ASTR-1010ASTR-1010Planetary AstronomyPlanetary Astronomy

Day 7Day 7

AnnouncementsAnnouncementsRead Chapter 3

Homework Chapter 3 will be due Thurs Sept. 23

Exam 1 -- Thursday Sept. 23

Kepler’s 2Kepler’s 2ndnd Law Law Kepler’s 3 Kepler’s 3rdrd Law Law Ranking TasksRanking Tasks

Johannes Johannes KeplerKepler

1571 - 16301571 - 1630

Johannes Kepler 1571 - 1630 is Known for -

1. First telescope observations of the sun

2. First sun centered scientific model of the solar system or universe

3. Being the world’s best naked-eye astronomer

4. Creating first a theoretical model to explain planetary motions

5. Creating first a theoretical model for explaining gravity

Johannes Johannes KeplerKepler

1571 – 16301571 – 1630

Kepler’s Three Kepler’s Three Laws of Laws of

Planetary Planetary MotionMotion

Eccentricity, e •how squashed or out of round the ellipse is

•a number ranging from 0 for a circle to 1 for a straight line

e = 0.02

e = 0.7

e = 0.9

Kepler’s First LawKepler’s First Law: The orbit of a planet : The orbit of a planet about the Sun is an ellipse with the Sun at about the Sun is an ellipse with the Sun at

one focus.one focus.

What is the shape of Earth’s What is the shape of Earth’s orbit around the Sun?orbit around the Sun?

Earth, e = 0.017Earth, e = 0.017

The Second and Third LawsThe Second and Third Laws• The Second Law

tells us what a particular planet does when it orbits a Star

– The planet will move faster when it is close to the Sun and slower when it is farther from the Sun

• The Third Law how the orbital periods are related to the orbital distances for all the planets in the Solar System– planets that are in an orbit

located near the Sun have short orbital periods

– planets that are in an orbit located far from the Sun have long orbital periods

SECOND LAWSECOND LAW• A line drawn from the planet to the Sun

sweeps out equal areas in equal times• orbital speed is not constant for an

ellipse only for a circle• planets move faster when near the

Sun (perihelion)• planets move slower when they are far

from the Sun (aphelion)

Kepler’s Second LawKepler’s Second Law: A line joining a : A line joining a planet and the Sun sweeps out equal planet and the Sun sweeps out equal

areas in equal intervals of time.areas in equal intervals of time.

SECOND LAWSECOND LAW• The speed a planet travels during its

orbit is related to the distance from the star– When the planet is near the sun the planet goes

faster than when the planet is farther from the sun

Planet travels fast herePlanet travels slow here

Lecture TutorialLecture Tutorial

• But first …

• MOVE to a new seat.

• At LEAST 2 rows (front – back) of where you are now

• At LEAST 2 seats away from the people sitting next to you (front, back, sides)

Lecture – TutorialLecture – Tutorial

Kepler’s 2Kepler’s 2ndnd Law: pg 21 Law: pg 21

• Work with a partner!• Read the instructions and questions carefully.• Discuss the concepts and your answers with one

another.• Come to a consensus answer you both agree on.• If you get stuck or are not sure of your answer, ask

another group.• You have 10 minutes!

According to Kepler’s second law, a According to Kepler’s second law, a planet with an orbit like Earth’s would:planet with an orbit like Earth’s would:

A. move faster when further from the Sun.

B. move slower when closer to the Sun.

C. experience a dramatic change in orbital speed from month to month.

D. experience very little change in orbital speed over the course of the year.

E. none of the above.

Kepler’s second law says “a line joining a Kepler’s second law says “a line joining a planet and the Sun sweeps out equal areas planet and the Sun sweeps out equal areas

in equal amounts of time.” Which of the in equal amounts of time.” Which of the following statements means nearly the following statements means nearly the

same thing?same thing?A. Planets move fastest when they are moving toward the Sun.B. Planets move equal distances throughout their orbit of the Sun. C. Planets move slowest when they are moving away from the

Sun. D. Planets travel farther in a given time when they are closer to

the Sun.E. Planets move the same speed at all points during their orbit of

the Sun.

THIRD LAWTHIRD LAW• The size of the orbit determines the

orbital period– planets that orbit near the Sun orbit with

shorter periods than planets that are far from the Sun

THIRD LAWTHIRD LAW• The size of the orbit determines the

orbital period– planets that orbit near the Sun orbit with

shorter periods than planets that are far from the Sun

a3AU= P2

years

THIRD LAWTHIRD LAW• The size of the orbit determines the

orbital period– planets that orbit near the Sun orbit with

shorter periods than planets that are far from the Sun

p = 1 yearp = ~ 12 years

THIRD LAWTHIRD LAW• The size of the orbit determines the

orbital period– planets that orbit near the Sun orbit with

shorter periods than planets that are far from the Sun

– MASS DOES NOT MATTER

Both have p = 1 year

Lecture – TutorialLecture – Tutorial

Kepler’s 3Kepler’s 3rdrd Law: pg 25 Law: pg 25

• Work with a partner!• Read the instructions and questions carefully.• Discuss the concepts and your answers with one

another.• Come to a consensus answer you both agree on.• If you get stuck or are not sure of your answer, ask

another group.• You have 10 minutes here… finish at home if need be

Which of the following best describes what Which of the following best describes what would happen to a planet’s orbital speed if would happen to a planet’s orbital speed if it’s mass were doubled but it stayed at the it’s mass were doubled but it stayed at the

same orbital distance?same orbital distance?

A. It would orbit half as fast.

B. It would orbit less that half as fast.

C. It would orbit twice as fast.

D. It would orbit more than twice as fast.

E. It would orbit with the same speed.

If a small weather satellite and the large If a small weather satellite and the large International Space Station are orbiting International Space Station are orbiting Earth at the same altitude above Earth’s Earth at the same altitude above Earth’s surface, which of the following is true?surface, which of the following is true?

A. The large space station has a longer orbital period.

B. The small weather satellite has a longer orbital period.

C. Each has the same orbital period