Question of the Day What is the Shape of Earth’s Orbit? New Sub-unit: Motions of Earth, Moon, Sun Question of the Day What is the Shape of Earth’s Orbit? “Mean distance from Earth to sun is 149.6 mil. km.” Inference about shape of Earth’s orbit? 52 http://chickscope.beckman.uiuc.edu/explore/eggmath/shape/pins.html

Planetary Orbits Not circular Slightly Elliptical (oval)

Ellipse: Oval shape Foci: “Centers” Focal Distance: (D) Distance between foci Major Axis: (L) Line intersects foci

Eccentricity Degree of flattening or “ovalness” How far from circle

Eccentricity = D (distance between foci) L (length of major axis)

Eccentricity Value Range from 0  1 Line = 1 Circle = 0 http://chickscope.beckman.uiuc.edu/explore/eggmath/shape/pins.html

Is the sun really the center of our solar system? Question of the Day Is the sun really the center of our solar system? How could we make this more elliptical? Increase distance between foci, and ellipse will become more eccentric. 53

Orbits Orbital Path: Planet around sun Perihelion: Closest to sun Aphelion – Farthest from sun (“Away”)

Earth’s perihelion: 147,000,000 km Earth’s aphelion: 152,000,000 km Perihelion - January! Aphelion - July!

Sun is NOT center of Earth’s Orbit. It’s 1 of 2 foci Sun is NOT center of Earth’s Orbit. It’s 1 of 2 foci. Other focus is “ghost.”

What is shape of Earth’s orbit? Slightly eccentric ellipse. What does Earth’s orbit look like on this scale? A circle

Why is Earth’s orbit like a “curveball,” not a “fastball”? Question of the Day Why is Earth’s orbit like a “curveball,” not a “fastball”? Warm-Up Perihelion = ? Aphelion = ? Revolution = ? Rotation = ? 55

Why is Earth’s orbit like a “curveball,” not a “fastball”? Question of the Day Why is Earth’s orbit like a “curveball,” not a “fastball”? Warm-Up Perihelion = Closest to sun Aphelion = Farthest (AWAY) from sun Rotation = Spin on axis Revolution = Orbit around sun

Inertia vs. Gravitation Inertia - object at rest stays at rest, & object in motion stays in motion, unless an opposing force. Gravitation - attractive force that exists between any 2 objects. Greater mass = greater attraction Closer objects = greater attraction

Orbit is dynamic equilibrium between inertia vs. gravity

Johannes Kepler (1571 – 1630) German mathematician Discovered Laws of Planetary Motion

1st Law All planets orbit in an ______ (oval) w/ Sun at 1 focus

1st Law All planets orbit in an ellipse (oval) w/ Sun at 1 focus

Planetary orbits are close to circular

Comets are most eccentric Ex. Halley’s Comet e = .967

2nd Law Planet’s speed (orbital velocity) changes as it orbits Sun Fastest when Near Sun

Equal areas swept out in equal time periods

Areas in red are equal. Time required for planet to travel from A to B is 21 days. How long will it take to travel from C to D?

3rd Law Farther from Sun, the longer or shorter the time for 1 orbit?

Farther from Sun, the the time for 1 orbit?

3rd Law Farther from Sun, the longer the time for 1 orbit 1. Outer (Jovian) Planets Slower 2. Inner (Terrestrial) Planets Faster

Gravity M1 M2 D2 M = Mass D = Distance between centers of gravity of two objects Direct or Indirect? 1) Gravity vs. Mass 2) Gravity vs. Distance

Question of the Day Do stars really move? Match: 1) Revolution 2) Rotation 3) Spin on axis 4) Orbit A B 56

1st Law All planets orbit in an (oval) w/ the Sun at 1 focus (center point)

2nd Law A planet’s orbital speed is fastest when it is _________ the Sun

3rd Law The farther a planet is from the Sun, the the period for 1 orbit

1. Apparent Motion of Stars Stars are stationary!!! Appear to move across sky

Star Trail (Eastern Horizon)

Star Trail (Western Horizon)

Star Trail (Northern Horizon)

A. Daily motion Caused by Earth’s rotation 15º / hr. Counterclockwise

B. Seasonal Changes Constellations visible different times of year Caused by Earth’s revolution Ex. Orion  autumn/winter

2. Apparent Motion of Planets Same daily motion as stars - Earth’s Rotation Retrograde (backward) motion - Earth’s Revolution

