Observational Astronomy 1

7/28/2019 Observational Astronomy 1

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18-Apr-13

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.


16/3018-Apr-13

What time of day does the first quarter moon

set?

A: 6am

B: noon

C: 6pm

D: midnightE: Never sets


17/3018-Apr-13

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

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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

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Observational Astronomy 1