Chapter 20 – Planetary Motion

What would happen if the Earth’s orbit changed?

I. History of our position in the universe

B. 2nd Century A.D. – Ptolemy – Earth is the

center of the universe (geocentric

model) and orbits of objects (moon, sun,

planets) are perfect circles.

B. Other early observations:

• Star positions in the sky changed through the night relative to the earth; but stars did not move relative to one another. Ex: constellations.

Constellations

2. “Wanderers” moved about the stars – other planets in their orbits

C. Early 1500’s – Nicolaus Copernicus –

Earth and other planets orbit the

sun (heliocentric model) in the same

direction and each planet takes a

different amount of time to orbit the sun.

D. 1700’s – Johannes Kepler –

• Orbits are not perfect circles but an ellipse (egg shaped) and run counter-clockwise

2. Developed three laws to help understand

planetary orbits around the sun:

a. The path of each planet about the sun is an ellipse with the sun at one focus.

b. Each planet moves so that an imaginary line drawn from the sun to the planet sweeps out equal areas in equal periods of time.

c. The ratio of the period of rotation squared

“T” of any two planets revolving about the sun

is equal to the ratio of the cubes of their

mean distance “R” from the sun.

E. Late 1700’s – Issac Newton – Developed Law of Inertia (an object’s motion will not change unless the object is acted upon by an outside force). Also, orbits are a result of a planet’s tendency to move in a straight line (direction), and the gravity of the sun and other celestial bodies (ex: moon).

II. Movement of Celestial Bodies A. Period of Revolution – making 1

complete orbit around the sun or planet (for moons) = 1 year; Ex.

earth=365.26 days; Mercury=88 days; Pluto=248 earth years. Planets revolve in a

counterclockwise direction around the sun.

B. Period of Rotation – movement of planet around its axis = 1 day (dayand night cycle); Ex. earth=24 hours; Mercury=59 earth days; Pluto=6earth days.

C. Earth is tilted 23.5 degrees on its axis.

• The tilt is the reason day and night are not 12 hours each and why we have seasons.

a. The side of the planet tilted towards the sun receives more direct sunlight and has longer days during the summer.

C. Earth is tilted 23.5 degrees on its axis.

• The tilt is the reason day and night are not 12 hours each and why we have seasons.

2. The side of the planet tilted toward the sun, is receiving more direct sunlight and longer days (summer).

b. The side of the planet tilted away from the sun, is receiving less direct sunlight and shorter days (winter).

c. During the period of revolution where the earth is not tilted towards or away from sun are autumn and spring.

d. Any planet that is tilted on its axis will have seasons.e. These changes of the seasons are marked by specific dates:

i. Summer solstice (June 20/21) – the sun appears at itshighest point in the sky. Longest day of the yearii. Winter solstice (December 21/22) – the N. Hemisphere is atfull tilt away from sun. Shortest day of the year.

iii. Autumnal equinox – (day and night equal) September 22/23

iv. Vernal equinox – (day and night equal) March 20/21

2. Retrograde rotation occurs when planets

rotate from east to west; normal rotation

direction is west to east. Ex. Venus.

Review Questions

• 1. How is the earth positioned relative to the sun during the summer in the Northern Hemisphere? (you can draw this)

The earth is tilted toward the sun.

2. Why is it necessary that we have an extra day every four years? (hint:think about the period of revolution for the earth)

Because is takes 365.26 days for the earth to orbit the sun.

3. Describe how living conditions on earth might change if the earth’s axis werestraight up and down instead of tilted.

There would not be a change of seasons.

4. What two factors dictate the elliptical orbit pattern of all the planets?What would happen to the orbit of the planets if the sun were to shrink?

Inertia and gravity

The force of gravity would be less therefore

the orbit would increase.

III. The magnetic field of the earth

C. Magnetic field is created from invisible lines of force connecting the 2 magnetic

poles (not geographic poles) of the earth.