Retrograde Motion Earth – green Mars – red Sun – yellow

3. Apparent Motion of Moon E to W daily - Earth’s rotation Rises 50 minutes later each day - moon’s orbit around Earth

4. Apparent Motion of Sun A. Daily Motion – Earth’s rotation Arc-shaped path E to W 15º / hr.

B. Yearly Motion of Sun Due to Earth’s revolution Altitude above horizon changes - due to Earth’s tilt

Geocentric Model Earth-centered Earth is stationary Sun & Planets orbit Earth Could not explain retrograde motion

Heliocentric Model Sun-centered Earth rotates on axis Earth orbits Sun

Question of the Day How do we know Earth rotates & revolves? What is X? What is Y? Heliocentric Geocentric X Y 57

Earth’s Rotation 360° counterclockwise - 24 hrs. (day) 15° / hr. Axis - imaginary line from N. Pole to S. Pole Axis tilted 23.5°

Evidence of Earth’s Rotation 1. Foucault Pendulum Freely swinging pendulum whose path appears to change.

2. Coriolis Effect Deflection of moving particles (ex 2. Coriolis Effect Deflection of moving particles (ex., wind) to right in N. Hemisphere http://www.youtube.com/watch?v=_36MiCUS1ro http://www.youtube.com/watch?v=mcPs_OdQOYU&feature=related

3. Day & Night (sunrise, sunset) The prime meridian SUNLIGHT A time meridian A standard time zone is 15° wide.

Earth’s Revolution Elliptical orbit with sun as 1 of 2 foci Counterclockwise 1 revolution = 365.26 days (1 year)

Evidence of Earth’s Revolution 1. Changing Seasons

2. Changing Length of Day

Evidence of Earth’s Revolution 3. Changing Constellations

Question of the Day What’s are the reasons for the seasons? 1. Summer solstice ___ 2. Winter solstice ___ 3. Vernal & autumnal equinoxes ___ A) Day & night are equal B) Sun - longest path, highest in sky C) Sun - shortest path, lowest in sky 58

Most Important Point! Earth’s tilt & orientation stay the same as it orbits Sun Axis always points to Polaris

Reasons for the Seasons? Earth’s Tilt & Revolution SUN

Tropics

Tropic of Cancer (23.5º N) Tropic of Capricorn (23.5º S)

Summer Solstice (June 21) Direct sun over Tropic of Cancer Longest day (in N. Hemisphere)

Summer Solstice Sun at highest altitude Sunrise/set NORTH of Due East-West

Winter Solstice (December 21) Direct sun over Tropic of Capricorn Shortest day

Winter Solstice Sun at lowest altitude Sunrise/set SOUTH of due East-West

Vernal Equinox (March 21) & Autumnal Equinox (Sept. 21) Direct sun over Equator (0º) Equal day and night everywhere

Vernal (Spring) & Autumnal Equinoxes Sun at middle altitude Sunrise/Sunset due East-West

Question of the Day Why does the sun’s path change with the seasons? 1. In which horizon are these star trails possible: N, S, E, W? 2. What star is at center? 3. Rotation or Revolution? 59

Daily Motion of the Sun Due to Earth’s _______ Moves in an ___-shaped path East to West or West to East direction? Rate = ___ degrees/hr

Seasonal Motions of the Sun Caused by Earth’s _____________ around the _____ .

1. Changes in Sun’s Altitude (angle above horizon) Increases Dec. 21 (Winter Solstice) to June 21 (Summer Solstice) Decreases June 21  Dec. 21

Solar Noon Local time when sun reaches highest point that day

Earth’s Revolution

2. Location of Sunrise/Sunset March 21 (vernal equinox) & Sept. 21 (autumnal equinox), sunrise/set = due east to due west March 21  Sept 21, sunrise/set more north Sept 21  March 21, sunrise/set more south

Sun is never directly overhead at 90º altitude (zenith) except in tropics!

Earth’s Revolution

3. Changes in Path Length As altitude increases, path length ________ _________ relationship with length of day.

Earth’s Revolution

Question of the Day What factors affect apparent path of sun? 1) Which shows summer solstice, winter solstice, equinoxes: A, B, C? 2) Which shows longest day, shortest day, same day as night? B C 60 http://astro.unl.edu/naap/motion1/animations/seasons_ecliptic.html

Celestial Sphere Imaginary sphere on which all celestial objects appear

Altitude # degrees above horizon Vertical measurement 0 degrees = horizon (Reference Pt.) 90 degrees = zenith or overhead

Azimuth # degrees from due north Horizontal North = 0º (Reference Pt.) East = 90º South = 180º West = 270º

What is reference point for altitude? What is reference point for azimuth?