Astronomy 1010-H Planetary Astronomy Fall_2015 Day-13.

Astronomy 1010-HFall_2015Day-13

Planetary Astronomy

Course Announcements• How is the sunset/sunrise observing going?• 1st Quarter night – Mon. 9/21 -7:30pm – on campus

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

Lecture – TutorialKepler’s 2nd 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.If you get really stuck or don’t understand what

the Lecture Tutorial is asking, ask one of us for help.

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

According to Kepler’s second law, a 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 planet and the Sun sweeps out equal areas in equal amounts of time.” Which of the following statements means nearly the 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 LAWThe size of the orbit determines the orbital

periodplanets that orbit near the Sun orbit with shorter

periods than planets that are far from the Sun

p = 1 yearp = ~ 12 years

THIRD LAW

The size of the orbit determines the orbital periodplanets that orbit near the Sun orbit with shorter

periods than planets that are far from the SunMASS DOES NOT MATTERMASS DOES NOT MATTER

Both have p = 1 year

THIRD LAWThe size of the orbit determines the orbital periodplanets that orbit near the Sun orbit with

shorter periods than planets that are far from the Sun

a3AU= P2

years

Kepler’s third law in its simplest form utilizes nonstandard units—the periods are in years, while the distances are in AU.

The relationship does not change if standard units are used.

The equation is just more complicated.

MATH TOOLS 3.2MATH TOOLS 3.2

Consequences:• Distant planets take

longer to orbit the Sun.

• Distant planets travel at slower speeds.

Lecture – TutorialKepler’s 3rd Law: pg 25





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

the same altitude above Earth’s 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

Copernicus was able to use right-triangle trigonometry and observations of planets at opposition or conjunction to very accurately find their distances relative to the Earth–Sun distance.

CONNECTIONS 3.1CONNECTIONS 3.1

These distances were very accurate compared to our modern values.

He was not able to discern our distance from the Sun this way, so the distances are expressed in units of our distance (1 AU).

CONNECTIONS 3.1CONNECTIONS 3.1

The synodic (S) and sidereal (P) periods of the planets can be related to Earth’s (E) sidereal period (365.25 days).

Inferior planets:

Superior planets:

Synodic periods are measurable from Earth.


Kepler’s third law in its simplest form utilizes nonstandard units—the periods are in years, while the distances are in AU.

The relationship does not change if standard units are used.

The equation is just more complicated.


Proportionality and inverse proportionality are ways to understand how one quantity behaves relative to another quantity.

It lets you get the gist of how the relationship works between those two quantities.

Sometimes, you need to know more than just the gist—you need to know the constant of proportionality, which exactly relates the quantities.


Galileo Galilei was the first scientist to observe the sky with a telescope.

Found four moons in orbit around Jupiter.

Saw that Venus had phases.• In a geocentric

model, Venus’s phase would not change.

Isaac Newton (1642 – 1727)The Baddest Dead White Guy of Them All

Isaac Newton (1642 – 1727)

1. Probably died a virgin

2. Was a priest and Lawyer

3. Had artificial wooden and silver noses

4. Probably died of Mercury poisoning

5. Rumored to have died when his bladder burst

6. Was blind at the time of his death

7. Was labelled a heretic by the church

Isaac Newton (1642 – 1727)

1. Probably died a virgin

2. Was a priest and Lawyer

3. Had artificial wooden and silver noses

4. Probably died of Mercury poisoning

5. Rumored to have died when his bladder burst

6. Was blind at the time of his death

7. Was labeled a heretic by the church

Using observations and investigations from Galileo, Isaac Newton discovered laws that apply to all objects.

Basis of classical mechanics.

Physical laws, not empirical science.

Newton’s First Law of Motion• A body remains at rest or moves in a

straight line at a constant speed unless acted upon by an outside (net) force.

• A rockets will coast in space along a straight line at constant speed.

• A hockey puck glides across the ice at constant speed until it hits something

Newton’s Second Law of Motion• (net)Force = mass x acceleration or

Fnet = m x a

• Acceleration is the rate of change in velocity – or how quickly your motion is changing.

• Three accelerators in your car!!

Acceleration is force divided by mass, or

Mass resists changes in motion. Greater forces mean greater accelerations.

Fa

m

Proportionality and inverse proportionality are ways to understand how one quantity behaves relative to another quantity.

It lets you get the gist of how the relationship works between those two quantities.

Sometimes, you need to know more than just the gist—you need to know the constant of proportionality, which exactly relates the quantities.


Knowing the unbalanced force an object experiences allows you to find its acceleration, and vice versa.


Newton’s Third Law of Motion• Whenever one body exerts a force on a

second body, the second body exerts an equal and opposite force on the first body.

• Don’t need a rocket launch pad!• The Bug and the Windshield – who is

having the worse day?

Newton’s third law of motion: For every force, there is an equal and opposite force.

The two forces have the same size.

The two have opposite directions.

Newton’s Law of Gravitation

• Newton’s law of gravitation states: Two bodies attract each other with a force that is directly proportional the product of their masses and is inversely proportional to the square of the distance between them.

2

21

d

mGmFgrav

What the ….? I thought I understood gravity?

Newton’s Law of Gravitation

• To figure out the gravitational force just multiply the mass of the two things together then divide by the distance they are apart (squared).

2

21

d

mGmFgrav

m1

m2

d

Newton’s Law of Gravitation• Newton’s law of gravitation states: Two bodies attract each other

with a force that is directly proportional the product of their masses and is inversely proportional to the square of the distance between them.

gmF

m

mkgkgNm

F

d

mGmF

grav

object

grav

grav

26

242

211

1038.6

1097.51067.6

2

21

g ~ 10 m/s2 “the acceleration of gravity” & g x m is your weight!

The gravitational acceleration at the surface of Earth, g, can be solved for by using the formula for the gravitational force and Newton’s second law.

The m cancels. g is the same for all

objects at the same R.

MATH TOOLS 4.1

Gravity works on every part of every body. Therefore, self-gravity exists within a planet. This produces internal forces, which hold the

planet together.

CONNECTIONS 4.1

There’s a special case: spherically symmetric bodies.

Force from a spherically symmetric body is the same as from a point mass at the center.

CONNECTIONS 4.1

Lecture – TutorialNewton’s Law of Gravity: pg 29





Two-Minute Essay My name is…

Definitions & Terms -1• Season: A time

Astronomy 1010-H Planetary Astronomy Fall_2015 Day-13.

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