semi-major
axis. P2 is
proportional to a3
or
P2 ? a3 (for circular orbits,
abradius). Translation the larger a planet's
orbit, the longer the period.
a
b