Charting the Heavens: Foundations of Astronomy Learning Goals Describe the Celestial Sphere and how astronomers use angular measurement to locate objects.

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.

The Earth's rotation axis is tilted with respect to its orbit around the Sun => seasons.

Summer Winter

Scorpius Orion

Tilt is 23.5o

DayNight Day NightSun high in northern sky

Sun low in northern sky

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?

See Tutorial on book’s websitefor animation.

The Motion of the Moon

DEMO - Phases of the Moon

Cycle of phases slightly longer than time it takes Moon to do a complete orbit around Earth.

Cycle of phases or "synodic month"

Orbit time or "sidereal month"

29.5 days 27.3 days

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

Solar Eclipses

Total

Diamond ring effect - just before or after total

Partial

Annular - why do these occur?

Lunar Eclipse

Why don't we get eclipses every month?

Moon's orbit tilted compared to Earth-Sun orbital plane:

SunEarthMoon

Moon's orbit slightly elliptical:

Earth

Moon

Side view

Top view, exaggerated ellipse

Distance varies by ~12%

5.2o

Types of Solar Eclipses Explained

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.

Recent and upcoming total and annular solar eclipses

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.

Clicker Review:

What time of day does the first quarter moon set?

A: 6am

B: noon

C: 6pm

D: midnight

E: Never sets

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

Brainstorm: What is a model and how is it useful?

"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.

Planets generally move in one direction relative to the stars, but sometimes they appear to loop back. This is "retrograde motion".

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)

"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, they’re elliptical, so it didn’t fit the data well.

● Not generally accepted then.

Copernicus 1473-1543

Illustration from Copernicus' work showing heliocentric model.

Planets generally move in one direction relative to the stars, but sometimes they appear to loop back. This is "retrograde motion".

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

67

Earth

Mars

Apparent motion of Mars against "fixed" stars

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*

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*

January

July

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, inconsistent with geocentric model.

Kepler (1571-1630)

Used Tycho Brahe's precise data on apparent planet motions and relative distances.

Deduced three laws of planetary motion.

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

Kepler's Second Law

A line connecting the Sun and a planet sweeps out equal areas in equal times.

Translation: planets move fasterwhen closer to the Sun.

slower faster

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.

ab