Actual Motions of EarthActual Motions of Earth

Models that Help Explain Models that Help Explain

Apparent Celestial MotionsApparent Celestial Motions

• Geocentric model- Earth is stationary in the center of the universe and all celestial objects revolve around it.

• Heliocentric model- the sun is the center• Heliocentric model- the sun is the centerof the solar system and all parts revolve around it.

Answer the questions on your notes near the explanation of each model.

Geocentric system

Actual Earth MotionsActual Earth Motions

• Rotation- the spinning of Earth on its axis, the imaginary line that runs from the north pole to the south pole.– Earth’s axis is tilted 23 ½ ° with the – Earth’s axis is tilted 23 ½ ° with the

northern end pointing toward the North Star (Polaris).

– Earth rotates west to east (counterclockwise)

– Causes daily motion of celestial objects like sun, moon & stars

Actual Motion of the EarthActual Motion of the Earth

Rotation

Evidence of Earth’s RotationEvidence of Earth’s Rotation

1. The Foucault Pendulum- a freely swinging pendulum whose path appears to change in a predictable way (if the Earth didn’t rotate, it would swing in the Earth didn’t rotate, it would swing in the same way)

Great info

2. The Coriolis Effect- the tendency of all particles of matter moving at Earth’s surface to be deflected, or curve away, from a straight line path.

Evidence of Earth’s RotationEvidence of Earth’s Rotation

– Deflection is to the right in the Northern

hemisphere

– Deflection is to the left in the Southern

Hemisphere .

Actual Earth MotionsActual Earth Motions

• Revolution- a planet’s movement around the sun in a path called an orbit (the movement of one body around another).– Causes seasonal motion of celestial – Causes seasonal motion of celestial

objects, like sun & constellations

– The planets revolve counterclockwisearound the sun as seen from Earth’s north pole.

Actual Motion of the EarthActual Motion of the Earth

Revolution

Evidence of Earth’s Revolution Around Evidence of Earth’s Revolution Around

the Sunthe Sun• Seasons - If Earth didn’t revolve around the

sun, the same part of Earth would be tilted

toward the sun and receive the more direct

rays. THE SEASONS WOULD NOT

CHANGE.– Summer is when we tilt toward the sun & the sun is – Summer is when we tilt toward the sun & the sun is

higher in the sky

– Winter is when we tilt away from the sun & the sun is lower in the sky

• Constellations - groups of stars that form a

pattern

– We see different constellations at differenttimes of the year.

Reasons for the SeasonsReasons for the Seasons

Reasons for the SeasonsReasons for the Seasons

Evidence of Earth’s Revolution Evidence of Earth’s Revolution

Around the SunAround the Sun

ConstellationsConstellations

Which constellations are both visible at midnight to an observer in Virginia when Earth is located at position D?1. Aries & Taurus1. Aries & Taurus2. Pisces & Libra3. Leo & Virgo4. Aquarius &

Scorpio

RetrogradeRetrograde Motion Motion -- PtolemyPtolemyWhen the planets move backwards

in the sky

Very difficult to fit retrograde

motion into geocentric view –

requires planets to travel in circles on circles

Retrograde Motion Retrograde Motion -- ActualActual

In actuality, in a heliocentric view, Earth is simply

passing or being passed by other planets, creating the odd backwards movement

http://www.lasalle.edu/~smithsc/Astronomy/retrograd.html

Earth’s MotionsEarth’s Motions

• Rotation– Polaris

http://ecuip.lib.uchicago.edu/diglib/science/cultural_astronomy/interactives/polaris/polaris.swf

• Revolution– Seasons

http://astro.unl.edu/classaction/animations/coordsmotion/eclipticsimulator.html

polaris.swf – Direct/Indirect rays http://astro.unl.edu/classaction/animations/coordsmotion/sunsrays.html

– Motion of Sun http://astro.unl.edu/classaction/animations/coordsmotion/sunmotions.html

Motion of Celestial Objects Motion of Celestial Objects --

ReviewReview

1. What two motions of the Earth cause the apparent motion of celestial objects?

2. What is one proof for each of these 2. What is one proof for each of these motions? Indicate which is which.

3. When is the altitude (height) of the sun higher or lower?