Chapter 23

Sun, Earth, Moon System

• Position?• Center of the universe?• No, the Sun is the center of our solar system; Earth

travels around the Sun.• Shape?

• Flat? Would you fall off if you went too far?• No, the Earth is a sphere. Spheres are round, 3-D,

and all points on the surface a equidistant (??) from the center.

• EVIDENCE: Aristotle: » Earth casts curved shadow» Appearance of approaching ships» Pictures from space probes and satellites.

Motion

• Earth rotates around an imaginary line called the axis.

• This causes day and night. • One rotation = 24 hours• Rotates 365 times in a

year

• Earth also revolves around the Sun

• Revolution = 1 year

Rotation causes…..• Day and night

– Rotates counterclockwise or East to West

• Changes in position of Sun, stars, planets– Sun rises in the East

sets in the West– Observed position

changes are due to rotation of Earth.

Revolution

One trip around the Sun!!!

** Revolution = length of a year

(Closer planets have shorter years, farther planets have longer years)

** Orbit = path the Earth takes around the Sun

* Ellipse = shape of the Earth’s orbit.

• http://www.mathsisfun.com/earth-orbit.html

Get your partner to say the following!!

• SPHERE• REVOLUTION• ROTATION• YEAR• ARISTOTLE• EAST TO WEST• ELLIPSE• ORBIT

Revolution

• Closest to Sun in January????

• Farthest in July???• Then what causes

seasons???

Tilt

• The Earth is tilted on its axis at about 23.5o

• Daylight hours are longer for the hemisphere tilted toward the SUN!!

• This hemisphere also receives more INTENSE RADIATION from the Sun.

http://astro.unl.edu/naap/motion1/animations/seasons_ecliptic.html

• http://highered.mcgraw-hill.com/sites/007299181x/student_view0/chapter2/seasons_interactive.html

Equinoxes

• Sun is directly over the equator. • Equal hours of day and night. • Neither hemisphere is pointed toward Sun• Happens twice/year

– N. Hemisphere

»Spring = March 21 or 22

(Vernal Equinox)»Autumn = September 21 or 22

(Autumnal Equinox)

Solstice

• Point at which Sun reaches greatest distance north or south of equator.

• Happens twice/year»June 21 or 22 (Summer Solstice)»MOST DAYLIGHT»December 21 or 22 (Winter Solstice)»MOST NIGHT

Get your partner to say the following:

• Sphere

• Axis

• Rotation

• Revolution

• Ellipse

• Equinox

• Solstice

Get your partner to say the following:

• 24 hours

• 365 days

• Autumn

• Orbit

• Tropic of Cancer

• June 21 or 22

• March 21 or 22