observational astronomy 1
TRANSCRIPT
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IESO
Observational Astronomy
Part 1
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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 starsin 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 => seasons.
Summer Winter
ScorpiusOrion
Tilt is 23.5o
DayNight Day NightSun high in
northern sky
Sun low in
northern sky
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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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The Motion of the Moon
DEMO - Phases of the Moon
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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.
Sun EarthMoon
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Solar Eclipses
Total
Diamond ring effect - just beforeor after total
Partial
Annular - why do these occur?
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LunarEclipse
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Why don't we get
eclipses everymonth?
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Moon's orbit tilted compared to Earth-Sun orbital plane:
Sun EarthMoon
Moon's orbit slightly elliptical:
Earth
Moon
Side view
Top view, exaggerated ellipse
Distance varies by ~12%
5.2o
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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.
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.
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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What time of day does the first quarter moon
set?
A: 6am
B: noon
C: 6pm
D: midnightE: 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: KeplerD: 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
Aristotle vs. Aristarchus (3rd century B.C.):
Aristotle: Sun, Moon, Planets and Stars rotate around fixed Earth.
Ancient Greek astronomers knew of Sun, Moon, Mercury, Venus,
Mars, Jupiter and Saturn.
Aristotle: But there's no wind or parallax (apparent movement of
stars).
Difficulty with Aristotle's "Geocentric" model: "Retrograde motion of the
planets".
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 => apparent motion of stars.
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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' workshowing 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".
1
2
3
4
56
7
12
3
4
5
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Earth
Mars
Apparent motion
of Mars against
"fixed" stars
*
*
* *
*
*
January
July
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Galileo (1564-1642)
Built his own telescope (1609).
Discovered four moons orbiting Jupiter
=> Earth is not center of all things!
Co-discovered sunspots. Deduced Sun
rotated on its axis.
Discovered phases of Venus, inconsistentwith 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.
Translation: planets move faster
when closer to the Sun.
slower faster
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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, a=b=radius).
Translation: the larger a planet's orbit,
the longer the period.
a
b