B. Created from the movement of materials in the inner core of the

earth, primarily iron and nickel

C. Slowly changes over time and completely reverses the field after aperiod of time.

D. Importance of the magnetic field:

a. Is important for navigation around the planet.

b. Shields earth from cosmic radiation

E. Field creates the magnetosphere – area of space that is affected by themagnetic field. Begins 1000km in earth’s atmosphere and extends 64,000km into space on side facing sun and millions of kilometers into space on side of earth facing away from the sun (due to solar wind, blowing the sphere out into space)

F. Van Allen Radiation Belts 1. The inner part of the magnetosphere where charged particles of solar winds are trapped.

• Inner belt has positive particles, such as protons.

2. Outer belt has negative particles, such as electrons

3. When these particles interact with the

upper atmosphere layer it creates aurora

(giving off light). Ex. northern lights

IV. The Moon, our most important satellite

A. ¼ the size of earth, 1/6 less gravity.

B. 384,403km away from earth. This was determined when astronautsplaced a small mirror on the moon and a laser was bounced off themirror from earth. The time to travel to the moon was timed andused to determine the distance, based on the speed of light.

C. Moon and earth are composed of the mostly the same material, which suggeststhey formed around the same time.

D. Temperature range of the moon is 100 degrees Celsius during the dayand –175 degrees Celsius at night.

E. Topography of the moon consists of

craters (made by meter strikes),

highlands (extinct volcanoes), rills

(cracks, either from lava flow or cracks

created from moonquakes)

F. Rotation around axis is around 27 days and revolution around earth isaround 27 days. The same side of the moon is ALWAYS facing earth because its rate of rotation is equal to its revolution.

• Perigee is when the orbit of the moon brings it closest to the earth

2. Apogee is when the orbit of the moon is farthest from the earth

G. The Moon appears to move west across the sky (because of the rotation of the earth) but actually moves eastward (retrograde rotation). You can observe the moon relative to stars and you will observe the eastward motion of the earth.

V. Outcome of movements of earth, moon and sun

A. Phases of the moon - The phases are the result of what sunlight is reflected by the moon.

• When the moon is full the earth is

between the sun and the moon, we see

the full side of the moon.

2. When the moon is new, the moon is

between the earth and the sun, no light is

reflected.

3. A waxing moon is the phase where more of the moon is gradually seen over two weeks.

4. A waning moon is the phase where less

of the moon gradually disappears over

two weeks.

B. Eclipses – As the moon and earth revolve together around the sun,sometimes one of the celestial objects block the suns rays. There aretwo types.

• Solar Eclipse – The new moon is between the sun and earth.

These do not occur once a month due to the plane of orbit of the moon around the earth. Label the bodies below: sun, moon, earth

Solar Eclipse

Solar Eclipse Paths

2. Lunar Eclipse – When the earth is between the sun and the full moon, causing the earth’s shadow to fall on the moon (looks dark and coppery color, due to the bending of light waves). Label the bodies below: sun, moon, earth

a. If you are in the inner shadow of an eclipse it is called the umbra

b. If you are in the outer shadow of an eclipse it is called the penumbra.

3. Tides – Caused by the gravitational pull between the earth and the moon.

a. Moon pulls on earth through the orbit, causing the rise and fallof the oceans.

b. Tides go between high and low every 6 hours. There is a 50 minutes delay each day due to the moon rising 50 minutes latereach day.

c. The sides of the earth in a direct line with the moon experiencehigh tides, while the sides of the earthat 90 degrees with the moon experience low tides

d. Full moon and new moons result in higher tides (spring tides) because the moon and sun are in direct line with the earth, increasing the gravitational pull. First quarter and last quarter of the moon phases are the high tides that are lower than usual(neap tides) because the moon and sun are at 90o with the earth, there is less gravitational pull

Review Questions

1. Draw the position of the sun, moon and earth of a new moon and a full moon.

Remember that the moon revolves around the earth in a counterclockwise direction.

Position of sun, earth and moon at full moon.

2. Why does the lunar eclipse occur either two weeks before or after the totalsolar eclipse?

What did you Learn?Copy and answer the questions

• What is a planet?• What is an orbit? How is it shaped?• Name two physical forces that affect a planet’s orbit.• What do you know about the Earth’s rotation, revolution,

and tilt?• What do you know about seasonal changes?• How does the magnetic field protect the Earth?• How does the moon affect the ocean tides? How often do

they change?• What do you know of the phases of the moon?• What is a solar eclipse? A lunar eclipse?• What did you learn about planetary motion that is not listed

above?